DRM Internals¶

This chapter documents DRM internals relevant to driver authors and developers working to add support for the latest features to existing drivers.

First, we go over some typical driver initialization requirements, like setting up command buffers, creating an initial output configuration, and initializing core services. Subsequent sections cover core internals in more detail, providing implementation notes and examples.

The DRM layer provides several services to graphics drivers, many of them driven by the application interfaces it provides through libdrm, the library that wraps most of the DRM ioctls. These include vblank event handling, memory management, output management, framebuffer management, command submission & fencing, suspend/resume support, and DMA services.

Driver Initialization¶

At the core of every DRM driver is a struct drm_driver structure. Drivers typically statically initialize a drm_driver structure, and then pass it to drm_dev_alloc() to allocate a device instance. After the device instance is fully initialized it can be registered (which makes it accessible from userspace) using drm_dev_register().

The struct drm_driver structure contains static information that describes the driver and features it supports, and pointers to methods that the DRM core will call to implement the DRM API. We will first go through the struct drm_driver static information fields, and will then describe individual operations in details as they get used in later sections.

Driver Information¶

Major, Minor and Patchlevel¶

int major; int minor; int patchlevel; The DRM core identifies driver versions by a major, minor and patch level triplet. The information is printed to the kernel log at initialization time and passed to userspace through the DRM_IOCTL_VERSION ioctl.

The major and minor numbers are also used to verify the requested driver API version passed to DRM_IOCTL_SET_VERSION. When the driver API changes between minor versions, applications can call DRM_IOCTL_SET_VERSION to select a specific version of the API. If the requested major isn’t equal to the driver major, or the requested minor is larger than the driver minor, the DRM_IOCTL_SET_VERSION call will return an error. Otherwise the driver’s set_version() method will be called with the requested version.

Name, Description and Date¶

char *name; char *desc; char *date; The driver name is printed to the kernel log at initialization time, used for IRQ registration and passed to userspace through DRM_IOCTL_VERSION.

The driver description is a purely informative string passed to userspace through the DRM_IOCTL_VERSION ioctl and otherwise unused by the kernel.

The driver date, formatted as YYYYMMDD, is meant to identify the date of the latest modification to the driver. However, as most drivers fail to update it, its value is mostly useless. The DRM core prints it to the kernel log at initialization time and passes it to userspace through the DRM_IOCTL_VERSION ioctl.

Module Initialization¶

This library provides helpers registering DRM drivers during module initialization and shutdown. The provided helpers act like bus-specific module helpers, such as module_pci_driver(), but respect additional parameters that control DRM driver registration.

Below is an example of initializing a DRM driver for a device on the PCI bus.

struct pci_driver my_pci_drv = {
};

drm_module_pci_driver(my_pci_drv);

The generated code will test if DRM drivers are enabled and register the PCI driver my_pci_drv. For more complex module initialization, you can still use module_init() and module_exit() in your driver.

Managing Ownership of the Framebuffer Aperture¶

A graphics device might be supported by different drivers, but only one driver can be active at any given time. Many systems load a generic graphics drivers, such as EFI-GOP or VESA, early during the boot process. During later boot stages, they replace the generic driver with a dedicated, hardware-specific driver. To take over the device the dedicated driver first has to remove the generic driver. DRM aperture functions manage ownership of DRM framebuffer memory and hand-over between drivers.

DRM drivers should call drm_aperture_remove_conflicting_framebuffers() at the top of their probe function. The function removes any generic driver that is currently associated with the given framebuffer memory. If the framebuffer is located at PCI BAR 0, the rsp code looks as in the example given below.

static const struct drm_driver example_driver = {
        ...
};

static int remove_conflicting_framebuffers(struct pci_dev *pdev)
{
        resource_size_t base, size;
        int ret;

        base = pci_resource_start(pdev, 0);
        size = pci_resource_len(pdev, 0);

        return drm_aperture_remove_conflicting_framebuffers(base, size,
                                                            &example_driver);
}

static int probe(struct pci_dev *pdev)
{
        int ret;

        // Remove any generic drivers...
        ret = remove_conflicting_framebuffers(pdev);
        if (ret)
                return ret;

        // ... and initialize the hardware.
        ...

        drm_dev_register();

        return 0;
}

PCI device drivers should call drm_aperture_remove_conflicting_pci_framebuffers() and let it detect the framebuffer apertures automatically. Device drivers without knowledge of the framebuffer’s location shall call drm_aperture_remove_framebuffers(), which removes all drivers for known framebuffer.

Drivers that are susceptible to being removed by other drivers, such as generic EFI or VESA drivers, have to register themselves as owners of their given framebuffer memory. Ownership of the framebuffer memory is achieved by calling devm_aperture_acquire_from_firmware(). On success, the driver is the owner of the framebuffer range. The function fails if the framebuffer is already owned by another driver. See below for an example.

static int acquire_framebuffers(struct drm_device *dev, struct platform_device *pdev)
{
        resource_size_t base, size;

        mem = platform_get_resource(pdev, IORESOURCE_MEM, 0);
        if (!mem)
                return -EINVAL;
        base = mem->start;
        size = resource_size(mem);

        return devm_acquire_aperture_from_firmware(dev, base, size);
}

static int probe(struct platform_device *pdev)
{
        struct drm_device *dev;
        int ret;

        // ... Initialize the device...
        dev = devm_drm_dev_alloc();
        ...

        // ... and acquire ownership of the framebuffer.
        ret = acquire_framebuffers(dev, pdev);
        if (ret)
                return ret;

        drm_dev_register(dev, 0);

        return 0;
}

The generic driver is now subject to forced removal by other drivers. This only works for platform drivers that support hot unplug. When a driver calls drm_aperture_remove_conflicting_framebuffers() et al. for the registered framebuffer range, the aperture helpers call platform_device_unregister() and the generic driver unloads itself. It may not access the device’s registers, framebuffer memory, ROM, etc afterwards.

int drm_aperture_remove_framebuffers(const struct drm_driver *req_driver)¶: remove all existing framebuffers

Parameters

const struct drm_driver *req_driver: requesting DRM driver

Description

This function removes all graphics device drivers. Use this function on systems that can have their framebuffer located anywhere in memory.

Return

0 on success, or a negative errno code otherwise

int devm_aperture_acquire_from_firmware(struct drm_device *dev, resource_size_t base, resource_size_t size)¶: Acquires ownership of a firmware framebuffer on behalf of a DRM driver.

Parameters

struct drm_device *dev: the DRM device to own the framebuffer memory
resource_size_t base: the framebuffer’s byte offset in physical memory
resource_size_t size: the framebuffer size in bytes

Description

Installs the given device as the new owner of the framebuffer. The function expects the framebuffer to be provided by a platform device that has been set up by firmware. Firmware can be any generic interface, such as EFI, VESA, VGA, etc. If the native hardware driver takes over ownership of the framebuffer range, the firmware state gets lost. Aperture helpers will then unregister the platform device automatically. Acquired apertures are released automatically if the underlying device goes away.

The function fails if the framebuffer range, or parts of it, is currently owned by another driver. To evict current owners, callers should use drm_aperture_remove_conflicting_framebuffers() et al. before calling this function. The function also fails if the given device is not a platform device.

Return

0 on success, or a negative errno value otherwise.

int drm_aperture_remove_conflicting_framebuffers(resource_size_t base, resource_size_t size, const struct drm_driver *req_driver)¶: remove existing framebuffers in the given range

Parameters

resource_size_t base: the aperture’s base address in physical memory
resource_size_t size: aperture size in bytes
const struct drm_driver *req_driver: requesting DRM driver

Description

This function removes graphics device drivers which use the memory range described by base and size.

