Gadget Testing¶

This file summarizes information on basic testing of USB functions provided by gadgets.

1. ACM function¶

The function is provided by usb_f_acm.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “acm”. The ACM function provides just one attribute in its function directory:

port_num

The attribute is read-only.

There can be at most 4 ACM/generic serial/OBEX ports in the system.

Testing the ACM function¶

On the host:

cat > /dev/ttyACM<X>

On the device:

cat /dev/ttyGS<Y>

then the other way round

On the device:

cat > /dev/ttyGS<Y>

On the host:

cat /dev/ttyACM<X>

2. ECM function¶

The function is provided by usb_f_ecm.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “ecm”. The ECM function provides these attributes in its function directory:

ifname

network device interface name associated with this function instance

qmult

queue length multiplier for high and super speed

host_addr

MAC address of host’s end of this Ethernet over USB link

dev_addr

MAC address of device’s end of this Ethernet over USB link

and after creating the functions/ecm.<instance name> they contain default values: qmult is 5, dev_addr and host_addr are randomly selected. The ifname can be written to if the function is not bound. A write must be an interface pattern such as “usb%d”, which will cause the net core to choose the next free usbX interface. By default, it is set to “usb%d”.

Testing the ECM function¶

Configure IP addresses of the device and the host. Then:

On the device:

ping <host's IP>

On the host:

ping <device's IP>

3. ECM subset function¶

The function is provided by usb_f_ecm_subset.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “geth”. The ECM subset function provides these attributes in its function directory:

ifname

network device interface name associated with this function instance

qmult

queue length multiplier for high and super speed

host_addr

MAC address of host’s end of this Ethernet over USB link

dev_addr

MAC address of device’s end of this Ethernet over USB link

and after creating the functions/ecm.<instance name> they contain default values: qmult is 5, dev_addr and host_addr are randomly selected. The ifname can be written to if the function is not bound. A write must be an interface pattern such as “usb%d”, which will cause the net core to choose the next free usbX interface. By default, it is set to “usb%d”.

Testing the ECM subset function¶

Configure IP addresses of the device and the host. Then:

On the device:

ping <host's IP>

On the host:

ping <device's IP>

4. EEM function¶

The function is provided by usb_f_eem.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “eem”. The EEM function provides these attributes in its function directory:

ifname

network device interface name associated with this function instance

qmult

queue length multiplier for high and super speed

host_addr

MAC address of host’s end of this Ethernet over USB link

dev_addr

MAC address of device’s end of this Ethernet over USB link

and after creating the functions/eem.<instance name> they contain default values: qmult is 5, dev_addr and host_addr are randomly selected. The ifname can be written to if the function is not bound. A write must be an interface pattern such as “usb%d”, which will cause the net core to choose the next free usbX interface. By default, it is set to “usb%d”.

Testing the EEM function¶

Configure IP addresses of the device and the host. Then:

On the device:

ping <host's IP>

On the host:

ping <device's IP>

5. FFS function¶

The function is provided by usb_f_fs.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “ffs”. The function directory is intentionally empty and not modifiable.

After creating the directory there is a new instance (a “device”) of FunctionFS available in the system. Once a “device” is available, the user should follow the standard procedure for using FunctionFS (mount it, run the userspace process which implements the function proper). The gadget should be enabled by writing a suitable string to usb_gadget/<gadget>/UDC.

The FFS function provides just one attribute in its function directory:

ready

The attribute is read-only and signals if the function is ready (1) to be used, E.G. if userspace has written descriptors and strings to ep0, so the gadget can be enabled.

