Backlight support¶

The backlight core supports implementing backlight drivers.

A backlight driver registers a driver using devm_backlight_device_register(). The properties of the backlight driver such as type and max_brightness must be specified. When the core detect changes in for example brightness or power state the update_status() operation is called. The backlight driver shall implement this operation and use it to adjust backlight.

Several sysfs attributes are provided by the backlight core:

- brightness         R/W, set the requested brightness level
- actual_brightness  RO, the brightness level used by the HW
- max_brightness     RO, the maximum  brightness level supported

See Documentation/ABI/stable/sysfs-class-backlight for the full list.

The backlight can be adjusted using the sysfs interface, and the backlight driver may also support adjusting backlight using a hot-key or some other platform or firmware specific way.

The driver must implement the get_brightness() operation if the HW do not support all the levels that can be specified in brightness, thus providing user-space access to the actual level via the actual_brightness attribute.

When the backlight changes this is reported to user-space using an uevent connected to the actual_brightness attribute. When brightness is set by platform specific means, for example a hot-key to adjust backlight, the driver must notify the backlight core that brightness has changed using backlight_force_update().

The backlight driver core receives notifications from fbdev and if the event is FB_EVENT_BLANK and if the value of blank, from the FBIOBLANK ioctrl, results in a change in the backlight state the update_status() operation is called.

enum backlight_update_reason¶: what method was used to update backlight

Constants

BACKLIGHT_UPDATE_HOTKEY: The backlight was updated using a hot-key.
BACKLIGHT_UPDATE_SYSFS: The backlight was updated using sysfs.

Description

A driver indicates the method (reason) used for updating the backlight when calling backlight_force_update().

enum backlight_type¶: the type of backlight control

Constants

BACKLIGHT_RAW: The backlight is controlled using hardware registers.
BACKLIGHT_PLATFORM: The backlight is controlled using a platform-specific interface.
BACKLIGHT_FIRMWARE: The backlight is controlled using a standard firmware interface.
BACKLIGHT_TYPE_MAX: Number of entries.

Description

The type of interface used to control the backlight.

struct backlight_ops¶: backlight operations

Definition:

struct backlight_ops {
    unsigned int options;
#define BL_CORE_SUSPENDRESUME   (1 << 0);
    int (*update_status)(struct backlight_device *);
    int (*get_brightness)(struct backlight_device *);
    bool (*controls_device)(struct backlight_device *bd, struct device *display_dev);
};

Members

options

Configure how operations are called from the core.

The options parameter is used to adjust the behaviour of the core. Set BL_CORE_SUSPENDRESUME to get the update_status() operation called upon suspend and resume.

update_status

Operation called when properties have changed.

Notify the backlight driver some property has changed. The update_status operation is protected by the update_lock.

The backlight driver is expected to use backlight_is_blank() to check if the display is blanked and set brightness accordingly. update_status() is called when any of the properties has changed.

RETURNS:

0 on success, negative error code if any failure occurred.

get_brightness

Return the current backlight brightness.

The driver may implement this as a readback from the HW. This operation is optional and if not present then the current brightness property value is used.

RETURNS:

A brightness value which is 0 or a positive number. On failure a negative error code is returned.

controls_device

Check against the display device

Check if the backlight controls the given display device. This operation is optional and if not implemented it is assumed that the display is always the one controlled by the backlight.

RETURNS:

If display_dev is NULL or display_dev matches the device controlled by the backlight, return true. Otherwise return false.

Description

The backlight operations are specified when the backlight device is registered.

struct backlight_properties¶: backlight properties

Definition:

struct backlight_properties {
    int brightness;
    int max_brightness;
    int power;
#define BACKLIGHT_POWER_ON              (0);
#define BACKLIGHT_POWER_OFF             (4);
#define BACKLIGHT_POWER_REDUCED         (1) ;
    enum backlight_type type;
    unsigned int state;
#define BL_CORE_SUSPENDED       (1 << 0)        ;
#define BL_CORE_FBBLANK         (1 << 1)        ;
    enum backlight_scale scale;
};

Members

brightness

The current brightness requested by the user.

The backlight core makes sure the range is (0 to max_brightness) when the brightness is set via the sysfs attribute: /sys/class/backlight/<backlight>/brightness.

This value can be set in the backlight_properties passed to devm_backlight_device_register() to set a default brightness value.

max_brightness

The maximum brightness value.

This value must be set in the backlight_properties passed to devm_backlight_device_register() and shall not be modified by the driver after registration.

power

The current power mode.

User space can configure the power mode using the sysfs attribute: /sys/class/backlight/<backlight>/bl_power When the power property is updated update_status() is called.

The possible values are: (0: full on, 4: full off), see BACKLIGHT_POWER constants.

When the backlight device is enabled, power is set to BACKLIGHT_POWER_ON. When the backlight device is disabled, power is set to BACKLIGHT_POWER_OFF.

type

The type of backlight supported.

The backlight type allows userspace to make appropriate policy decisions based on the backlight type.

This value must be set in the backlight_properties passed to devm_backlight_device_register().

state

The state of the backlight core.

The state is a bitmask. BL_CORE_FBBLANK is set when the display is expected to be blank. BL_CORE_SUSPENDED is set when the driver is suspended.

backlight drivers are expected to use backlight_is_blank() in their update_status() operation rather than reading the state property.

The state is maintained by the core and drivers may not modify it.

scale

The type of the brightness scale.