Return

0 on success, or a negative errno code otherwise

int drm_aperture_remove_conflicting_pci_framebuffers(struct pci_dev *pdev, const struct drm_driver *req_driver)¶: remove existing framebuffers for PCI devices

Parameters

struct pci_dev *pdev: PCI device
const struct drm_driver *req_driver: requesting DRM driver

Description

This function removes graphics device drivers using the memory range configured for any of pdev’s memory bars. The function assumes that a PCI device with shadowed ROM drives a primary display and so kicks out vga16fb.

Return

0 on success, or a negative errno code otherwise

Device Instance and Driver Handling¶

A device instance for a drm driver is represented by struct drm_device. This is allocated and initialized with devm_drm_dev_alloc(), usually from bus-specific ->probe() callbacks implemented by the driver. The driver then needs to initialize all the various subsystems for the drm device like memory management, vblank handling, modesetting support and initial output configuration plus obviously initialize all the corresponding hardware bits. Finally when everything is up and running and ready for userspace the device instance can be published using drm_dev_register().

There is also deprecated support for initializing device instances using bus-specific helpers and the drm_driver.load callback. But due to backwards-compatibility needs the device instance have to be published too early, which requires unpretty global locking to make safe and is therefore only support for existing drivers not yet converted to the new scheme.

When cleaning up a device instance everything needs to be done in reverse: First unpublish the device instance with drm_dev_unregister(). Then clean up any other resources allocated at device initialization and drop the driver’s reference to drm_device using drm_dev_put().

Note that any allocation or resource which is visible to userspace must be released only when the final drm_dev_put() is called, and not when the driver is unbound from the underlying physical struct device. Best to use drm_device managed resources with drmm_add_action(), drmm_kmalloc() and related functions.

devres managed resources like devm_kmalloc() can only be used for resources directly related to the underlying hardware device, and only used in code paths fully protected by drm_dev_enter() and drm_dev_exit().

Display driver example¶

The following example shows a typical structure of a DRM display driver. The example focus on the probe() function and the other functions that is almost always present and serves as a demonstration of devm_drm_dev_alloc().

struct driver_device {
        struct drm_device drm;
        void *userspace_facing;
        struct clk *pclk;
};

static const struct drm_driver driver_drm_driver = {
        [...]
};

static int driver_probe(struct platform_device *pdev)
{
        struct driver_device *priv;
        struct drm_device *drm;
        int ret;

        priv = devm_drm_dev_alloc(&pdev->dev, &driver_drm_driver,
                                  struct driver_device, drm);
        if (IS_ERR(priv))
                return PTR_ERR(priv);
        drm = &priv->drm;

        ret = drmm_mode_config_init(drm);
        if (ret)
                return ret;

        priv->userspace_facing = drmm_kzalloc(..., GFP_KERNEL);
        if (!priv->userspace_facing)
                return -ENOMEM;

        priv->pclk = devm_clk_get(dev, "PCLK");
        if (IS_ERR(priv->pclk))
                return PTR_ERR(priv->pclk);

        // Further setup, display pipeline etc

        platform_set_drvdata(pdev, drm);

        drm_mode_config_reset(drm);

        ret = drm_dev_register(drm);
        if (ret)
                return ret;

        drm_fbdev_generic_setup(drm, 32);

        return 0;
}

// This function is called before the devm_ resources are released
static int driver_remove(struct platform_device *pdev)
{
        struct drm_device *drm = platform_get_drvdata(pdev);

        drm_dev_unregister(drm);
        drm_atomic_helper_shutdown(drm)

        return 0;
}

// This function is called on kernel restart and shutdown
static void driver_shutdown(struct platform_device *pdev)
{
        drm_atomic_helper_shutdown(platform_get_drvdata(pdev));
}

static int __maybe_unused driver_pm_suspend(struct device *dev)
{
        return drm_mode_config_helper_suspend(dev_get_drvdata(dev));
}

static int __maybe_unused driver_pm_resume(struct device *dev)
{
        drm_mode_config_helper_resume(dev_get_drvdata(dev));

        return 0;
}

static const struct dev_pm_ops driver_pm_ops = {
        SET_SYSTEM_SLEEP_PM_OPS(driver_pm_suspend, driver_pm_resume)
};

static struct platform_driver driver_driver = {
        .driver = {
                [...]
                .pm = &driver_pm_ops,
        },
        .probe = driver_probe,
        .remove = driver_remove,
        .shutdown = driver_shutdown,
};
module_platform_driver(driver_driver);

Drivers that want to support device unplugging (USB, DT overlay unload) should use drm_dev_unplug() instead of drm_dev_unregister(). The driver must protect regions that is accessing device resources to prevent use after they’re released. This is done using drm_dev_enter() and drm_dev_exit(). There is one shortcoming however, drm_dev_unplug() marks the drm_device as unplugged before drm_atomic_helper_shutdown() is called. This means that if the disable code paths are protected, they will not run on regular driver module unload, possibly leaving the hardware enabled.

enum switch_power_state¶: power state of drm device

Constants

DRM_SWITCH_POWER_ON: Power state is ON
DRM_SWITCH_POWER_OFF: Power state is OFF
DRM_SWITCH_POWER_CHANGING: Power state is changing
DRM_SWITCH_POWER_DYNAMIC_OFF: Suspended

struct drm_device¶: DRM device structure

Definition:

struct drm_device {
    int if_version;
    struct kref ref;
    struct device *dev;
    struct {
        struct list_head resources;
        void *final_kfree;
        spinlock_t lock;
    } managed;
    const struct drm_driver *driver;
    void *dev_private;
    struct drm_minor *primary;
    struct drm_minor *render;
    struct drm_minor *accel;
    bool registered;
    struct drm_master *master;
    u32 driver_features;
    bool unplugged;
    struct inode *anon_inode;
    char *unique;
    struct mutex struct_mutex;
    struct mutex master_mutex;
    atomic_t open_count;
    struct mutex filelist_mutex;
    struct list_head filelist;
    struct list_head filelist_internal;
    struct mutex clientlist_mutex;
    struct list_head clientlist;
    bool vblank_disable_immediate;
    struct drm_vblank_crtc *vblank;
    spinlock_t vblank_time_lock;
    spinlock_t vbl_lock;
    u32 max_vblank_count;
    struct list_head vblank_event_list;
    spinlock_t event_lock;
    unsigned int num_crtcs;
    struct drm_mode_config mode_config;
    struct mutex object_name_lock;
    struct idr object_name_idr;
    struct drm_vma_offset_manager *vma_offset_manager;
    struct drm_vram_mm *vram_mm;
    enum switch_power_state switch_power_state;
    struct drm_fb_helper *fb_helper;
    struct dentry *debugfs_root;
};

Members

if_version

Highest interface version set

ref

Object ref-count

dev

Device structure of bus-device

managed

Managed resources linked to the lifetime of this drm_device as tracked by ref.

driver

DRM driver managing the device

dev_private

DRM driver private data. This is deprecated and should be left set to NULL.