Testing the FFS function¶

On the device: start the function’s userspace daemon, enable the gadget

On the host: use the USB function provided by the device

6. HID function¶

The function is provided by usb_f_hid.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “hid”. The HID function provides these attributes in its function directory:

protocol

HID protocol to use

report_desc

data to be used in HID reports, except data passed with /dev/hidg<X>

report_length

HID report length

subclass

HID subclass to use

For a keyboard the protocol and the subclass are 1, the report_length is 8, while the report_desc is:

$ hd my_report_desc
00000000  05 01 09 06 a1 01 05 07  19 e0 29 e7 15 00 25 01  |..........)...%.|
00000010  75 01 95 08 81 02 95 01  75 08 81 03 95 05 75 01  |u.......u.....u.|
00000020  05 08 19 01 29 05 91 02  95 01 75 03 91 03 95 06  |....).....u.....|
00000030  75 08 15 00 25 65 05 07  19 00 29 65 81 00 c0     |u...%e....)e...|
0000003f

Such a sequence of bytes can be stored to the attribute with echo:

$ echo -ne \\x05\\x01\\x09\\x06\\xa1.....

Testing the HID function¶

Device:

create the gadget
connect the gadget to a host, preferably not the one used to control the gadget
run a program which writes to /dev/hidg<N>, e.g. a userspace program found in Linux USB HID gadget driver:
```
$ ./hid_gadget_test /dev/hidg0 keyboard
```

Host:

observe the keystrokes from the gadget

7. LOOPBACK function¶

The function is provided by usb_f_ss_lb.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “Loopback”. The LOOPBACK function provides these attributes in its function directory:

qlen

depth of loopback queue

bulk_buflen

buffer length

Testing the LOOPBACK function¶

device: run the gadget

host: test-usb (tools/usb/testusb.c)

8. MASS STORAGE function¶

The function is provided by usb_f_mass_storage.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “mass_storage”. The MASS STORAGE function provides these attributes in its directory: files:

stall

Set to permit function to halt bulk endpoints. Disabled on some USB devices known not to work correctly. You should set it to true.

num_buffers

Number of pipeline buffers. Valid numbers are 2..4. Available only if CONFIG_USB_GADGET_DEBUG_FILES is set.

and a default lun.0 directory corresponding to SCSI LUN #0.

A new lun can be added with mkdir:

$ mkdir functions/mass_storage.0/partition.5

Lun numbering does not have to be continuous, except for lun #0 which is created by default. A maximum of 8 luns can be specified and they all must be named following the <name>.<number> scheme. The numbers can be 0..8. Probably a good convention is to name the luns “lun.<number>”, although it is not mandatory.

In each lun directory there are the following attribute files:

file

The path to the backing file for the LUN. Required if LUN is not marked as removable.

ro

Flag specifying access to the LUN shall be read-only. This is implied if CD-ROM emulation is enabled as well as when it was impossible to open “filename” in R/W mode.

removable

Flag specifying that LUN shall be indicated as being removable.

cdrom

Flag specifying that LUN shall be reported as being a CD-ROM.

nofua

Flag specifying that FUA flag in SCSI WRITE(10,12)

forced_eject

This write-only file is useful only when the function is active. It causes the backing file to be forcibly detached from the LUN, regardless of whether the host has allowed it. Any non-zero number of bytes written will result in ejection.

Testing the MASS STORAGE function¶

device: connect the gadget, enable it host: dmesg, see the USB drives appear (if system configured to automatically mount)

9. MIDI function¶

The function is provided by usb_f_midi.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “midi”. The MIDI function provides these attributes in its function directory:

buflen

MIDI buffer length

id

ID string for the USB MIDI adapter

in_ports

number of MIDI input ports

index

index value for the USB MIDI adapter

out_ports

number of MIDI output ports

qlen

USB read request queue length

Testing the MIDI function¶

There are two cases: playing a mid from the gadget to the host and playing a mid from the host to the gadget.