Description

This structure defines all the properties of a backlight.

struct backlight_device¶: backlight device data

Definition:

struct backlight_device {
    struct backlight_properties props;
    struct mutex update_lock;
    struct mutex ops_lock;
    const struct backlight_ops *ops;
    struct notifier_block fb_notif;
    struct list_head entry;
    struct device dev;
    bool fb_bl_on[FB_MAX];
    int use_count;
};

Members

props

Backlight properties

update_lock

The lock used when calling the update_status() operation.

update_lock is an internal backlight lock that serialise access to the update_status() operation. The backlight core holds the update_lock when calling the update_status() operation. The update_lock shall not be used by backlight drivers.

ops_lock

The lock used around everything related to backlight_ops.

ops_lock is an internal backlight lock that protects the ops pointer and is used around all accesses to ops and when the operations are invoked. The ops_lock shall not be used by backlight drivers.

ops

Pointer to the backlight operations.

If ops is NULL, the driver that registered this device has been unloaded, and if class_get_devdata() points to something in the body of that driver, it is also invalid.

fb_notif

The framebuffer notifier block

entry

List entry of all registered backlight devices

dev

Parent device.

fb_bl_on

The state of individual fbdev’s.

Multiple fbdev’s may share one backlight device. The fb_bl_on records the state of the individual fbdev.

use_count

The number of uses of fb_bl_on.

Description

This structure holds all data required by a backlight device.

int backlight_update_status(struct backlight_device *bd)¶: force an update of the backlight device status

Parameters

struct backlight_device *bd: the backlight device

int backlight_enable(struct backlight_device *bd)¶: Enable backlight

Parameters

struct backlight_device *bd: the backlight device to enable

int backlight_disable(struct backlight_device *bd)¶: Disable backlight

Parameters

struct backlight_device *bd: the backlight device to disable

bool backlight_is_blank(const struct backlight_device *bd)¶: Return true if display is expected to be blank

Parameters

const struct backlight_device *bd: the backlight device

Description

Display is expected to be blank if any of these is true:

1) if power in not UNBLANK
2) if state indicate BLANK or SUSPENDED

Returns true if display is expected to be blank, false otherwise.

int backlight_get_brightness(const struct backlight_device *bd)¶: Returns the current brightness value

Parameters

const struct backlight_device *bd: the backlight device

Description

Returns the current brightness value, taking in consideration the current state. If backlight_is_blank() returns true then return 0 as brightness otherwise return the current brightness property value.

Backlight drivers are expected to use this function in their update_status() operation to get the brightness value.

void *bl_get_data(struct backlight_device *bl_dev)¶: access devdata

Parameters

struct backlight_device *bl_dev: pointer to backlight device

Description

When a backlight device is registered the driver has the possibility to supply a void * devdata. bl_get_data() return a pointer to the devdata.

pointer to devdata stored while registering the backlight device.

Return

void backlight_force_update(struct backlight_device *bd, enum backlight_update_reason reason)¶: tell the backlight subsystem that hardware state has changed

Parameters

struct backlight_device *bd: the backlight device to update
enum backlight_update_reason reason: reason for update

Description

Updates the internal state of the backlight in response to a hardware event, and generates an uevent to notify userspace. A backlight driver shall call backlight_force_update() when the backlight is changed using, for example, a hot-key. The updated brightness is read using get_brightness() and the brightness value is reported using an uevent.

struct backlight_device *backlight_device_get_by_name(const char *name)¶: Get backlight device by name

Parameters

const char *name: Device name

Description

This function looks up a backlight device by its name. It obtains a reference on the backlight device and it is the caller’s responsibility to drop the reference by calling put_device().

Return

A pointer to the backlight device if found, otherwise NULL.

struct backlight_device *devm_backlight_device_register(struct device *dev, const char *name, struct device *parent, void *devdata, const struct backlight_ops *ops, const struct backlight_properties *props)¶: register a new backlight device

Parameters

struct device *dev: the device to register
const char *name: the name of the device
struct device *parent: a pointer to the parent device (often the same as dev)
void *devdata: an optional pointer to be stored for private driver use
const struct backlight_ops *ops: the backlight operations structure
const struct backlight_properties *props: the backlight properties

Description

Creates and registers new backlight device. When a backlight device is registered the configuration must be specified in the props parameter. See description of backlight_properties.

struct backlight on success, or an ERR_PTR on error

Return

void devm_backlight_device_unregister(struct device *dev, struct backlight_device *bd)¶: unregister backlight device

Parameters

struct device *dev: the device to unregister
struct backlight_device *bd: the backlight device to unregister

Description

Deallocates a backlight allocated with devm_backlight_device_register(). Normally this function will not need to be called and the resource management code will ensure that the resources are freed.

struct backlight_device *of_find_backlight_by_node(struct device_node *node)¶: find backlight device by device-tree node

Parameters

struct device_node *node: device-tree node of the backlight device

Description

Returns a pointer to the backlight device corresponding to the given DT node or NULL if no such backlight device exists or if the device hasn’t been probed yet.

This function obtains a reference on the backlight device and it is the caller’s responsibility to drop the reference by calling put_device() on the backlight device’s .dev field.

struct backlight_device *devm_of_find_backlight(struct device *dev)¶: find backlight for a device

Parameters

struct device *dev: the device

Description

This function looks for a property named ‘backlight’ on the DT node connected to dev and looks up the backlight device. The lookup is device managed so the reference to the backlight device is automatically dropped on driver detach.

A pointer to the backlight device if found. Error pointer -EPROBE_DEFER if the DT property is set, but no backlight device is found. NULL if there’s no backlight property.

Return

The Linux Kernel

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Backlight support¶