Instead of using this pointer it is recommended that drivers use devm_drm_dev_alloc() and embed struct drm_device in their larger per-device structure.

primary

Primary node. Drivers should not interact with this directly. debugfs interfaces can be registered with drm_debugfs_add_file(), and sysfs should be directly added on the hardware (and not character device node) struct device dev.

render

Render node. Drivers should not interact with this directly ever. Drivers should not expose any additional interfaces in debugfs or sysfs on this node.

accel

Compute Acceleration node

registered

Internally used by drm_dev_register() and drm_connector_register().

master

Currently active master for this device. Protected by master_mutex

driver_features

per-device driver features

Drivers can clear specific flags here to disallow certain features on a per-device basis while still sharing a single struct drm_driver instance across all devices.

unplugged

Flag to tell if the device has been unplugged. See drm_dev_enter() and drm_dev_is_unplugged().

anon_inode

inode for private address-space

unique

Unique name of the device

struct_mutex

Lock for others (not drm_minor.master and drm_file.is_master)

TODO: This lock used to be the BKL of the DRM subsystem. Move the: lock into i915, which is the only remaining user.

master_mutex

Lock for drm_minor.master and drm_file.is_master

open_count

Usage counter for outstanding files open, protected by drm_global_mutex

filelist_mutex

Protects filelist.

filelist

List of userspace clients, linked through drm_file.lhead.

filelist_internal

List of open DRM files for in-kernel clients. Protected by filelist_mutex.

clientlist_mutex

Protects clientlist access.

clientlist

List of in-kernel clients. Protected by clientlist_mutex.

vblank_disable_immediate

If true, vblank interrupt will be disabled immediately when the refcount drops to zero, as opposed to via the vblank disable timer.

This can be set to true it the hardware has a working vblank counter with high-precision timestamping (otherwise there are races) and the driver uses drm_crtc_vblank_on() and drm_crtc_vblank_off() appropriately. See also max_vblank_count and drm_crtc_funcs.get_vblank_counter.

vblank

Array of vblank tracking structures, one per struct drm_crtc. For historical reasons (vblank support predates kernel modesetting) this is free-standing and not part of struct drm_crtc itself. It must be initialized explicitly by calling drm_vblank_init().

vblank_time_lock

Protects vblank count and time updates during vblank enable/disable

vbl_lock

Top-level vblank references lock, wraps the low-level vblank_time_lock.

max_vblank_count

Maximum value of the vblank registers. This value +1 will result in a wrap-around of the vblank register. It is used by the vblank core to handle wrap-arounds.

If set to zero the vblank core will try to guess the elapsed vblanks between times when the vblank interrupt is disabled through high-precision timestamps. That approach is suffering from small races and imprecision over longer time periods, hence exposing a hardware vblank counter is always recommended.

This is the statically configured device wide maximum. The driver can instead choose to use a runtime configurable per-crtc value drm_vblank_crtc.max_vblank_count, in which case max_vblank_count must be left at zero. See drm_crtc_set_max_vblank_count() on how to use the per-crtc value.

If non-zero, drm_crtc_funcs.get_vblank_counter must be set.

vblank_event_list

List of vblank events

event_lock

Protects vblank_event_list and event delivery in general. See drm_send_event() and drm_send_event_locked().

num_crtcs

Number of CRTCs on this device

mode_config

Current mode config

object_name_lock

GEM information

object_name_idr

GEM information

vma_offset_manager

GEM information

vram_mm

VRAM MM memory manager

switch_power_state

Power state of the client. Used by drivers supporting the switcheroo driver. The state is maintained in the vga_switcheroo_client_ops.set_gpu_state callback

fb_helper

Pointer to the fbdev emulation structure. Set by drm_fb_helper_init() and cleared by drm_fb_helper_fini().

debugfs_root

Root directory for debugfs files.

Description

This structure represent a complete card that may contain multiple heads.

enum drm_driver_feature¶: feature flags

Constants

DRIVER_GEM: Driver use the GEM memory manager. This should be set for all modern drivers.
DRIVER_MODESET: Driver supports mode setting interfaces (KMS).
DRIVER_RENDER: Driver supports dedicated render nodes. See also the section on render nodes for details.
DRIVER_ATOMIC: Driver supports the full atomic modesetting userspace API. Drivers which only use atomic internally, but do not support the full userspace API (e.g. not all properties converted to atomic, or multi-plane updates are not guaranteed to be tear-free) should not set this flag.
DRIVER_SYNCOBJ: Driver supports drm_syncobj for explicit synchronization of command submission.
DRIVER_SYNCOBJ_TIMELINE: Driver supports the timeline flavor of drm_syncobj for explicit synchronization of command submission.
DRIVER_COMPUTE_ACCEL: Driver supports compute acceleration devices. This flag is mutually exclusive with DRIVER_RENDER and DRIVER_MODESET. Devices that support both graphics and compute acceleration should be handled by two drivers that are connected using auxiliary bus.
DRIVER_GEM_GPUVA: Driver supports user defined GPU VA bindings for GEM objects.
DRIVER_CURSOR_HOTSPOT: Driver supports and requires cursor hotspot information in the cursor plane (e.g. cursor plane has to actually track the mouse cursor and the clients are required to set hotspot in order for the cursor planes to work correctly).
DRIVER_USE_AGP: Set up DRM AGP support, see drm_agp_init(), the DRM core will manage AGP resources. New drivers don’t need this.
DRIVER_LEGACY: Denote a legacy driver using shadow attach. Do not use.
DRIVER_PCI_DMA: Driver is capable of PCI DMA, mapping of PCI DMA buffers to userspace will be enabled. Only for legacy drivers. Do not use.
DRIVER_SG: Driver can perform scatter/gather DMA, allocation and mapping of scatter/gather buffers will be enabled. Only for legacy drivers. Do not use.
DRIVER_HAVE_DMA: Driver supports DMA, the userspace DMA API will be supported. Only for legacy drivers. Do not use.
DRIVER_HAVE_IRQ: Legacy irq support. Only for legacy drivers. Do not use.

Description

See drm_driver.driver_features, drm_device.driver_features and drm_core_check_feature().

struct drm_driver¶: DRM driver structure

Definition:

struct drm_driver {
    int (*load) (struct drm_device *, unsigned long flags);
    int (*open) (struct drm_device *, struct drm_file *);
    void (*postclose) (struct drm_device *, struct drm_file *);
    void (*lastclose) (struct drm_device *);
    void (*unload) (struct drm_device *);
    void (*release) (struct drm_device *);
    void (*master_set)(struct drm_device *dev, struct drm_file *file_priv, bool from_open);
    void (*master_drop)(struct drm_device *dev, struct drm_file *file_priv);
    void (*debugfs_init)(struct drm_minor *minor);
    struct drm_gem_object *(*gem_create_object)(struct drm_device *dev, size_t size);
    int (*prime_handle_to_fd)(struct drm_device *dev, struct drm_file *file_priv, uint32_t handle, uint32_t flags, int *prime_fd);
    int (*prime_fd_to_handle)(struct drm_device *dev, struct drm_file *file_priv, int prime_fd, uint32_t *handle);
    struct drm_gem_object * (*gem_prime_import)(struct drm_device *dev, struct dma_buf *dma_buf);
    struct drm_gem_object *(*gem_prime_import_sg_table)(struct drm_device *dev,struct dma_buf_attachment *attach, struct sg_table *sgt);
    int (*dumb_create)(struct drm_file *file_priv,struct drm_device *dev, struct drm_mode_create_dumb *args);
    int (*dumb_map_offset)(struct drm_file *file_priv,struct drm_device *dev, uint32_t handle, uint64_t *offset);
    void (*show_fdinfo)(struct drm_printer *p, struct drm_file *f);
    int major;
    int minor;
    int patchlevel;
    char *name;
    char *desc;
    char *date;
    u32 driver_features;
    const struct drm_ioctl_desc *ioctls;
    int num_ioctls;
    const struct file_operations *fops;
};

Members

load

Backward-compatible driver callback to complete initialization steps after the driver is registered. For this reason, may suffer from race conditions and its use is deprecated for new drivers. It is therefore only supported for existing drivers not yet converted to the new scheme. See devm_drm_dev_alloc() and drm_dev_register() for proper and race-free way to set up a struct drm_device.

This is deprecated, do not use!

Returns:

Zero on success, non-zero value on failure.

open

Driver callback when a new struct drm_file is opened. Useful for setting up driver-private data structures like buffer allocators, execution contexts or similar things. Such driver-private resources must be released again in postclose.

Since the display/modeset side of DRM can only be owned by exactly one struct drm_file (see drm_file.is_master and drm_device.master) there should never be a need to set up any modeset related resources in this callback. Doing so would be a driver design bug.

Returns:

0 on success, a negative error code on failure, which will be promoted to userspace as the result of the open() system call.

postclose

One of the driver callbacks when a new struct drm_file is closed. Useful for tearing down driver-private data structures allocated in open like buffer allocators, execution contexts or similar things.

Since the display/modeset side of DRM can only be owned by exactly one struct drm_file (see drm_file.is_master and drm_device.master) there should never be a need to tear down any modeset related resources in this callback. Doing so would be a driver design bug.

lastclose

Called when the last struct drm_file has been closed and there’s currently no userspace client for the struct drm_device.

Modern drivers should only use this to force-restore the fbdev framebuffer using drm_fb_helper_restore_fbdev_mode_unlocked(). Anything else would indicate there’s something seriously wrong. Modern drivers can also use this to execute delayed power switching state changes, e.g. in conjunction with the VGA Switcheroo infrastructure.

This is called after postclose hook has been called.

NOTE:

All legacy drivers use this callback to de-initialize the hardware. This is purely because of the shadow-attach model, where the DRM kernel driver does not really own the hardware. Instead ownershipe is handled with the help of userspace through an inheritedly racy dance to set/unset the VT into raw mode.

Legacy drivers initialize the hardware in the firstopen callback, which isn’t even called for modern drivers.

unload

Reverse the effects of the driver load callback. Ideally, the clean up performed by the driver should happen in the reverse order of the initialization. Similarly to the load hook, this handler is deprecated and its usage should be dropped in favor of an open-coded teardown function at the driver layer. See drm_dev_unregister() and drm_dev_put() for the proper way to remove a struct drm_device.

The unload() hook is called right after unregistering the device.

release

Optional callback for destroying device data after the final reference is released, i.e. the device is being destroyed.

This is deprecated, clean up all memory allocations associated with a drm_device using drmm_add_action(), drmm_kmalloc() and related managed resources functions.

master_set

Called whenever the minor master is set. Only used by vmwgfx.

master_drop

Called whenever the minor master is dropped. Only used by vmwgfx.

debugfs_init

Allows drivers to create driver-specific debugfs files.

gem_create_object

constructor for gem objects

Hook for allocating the GEM object struct, for use by the CMA and SHMEM GEM helpers. Returns a GEM object on success, or an ERR_PTR()-encoded error code otherwise.

prime_handle_to_fd

PRIME export function. Only used by vmwgfx.

prime_fd_to_handle

PRIME import function. Only used by vmwgfx.

gem_prime_import

Import hook for GEM drivers.

This defaults to drm_gem_prime_import() if not set.

gem_prime_import_sg_table

Optional hook used by the PRIME helper functions drm_gem_prime_import() respectively drm_gem_prime_import_dev().

dumb_create

This creates a new dumb buffer in the driver’s backing storage manager (GEM, TTM or something else entirely) and returns the resulting buffer handle. This handle can then be wrapped up into a framebuffer modeset object.

Note that userspace is not allowed to use such objects for render acceleration - drivers must create their own private ioctls for such a use case.

Width, height and depth are specified in the drm_mode_create_dumb argument. The callback needs to fill the handle, pitch and size for the created buffer.

Called by the user via ioctl.

Returns:

Zero on success, negative errno on failure.

dumb_map_offset

Allocate an offset in the drm device node’s address space to be able to memory map a dumb buffer.

The default implementation is drm_gem_create_mmap_offset(). GEM based drivers must not overwrite this.

Called by the user via ioctl.

Returns:

Zero on success, negative errno on failure.

show_fdinfo

Print device specific fdinfo. See DRM client usage stats.

major

driver major number

minor

driver minor number

patchlevel

driver patch level

name

driver name

desc

driver description

date

driver date

driver_features

Driver features, see enum drm_driver_feature. Drivers can disable some features on a per-instance basis using drm_device.driver_features.

ioctls

Array of driver-private IOCTL description entries. See the chapter on IOCTL support in the userland interfaces chapter for the full details.

num_ioctls

Number of entries in ioctls.

fops

File operations for the DRM device node. See the discussion in file operations for in-depth coverage and some examples.

Description

This structure represent the common code for a family of cards. There will be one struct drm_device for each card present in this family. It contains lots of vfunc entries, and a pile of those probably should be moved to more appropriate places like drm_mode_config_funcs or into a new operations structure for GEM drivers.

devm_drm_dev_alloc¶

devm_drm_dev_alloc (parent, driver, type, member)

Resource managed allocation of a drm_device instance

Parameters

parent: Parent device object
driver: DRM driver
type: the type of the struct which contains struct drm_device
member: the name of the drm_device within type.

Description

This allocates and initialize a new DRM device. No device registration is done. Call drm_dev_register() to advertice the device to user space and register it with other core subsystems. This should be done last in the device initialization sequence to make sure userspace can’t access an inconsistent state.

The initial ref-count of the object is 1. Use drm_dev_get() and drm_dev_put() to take and drop further ref-counts.

It is recommended that drivers embed struct drm_device into their own device structure.

Note that this manages the lifetime of the resulting drm_device automatically using devres. The DRM device initialized with this function is automatically put on driver detach using drm_dev_put().

Return

Pointer to new DRM device, or ERR_PTR on failure.

bool drm_dev_is_unplugged(struct drm_device *dev)¶: is a DRM device unplugged

Parameters

struct drm_device *dev: DRM device

Description

This function can be called to check whether a hotpluggable is unplugged. Unplugging itself is singalled through drm_dev_unplug(). If a device is unplugged, these two functions guarantee that any store before calling drm_dev_unplug() is visible to callers of this function after it completes

WARNING: This function fundamentally races against drm_dev_unplug(). It is recommended that drivers instead use the underlying drm_dev_enter() and drm_dev_exit() function pairs.

bool drm_core_check_all_features(const struct drm_device *dev, u32 features)¶: check driver feature flags mask

Parameters

const struct drm_device *dev: DRM device to check
u32 features: feature flag(s) mask

Description

This checks dev for driver features, see drm_driver.driver_features, drm_device.driver_features, and the various enum drm_driver_feature flags.

Returns true if all features in the features mask are supported, false otherwise.

bool drm_core_check_feature(const struct drm_device *dev, enum drm_driver_feature feature)¶: check driver feature flags

Parameters

const struct drm_device *dev: DRM device to check
enum drm_driver_feature feature: feature flag

Description

This checks dev for driver features, see drm_driver.driver_features, drm_device.driver_features, and the various enum drm_driver_feature flags.

Returns true if the feature is supported, false otherwise.

bool drm_drv_uses_atomic_modeset(struct drm_device *dev)¶: check if the driver implements atomic_commit()

Parameters

struct drm_device *dev: DRM device

Description

This check is useful if drivers do not have DRIVER_ATOMIC set but have atomic modesetting internally implemented.

void drm_put_dev(struct drm_device *dev)¶: Unregister and release a DRM device

Parameters

struct drm_device *dev: DRM device

Description

Called at module unload time or when a PCI device is unplugged.

Cleans up all DRM device, calling drm_lastclose().

Note

Use of this function is deprecated. It will eventually go away completely. Please use drm_dev_unregister() and drm_dev_put() explicitly instead to make sure that the device isn’t userspace accessible any more while teardown is in progress, ensuring that userspace can’t access an inconsistent state.

bool drm_dev_enter(struct drm_device *dev, int *idx)¶: Enter device critical section

Parameters

struct drm_device *dev: DRM device
int *idx: Pointer to index that will be passed to the matching drm_dev_exit()

Description

This function marks and protects the beginning of a section that should not be entered after the device has been unplugged. The section end is marked with drm_dev_exit(). Calls to this function can be nested.

Return

True if it is OK to enter the section, false otherwise.

void drm_dev_exit(int idx)¶: Exit device critical section

Parameters

int idx: index returned from drm_dev_enter()

Description

This function marks the end of a section that should not be entered after the device has been unplugged.

void drm_dev_unplug(struct drm_device *dev)¶: unplug a DRM device

Parameters

struct drm_device *dev: DRM device

Description

This unplugs a hotpluggable DRM device, which makes it inaccessible to userspace operations. Entry-points can use drm_dev_enter() and drm_dev_exit() to protect device resources in a race free manner. This essentially unregisters the device like drm_dev_unregister(), but can be called while there are still open users of dev.

struct drm_device *drm_dev_alloc(const struct drm_driver *driver, struct device *parent)¶: Allocate new DRM device

Parameters

const struct drm_driver *driver: DRM driver to allocate device for
struct device *parent: Parent device object

Description

This is the deprecated version of devm_drm_dev_alloc(), which does not support subclassing through embedding the struct drm_device in a driver private structure, and which does not support automatic cleanup through devres.

Return

Pointer to new DRM device, or ERR_PTR on failure.

void drm_dev_get(struct drm_device *dev)¶: Take reference of a DRM device

Parameters

struct drm_device *dev: device to take reference of or NULL

Description

This increases the ref-count of dev by one. You must already own a reference when calling this. Use drm_dev_put() to drop this reference again.

This function never fails. However, this function does not provide any guarantee whether the device is alive or running. It only provides a reference to the object and the memory associated with it.

void drm_dev_put(struct drm_device *dev)¶: Drop reference of a DRM device

Parameters

struct drm_device *dev: device to drop reference of or NULL

Description

This decreases the ref-count of dev by one. The device is destroyed if the ref-count drops to zero.

int drm_dev_register(struct drm_device *dev, unsigned long flags)¶: Register DRM device

Parameters

struct drm_device *dev: Device to register
unsigned long flags: Flags passed to the driver’s .load() function

Description

Register the DRM device dev with the system, advertise device to user-space and start normal device operation. dev must be initialized via drm_dev_init() previously.

Never call this twice on any device!

NOTE

To ensure backward compatibility with existing drivers method this function calls the drm_driver.load method after registering the device nodes, creating race conditions. Usage of the drm_driver.load methods is therefore deprecated, drivers must perform all initialization before calling drm_dev_register().

Return

0 on success, negative error code on failure.

void drm_dev_unregister(struct drm_device *dev)¶: Unregister DRM device

Parameters

struct drm_device *dev: Device to unregister

Description

Unregister the DRM device from the system. This does the reverse of drm_dev_register() but does not deallocate the device. The caller must call drm_dev_put() to drop their final reference, unless it is managed with devres (as devices allocated with devm_drm_dev_alloc() are), in which case there is already an unwind action registered.

A special form of unregistering for hotpluggable devices is drm_dev_unplug(), which can be called while there are still open users of dev.

This should be called first in the device teardown code to make sure userspace can’t access the device instance any more.

Driver Load¶

Component Helper Usage¶

DRM drivers that drive hardware where a logical device consists of a pile of independent hardware blocks are recommended to use the component helper library. For consistency and better options for code reuse the following guidelines apply:

The entire device initialization procedure should be run from the component_master_ops.master_bind callback, starting with devm_drm_dev_alloc(), then binding all components with component_bind_all() and finishing with drm_dev_register().

The opaque pointer passed to all components through component_bind_all() should point at struct drm_device of the device instance, not some driver specific private structure.

The component helper fills the niche where further standardization of interfaces is not practical. When there already is, or will be, a standardized interface like drm_bridge or drm_panel, providing its own functions to find such components at driver load time, like drm_of_find_panel_or_bridge(), then the component helper should not be used.

Memory Manager Initialization¶

Every DRM driver requires a memory manager which must be initialized at load time. DRM currently contains two memory managers, the Translation Table Manager (TTM) and the Graphics Execution Manager (GEM). This document describes the use of the GEM memory manager only. See ? for details.

Miscellaneous Device Configuration¶

Another task that may be necessary for PCI devices during configuration is mapping the video BIOS. On many devices, the VBIOS describes device configuration, LCD panel timings (if any), and contains flags indicating device state. Mapping the BIOS can be done using the pci_map_rom() call, a convenience function that takes care of mapping the actual ROM, whether it has been shadowed into memory (typically at address 0xc0000) or exists on the PCI device in the ROM BAR. Note that after the ROM has been mapped and any necessary information has been extracted, it should be unmapped; on many devices, the ROM address decoder is shared with other BARs, so leaving it mapped could cause undesired behaviour like hangs or memory corruption.

Managed Resources¶

Inspired by struct device managed resources, but tied to the lifetime of struct drm_device, which can outlive the underlying physical device, usually when userspace has some open files and other handles to resources still open.

Release actions can be added with drmm_add_action(), memory allocations can be done directly with drmm_kmalloc() and the related functions. Everything will be released on the final drm_dev_put() in reverse order of how the release actions have been added and memory has been allocated since driver loading started with devm_drm_dev_alloc().

Note that release actions and managed memory can also be added and removed during the lifetime of the driver, all the functions are fully concurrent safe. But it is recommended to use managed resources only for resources that change rarely, if ever, during the lifetime of the drm_device instance.

void drmm_release_action(struct drm_device *dev, drmres_release_t action, void *data)¶: release a managed action from a drm_device

Parameters

struct drm_device *dev: DRM device
drmres_release_t action: function which would be called when dev is released
void *data: opaque pointer, passed to action

Description

This function calls the action previously added by drmm_add_action() immediately. The action is removed from the list of cleanup actions for dev, which means that it won’t be called in the final drm_dev_put().

void *drmm_kmalloc(struct drm_device *dev, size_t size, gfp_t gfp)¶: drm_device managed kmalloc()

Parameters

struct drm_device *dev: DRM device
size_t size: size of the memory allocation
gfp_t gfp: GFP allocation flags

Description

This is a drm_device managed version of kmalloc(). The allocated memory is automatically freed on the final drm_dev_put(). Memory can also be freed before the final drm_dev_put() by calling drmm_kfree().

char *drmm_kstrdup(struct drm_device *dev, const char *s, gfp_t gfp)¶: drm_device managed kstrdup()

Parameters

struct drm_device *dev: DRM device
const char *s: 0-terminated string to be duplicated
gfp_t gfp: GFP allocation flags

Description

This is a drm_device managed version of kstrdup(). The allocated memory is automatically freed on the final drm_dev_put() and works exactly like a memory allocation obtained by drmm_kmalloc().

void drmm_kfree(struct drm_device *dev, void *data)¶: drm_device managed kfree()

Parameters

struct drm_device *dev: DRM device
void *data: memory allocation to be freed

Description

This is a drm_device managed version of kfree() which can be used to release memory allocated through drmm_kmalloc() or any of its related functions before the final drm_dev_put() of dev.

drmm_add_action¶

drmm_add_action (dev, action, data)

add a managed release action to a drm_device

Parameters

dev: DRM device
action: function which should be called when dev is released
data: opaque pointer, passed to action

Description

This function adds the release action with optional parameter data to the list of cleanup actions for dev. The cleanup actions will be run in reverse order in the final drm_dev_put() call for dev.

drmm_add_action_or_reset¶

drmm_add_action_or_reset (dev, action, data)

add a managed release action to a drm_device

Parameters

dev: DRM device
action: function which should be called when dev is released
data: opaque pointer, passed to action

Description

Similar to drmm_add_action(), with the only difference that upon failure action is directly called for any cleanup work necessary on failures.

void *drmm_kzalloc(struct drm_device *dev, size_t size, gfp_t gfp)¶: drm_device managed kzalloc()

Parameters

struct drm_device *dev: DRM device
size_t size: size of the memory allocation
gfp_t gfp: GFP allocation flags

Description

This is a drm_device managed version of kzalloc(). The allocated memory is automatically freed on the final drm_dev_put(). Memory can also be freed before the final drm_dev_put() by calling drmm_kfree().

void *drmm_kmalloc_array(struct drm_device *dev, size_t n, size_t size, gfp_t flags)¶: drm_device managed kmalloc_array()

Parameters

struct drm_device *dev: DRM device
size_t n: number of array elements to allocate
size_t size: size of array member
gfp_t flags: GFP allocation flags

Description

This is a drm_device managed version of kmalloc_array(). The allocated memory is automatically freed on the final drm_dev_put() and works exactly like a memory allocation obtained by drmm_kmalloc().

void *drmm_kcalloc(struct drm_device *dev, size_t n, size_t size, gfp_t flags)¶: drm_device managed kcalloc()

Parameters

struct drm_device *dev: DRM device
size_t n: number of array elements to allocate
size_t size: size of array member
gfp_t flags: GFP allocation flags

Description

This is a drm_device managed version of kcalloc(). The allocated memory is automatically freed on the final drm_dev_put() and works exactly like a memory allocation obtained by drmm_kmalloc().

drmm_mutex_init¶

drmm_mutex_init (dev, lock)

drm_device-managed mutex_init()

Parameters

dev: DRM device
lock: lock to be initialized

Return

0 on success, or a negative errno code otherwise.

Description

This is a drm_device-managed version of mutex_init(). The initialized lock is automatically destroyed on the final drm_dev_put().

Open/Close, File Operations and IOCTLs¶

File Operations¶

Drivers must define the file operations structure that forms the DRM userspace API entry point, even though most of those operations are implemented in the DRM core. The resulting struct file_operations must be stored in the drm_driver.fops field. The mandatory functions are drm_open(), drm_read(), drm_ioctl() and drm_compat_ioctl() if CONFIG_COMPAT is enabled Note that drm_compat_ioctl will be NULL if CONFIG_COMPAT=n, so there’s no need to sprinkle #ifdef into the code. Drivers which implement private ioctls that require 32/64 bit compatibility support must provide their own file_operations.compat_ioctl handler that processes private ioctls and calls drm_compat_ioctl() for core ioctls.

In addition drm_read() and drm_poll() provide support for DRM events. DRM events are a generic and extensible means to send asynchronous events to userspace through the file descriptor. They are used to send vblank event and page flip completions by the KMS API. But drivers can also use it for their own needs, e.g. to signal completion of rendering.

For the driver-side event interface see drm_event_reserve_init() and drm_send_event() as the main starting points.

The memory mapping implementation will vary depending on how the driver manages memory. For GEM-based drivers this is drm_gem_mmap().

No other file operations are supported by the DRM userspace API. Overall the following is an example file_operations structure:

static const example_drm_fops = {
        .owner = THIS_MODULE,
        .open = drm_open,
        .release = drm_release,
        .unlocked_ioctl = drm_ioctl,
        .compat_ioctl = drm_compat_ioctl, // NULL if CONFIG_COMPAT=n
        .poll = drm_poll,
        .read = drm_read,
        .llseek = no_llseek,
        .mmap = drm_gem_mmap,
};

For plain GEM based drivers there is the DEFINE_DRM_GEM_FOPS() macro, and for DMA based drivers there is the DEFINE_DRM_GEM_DMA_FOPS() macro to make this simpler.

The driver’s file_operations must be stored in drm_driver.fops.

For driver-private IOCTL handling see the more detailed discussion in IOCTL support in the userland interfaces chapter.

struct drm_minor¶: DRM device minor structure

Definition:

struct drm_minor {
};

Members

Description

This structure represents a DRM minor number for device nodes in /dev. Entirely opaque to drivers and should never be inspected directly by drivers. Drivers instead should only interact with struct drm_file and of course struct drm_device, which is also where driver-private data and resources can be attached to.

struct drm_pending_event¶: Event queued up for userspace to read

Definition:

struct drm_pending_event {
    struct completion *completion;
    void (*completion_release)(struct completion *completion);
    struct drm_event *event;
    struct dma_fence *fence;
    struct drm_file *file_priv;
    struct list_head link;
    struct list_head pending_link;
};

Members

completion: Optional pointer to a kernel internal completion signalled when drm_send_event() is called, useful to internally synchronize with nonblocking operations.
completion_release: Optional callback currently only used by the atomic modeset helpers to clean up the reference count for the structure completion is stored in.
event: Pointer to the actual event that should be sent to userspace to be read using drm_read(). Can be optional, since nowadays events are also used to signal kernel internal threads with completion or DMA transactions using fence.
fence: Optional DMA fence to unblock other hardware transactions which depend upon the nonblocking DRM operation this event represents.
file_priv: struct drm_file where event should be delivered to. Only set when event is set.
link: Double-linked list to keep track of this event. Can be used by the driver up to the point when it calls drm_send_event(), after that this list entry is owned by the core for its own book-keeping.
pending_link: Entry on drm_file.pending_event_list, to keep track of all pending events for file_priv, to allow correct unwinding of them when userspace closes the file before the event is delivered.

Description

This represents a DRM event. Drivers can use this as a generic completion mechanism, which supports kernel-internal struct completion, struct dma_fence and also the DRM-specific struct drm_event delivery mechanism.

struct drm_file¶: DRM file private data

Definition:

struct drm_file {
    bool authenticated;
    bool stereo_allowed;
    bool universal_planes;
    bool atomic;
    bool aspect_ratio_allowed;
    bool writeback_connectors;
    bool was_master;
    bool is_master;
    bool supports_virtualized_cursor_plane;
    struct drm_master *master;
    spinlock_t master_lookup_lock;
    struct pid __rcu *pid;
    u64 client_id;
    drm_magic_t magic;
    struct list_head lhead;
    struct drm_minor *minor;
    struct idr object_idr;
    spinlock_t table_lock;
    struct idr syncobj_idr;
    spinlock_t syncobj_table_lock;
    struct file *filp;
    void *driver_priv;
    struct list_head fbs;
    struct mutex fbs_lock;
    struct list_head blobs;
    wait_queue_head_t event_wait;
    struct list_head pending_event_list;
    struct list_head event_list;
    int event_space;
    struct mutex event_read_lock;
    struct drm_prime_file_private prime;
};

Members

authenticated

Whether the client is allowed to submit rendering, which for legacy nodes means it must be authenticated.

stereo_allowed

True when the client has asked us to expose stereo 3D mode flags.

universal_planes

True if client understands CRTC primary planes and cursor planes in the plane list. Automatically set when atomic is set.

atomic

True if client understands atomic properties.

aspect_ratio_allowed

True, if client can handle picture aspect ratios, and has requested to pass this information along with the mode.

writeback_connectors

True if client understands writeback connectors

was_master

This client has or had, master capability. Protected by struct drm_device.master_mutex.

This is used to ensure that CAP_SYS_ADMIN is not enforced, if the client is or was master in the past.

is_master

This client is the creator of master. Protected by struct drm_device.master_mutex.

supports_virtualized_cursor_plane

This client is capable of handling the cursor plane with the restrictions imposed on it by the virtualized drivers.

This implies that the cursor plane has to behave like a cursor i.e. track cursor movement. It also requires setting of the hotspot properties by the client on the cursor plane.

master

Master this node is currently associated with. Protected by struct drm_device.master_mutex, and serialized by master_lookup_lock.

Only relevant if drm_is_primary_client() returns true. Note that this only matches drm_device.master if the master is the currently active one.

To update master, both drm_device.master_mutex and master_lookup_lock need to be held, therefore holding either of them is safe and enough for the read side.

When dereferencing this pointer, either hold struct drm_device.master_mutex for the duration of the pointer’s use, or use drm_file_get_master() if struct drm_device.master_mutex is not currently held and there is no other need to hold it. This prevents master from being freed during use.

See also authentication and is_master and the section on primary nodes and authentication.

master_lookup_lock

Serializes master.

pid

Process that is using this file.

Must only be dereferenced under a rcu_read_lock or equivalent.

Updates are guarded with dev->filelist_mutex and reference must be dropped after a RCU grace period to accommodate lockless readers.

client_id

A unique id for fdinfo

magic

Authentication magic, see authenticated.

lhead

List of all open files of a DRM device, linked into drm_device.filelist. Protected by drm_device.filelist_mutex.

minor

struct drm_minor for this file.

object_idr

Mapping of mm object handles to object pointers. Used by the GEM subsystem. Protected by table_lock.

table_lock

Protects object_idr.

syncobj_idr

Mapping of sync object handles to object pointers.

syncobj_table_lock

Protects syncobj_idr.

filp

Pointer to the core file structure.

driver_priv

Optional pointer for driver private data. Can be allocated in drm_driver.open and should be freed in drm_driver.postclose.

fbs

List of struct drm_framebuffer associated with this file, using the drm_framebuffer.filp_head entry.

Protected by fbs_lock. Note that the fbs list holds a reference on the framebuffer object to prevent it from untimely disappearing.

fbs_lock

Protects fbs.

blobs

User-created blob properties; this retains a reference on the property.

Protected by drm_mode_config.blob_lock;

event_wait

Waitqueue for new events added to event_list.

pending_event_list

List of pending struct drm_pending_event, used to clean up pending events in case this file gets closed before the event is signalled. Uses the drm_pending_event.pending_link entry.

Protect by drm_device.event_lock.

event_list

List of struct drm_pending_event, ready for delivery to userspace through drm_read(). Uses the drm_pending_event.link entry.

Protect by drm_device.event_lock.

event_space

Available event space to prevent userspace from exhausting kernel memory. Currently limited to the fairly arbitrary value of 4KB.

event_read_lock

Serializes drm_read().

prime

Per-file buffer caches used by the PRIME buffer sharing code.

Description

This structure tracks DRM state per open file descriptor.

bool drm_is_primary_client(const struct drm_file *file_priv)¶: is this an open file of the primary node

Parameters

const struct drm_file *file_priv: DRM file

Description

Returns true if this is an open file of the primary node, i.e. drm_file.minor of file_priv is a primary minor.

bool drm_is_render_client(const struct drm_file *file_priv)¶: is this an open file of the render node

Parameters

const struct drm_file *file_priv: DRM file

Description

Returns true if this is an open file of the render node, i.e. drm_file.minor of file_priv is a render minor.

Misc Utilities¶

Printer¶

A simple wrapper for dev_printk(), seq_printf(), etc. Allows same debug code to be used for both debugfs and printk logging.

For example:

void log_some_info(struct drm_printer *p)
{
        drm_printf(p, "foo=%d\n", foo);
        drm_printf(p, "bar=%d\n", bar);
}

#ifdef CONFIG_DEBUG_FS
void debugfs_show(struct seq_file *f)
{
        struct drm_printer p = drm_seq_file_printer(f);
        log_some_info(&p);
}
#endif

void some_other_function(...)
{
        struct drm_printer p = drm_info_printer(drm->dev);
        log_some_info(&p);
}

enum drm_debug_category¶: The DRM debug categories

Constants

DRM_UT_CORE: Used in the generic drm code: drm_ioctl.c, drm_mm.c, drm_memory.c, ...
DRM_UT_DRIVER: Used in the vendor specific part of the driver: i915, radeon, ... macro.
DRM_UT_KMS: Used in the modesetting code.
DRM_UT_PRIME: Used in the prime code.
DRM_UT_ATOMIC: Used in the atomic code.
DRM_UT_VBL: Used for verbose debug message in the vblank code.
DRM_UT_STATE: Used for verbose atomic state debugging.
DRM_UT_LEASE: Used in the lease code.
DRM_UT_DP: Used in the DP code.
DRM_UT_DRMRES: Used in the drm managed resources code.

Description

Each of the DRM debug logging macros use a specific category, and the logging is filtered by the drm.debug module parameter. This enum specifies the values for the interface.

Each DRM_DEBUG_<CATEGORY> macro logs to DRM_UT_<CATEGORY> category, except DRM_DEBUG() logs to DRM_UT_CORE.

Enabling verbose debug messages is done through the drm.debug parameter, each category being enabled by a bit:

drm.debug=0x1 will enable CORE messages

drm.debug=0x2 will enable DRIVER messages

drm.debug=0x3 will enable CORE and DRIVER messages

...

drm.debug=0x1ff will enable all messages

An interesting feature is that it’s possible to enable verbose logging at run-time by echoing the debug value in its sysfs node:

# echo 0xf > /sys/module/drm/parameters/debug

struct drm_printer¶: drm output “stream”

Definition:

struct drm_printer {
};

Members

Description

Do not use struct members directly. Use drm_printer_seq_file(), drm_printer_info(), etc to initialize. And drm_printf() for output.

void drm_vprintf(struct drm_printer *p, const char *fmt, va_list *va)¶: print to a drm_printer stream

Parameters

struct drm_printer *p: the drm_printer
const char *fmt: format string
va_list *va: the va_list

drm_printf_indent¶

drm_printf_indent (printer, indent, fmt, ...)

Print to a drm_printer stream with indentation

Parameters

printer: DRM printer
indent: Tab indentation level (max 5)
fmt: Format string
...: variable arguments

struct drm_print_iterator¶: local struct used with drm_printer_coredump

Definition:

struct drm_print_iterator {
    void *data;
    ssize_t start;
    ssize_t remain;
};

Members

data: Pointer to the devcoredump output buffer
start: The offset within the buffer to start writing
remain: The number of bytes to write for this iteration

struct drm_printer drm_coredump_printer(struct drm_print_iterator *iter)¶: construct a drm_printer that can output to a buffer from the read function for devcoredump

Parameters

struct drm_print_iterator *iter: A pointer to a struct drm_print_iterator for the read instance

Description

This wrapper extends drm_printf() to work with a dev_coredumpm() callback function. The passed in drm_print_iterator struct contains the buffer pointer, size and offset as passed in from devcoredump.

For example:

void coredump_read(char *buffer, loff_t offset, size_t count,
        void *data, size_t datalen)
{
        struct drm_print_iterator iter;
        struct drm_printer p;

        iter.data = buffer;
        iter.start = offset;
        iter.remain = count;

        p = drm_coredump_printer(&iter);

        drm_printf(p, "foo=%d\n", foo);
}

void makecoredump(...)
{
        ...
        dev_coredumpm(dev, THIS_MODULE, data, 0, GFP_KERNEL,
                coredump_read, ...)
}

Return

The drm_printer object

struct drm_printer drm_seq_file_printer(struct seq_file *f)¶: construct a drm_printer that outputs to seq_file

Parameters

struct seq_file *f: the struct seq_file to output to

Return

The drm_printer object

struct drm_printer drm_info_printer(struct device *dev)¶: construct a drm_printer that outputs to dev_printk()

Parameters

struct device *dev: the struct device pointer

Return

The drm_printer object

struct drm_printer drm_dbg_printer(struct drm_device *drm, enum drm_debug_category category, const char *prefix)¶: construct a drm_printer for drm device specific output

Parameters

struct drm_device *drm: the struct drm_device pointer, or NULL
enum drm_debug_category category: the debug category to use
const char *prefix: debug output prefix, or NULL for no prefix

Return

The drm_printer object

struct drm_printer drm_err_printer(struct drm_device *drm, const char *prefix)¶: construct a drm_printer that outputs to drm_err()

Parameters

struct drm_device *drm: the struct drm_device pointer
const char *prefix: debug output prefix, or NULL for no prefix

Return

The drm_printer object

DRM_DEV_ERROR¶

DRM_DEV_ERROR (dev, fmt, ...)

Error output.

Parameters

dev: device pointer
fmt: printf() like format string.
...: variable arguments

NOTE

this is deprecated in favor of drm_err() or dev_err().

DRM_DEV_ERROR_RATELIMITED¶

DRM_DEV_ERROR_RATELIMITED (dev, fmt, ...)

Rate limited error output.

Parameters

dev: device pointer
fmt: printf() like format string.
...: variable arguments

NOTE

this is deprecated in favor of drm_err_ratelimited() or dev_err_ratelimited().

Description

Like DRM_ERROR() but won’t flood the log.

DRM_DEV_DEBUG¶

DRM_DEV_DEBUG (dev, fmt, ...)

Debug output for generic drm code

Parameters

dev: device pointer
fmt: printf() like format string.
...: variable arguments

NOTE

this is deprecated in favor of drm_dbg_core().

DRM_DEV_DEBUG_DRIVER¶

DRM_DEV_DEBUG_DRIVER (dev, fmt, ...)

Debug output for vendor specific part of the driver

Parameters

dev: device pointer
fmt: printf() like format string.
...: variable arguments

NOTE

this is deprecated in favor of drm_dbg() or dev_dbg().

DRM_DEV_DEBUG_KMS¶

DRM_DEV_DEBUG_KMS (dev, fmt, ...)

Debug output for modesetting code

Parameters

dev: device pointer
fmt: printf() like format string.
...: variable arguments

NOTE

this is deprecated in favor of drm_dbg_kms().

void drm_puts(struct drm_printer *p, const char *str)¶: print a const string to a drm_printer stream

Parameters

struct drm_printer *p: the drm printer
const char *str: const string

Description

Allow drm_printer types that have a constant string option to use it.

void drm_printf(struct drm_printer *p, const char *f, ...)¶: print to a drm_printer stream

Parameters

struct drm_printer *p: the drm_printer
const char *f: format string
...: variable arguments

void drm_print_bits(struct drm_printer *p, unsigned long value, const char *const bits[], unsigned int nbits)¶: print bits to a drm_printer stream

Parameters

struct drm_printer *p: the drm_printer
unsigned long value: field value.
const char * const bits[]: Array with bit names.
unsigned int nbits: Size of bit names array.

Description

Print bits (in flag fields for example) in human readable form.

void drm_print_regset32(struct drm_printer *p, struct debugfs_regset32 *regset)¶: print the contents of registers to a drm_printer stream.

Parameters

struct drm_printer *p: the drm printer
struct debugfs_regset32 *regset: the list of registers to print.

Description

Often in driver debug, it’s useful to be able to either capture the contents of registers in the steady state using debugfs or at specific points during operation. This lets the driver have a single list of registers for both.

Utilities¶

Macros and inline functions that does not naturally belong in other places

for_each_if¶

for_each_if (condition)

helper for handling conditionals in various for_each macros

Parameters

condition: The condition to check

Description

Typical use:

#define for_each_foo_bar(x, y) \'
        list_for_each_entry(x, y->list, head) \'
                for_each_if(x->something == SOMETHING)

The for_each_if() macro makes the use of for_each_foo_bar() less error prone.

bool drm_can_sleep(void)¶: returns true if currently okay to sleep

Parameters

void: no arguments

Description

This function shall not be used in new code. The check for running in atomic context may not work - see linux/preempt.h.

FIXME: All users of drm_can_sleep should be removed (see TODO list)

Return

False if kgdb is active, we are in atomic context or irqs are disabled.

Unit testing¶

KUnit¶

KUnit (Kernel unit testing framework) provides a common framework for unit tests within the Linux kernel.

This section covers the specifics for the DRM subsystem. For general information about KUnit, please refer to Getting Started.

How to run the tests?¶

In order to facilitate running the test suite, a configuration file is present in drivers/gpu/drm/tests/.kunitconfig. It can be used by kunit.py as follows:

$ ./tools/testing/kunit/kunit.py run --kunitconfig=drivers/gpu/drm/tests \
        --kconfig_add CONFIG_VIRTIO_UML=y \
        --kconfig_add CONFIG_UML_PCI_OVER_VIRTIO=y

Note

The configuration included in .kunitconfig should be as generic as possible. CONFIG_VIRTIO_UML and CONFIG_UML_PCI_OVER_VIRTIO are not included in it because they are only required for User Mode Linux.

Legacy Support Code¶

The section very briefly covers some of the old legacy support code which is only used by old DRM drivers which have done a so-called shadow-attach to the underlying device instead of registering as a real driver. This also includes some of the old generic buffer management and command submission code. Do not use any of this in new and modern drivers.

Legacy Suspend/Resume¶

The DRM core provides some suspend/resume code, but drivers wanting full suspend/resume support should provide save() and restore() functions. These are called at suspend, hibernate, or resume time, and should perform any state save or restore required by your device across suspend or hibernate states.

int (*suspend) (struct drm_device *, pm_message_t state); int (*resume) (struct drm_device *); Those are legacy suspend and resume methods which only work with the legacy shadow-attach driver registration functions. New driver should use the power management interface provided by their bus type (usually through the struct device_driver dev_pm_ops) and set these methods to NULL.

Legacy DMA Services¶

This should cover how DMA mapping etc. is supported by the core. These functions are deprecated and should not be used.

The Linux Kernel

Contents

This Page

DRM Internals¶

Driver Initialization¶

Driver Information¶

Major, Minor and Patchlevel¶

Name, Description and Date¶

Module Initialization¶

Managing Ownership of the Framebuffer Aperture¶

Device Instance and Driver Handling¶

Display driver example¶

Driver Load¶

Component Helper Usage¶

Memory Manager Initialization¶

Miscellaneous Device Configuration¶

Managed Resources¶

Open/Close, File Operations and IOCTLs¶

File Operations¶

Misc Utilities¶

Printer¶

Utilities¶

Unit testing¶

KUnit¶

How to run the tests?¶

Legacy Support Code¶

Legacy Suspend/Resume¶

Legacy DMA Services¶