Playing a mid from the gadget to the host:

host:

$ arecordmidi -l
 Port    Client name                      Port name
 14:0    Midi Through                     Midi Through Port-0
 24:0    MIDI Gadget                      MIDI Gadget MIDI 1
$ arecordmidi -p 24:0 from_gadget.mid

gadget:

$ aplaymidi -l
 Port    Client name                      Port name
 20:0    f_midi                           f_midi

$ aplaymidi -p 20:0 to_host.mid

Playing a mid from the host to the gadget

gadget:

$ arecordmidi -l
 Port    Client name                      Port name
 20:0    f_midi                           f_midi

$ arecordmidi -p 20:0 from_host.mid

host:

$ aplaymidi -l
 Port    Client name                      Port name
 14:0    Midi Through                     Midi Through Port-0
 24:0    MIDI Gadget                      MIDI Gadget MIDI 1

$ aplaymidi -p24:0 to_gadget.mid

The from_gadget.mid should sound identical to the to_host.mid.

The from_host.id should sound identical to the to_gadget.mid.

MIDI files can be played to speakers/headphones with e.g. timidity installed:

$ aplaymidi -l
 Port    Client name                      Port name
 14:0    Midi Through                     Midi Through Port-0
 24:0    MIDI Gadget                      MIDI Gadget MIDI 1
128:0    TiMidity                         TiMidity port 0
128:1    TiMidity                         TiMidity port 1
128:2    TiMidity                         TiMidity port 2
128:3    TiMidity                         TiMidity port 3

$ aplaymidi -p 128:0 file.mid

MIDI ports can be logically connected using the aconnect utility, e.g.:

$ aconnect 24:0 128:0 # try it on the host

After the gadget’s MIDI port is connected to timidity’s MIDI port, whatever is played at the gadget side with aplaymidi -l is audible in host’s speakers/headphones.

10. NCM function¶

The function is provided by usb_f_ncm.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “ncm”. The NCM function provides these attributes in its function directory:

ifname

network device interface name associated with this function instance

qmult

queue length multiplier for high and super speed

host_addr

MAC address of host’s end of this Ethernet over USB link

dev_addr

MAC address of device’s end of this Ethernet over USB link

max_segment_size

Segment size required for P2P connections. This will set MTU to 14 bytes

and after creating the functions/ncm.<instance name> they contain default values: qmult is 5, dev_addr and host_addr are randomly selected. The ifname can be written to if the function is not bound. A write must be an interface pattern such as “usb%d”, which will cause the net core to choose the next free usbX interface. By default, it is set to “usb%d”.

Testing the NCM function¶

Configure IP addresses of the device and the host. Then:

On the device:

ping <host's IP>

On the host:

ping <device's IP>

11. OBEX function¶

The function is provided by usb_f_obex.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “obex”. The OBEX function provides just one attribute in its function directory:

port_num

The attribute is read-only.

There can be at most 4 ACM/generic serial/OBEX ports in the system.

Testing the OBEX function¶

On device:

seriald -f /dev/ttyGS<Y> -s 1024

On host:

serialc -v <vendorID> -p <productID> -i<interface#> -a1 -s1024 \
        -t<out endpoint addr> -r<in endpoint addr>

where seriald and serialc are Felipe’s utilities found here:

https://github.com/felipebalbi/usb-tools.git master

12. PHONET function¶

The function is provided by usb_f_phonet.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “phonet”. The PHONET function provides just one attribute in its function directory:

ifname

network device interface name associated with this function instance

Testing the PHONET function¶

It is not possible to test the SOCK_STREAM protocol without a specific piece of hardware, so only SOCK_DGRAM has been tested. For the latter to work, in the past I had to apply the patch mentioned here:

http://www.spinics.net/lists/linux-usb/msg85689.html

These tools are required:

git://git.gitorious.org/meego-cellular/phonet-utils.git

On the host:

$ ./phonet -a 0x10 -i usbpn0
$ ./pnroute add 0x6c usbpn0
$./pnroute add 0x10 usbpn0
$ ifconfig usbpn0 up

On the device:

$ ./phonet -a 0x6c -i upnlink0
$ ./pnroute add 0x10 upnlink0
$ ifconfig upnlink0 up

Then a test program can be used:

http://www.spinics.net/lists/linux-usb/msg85690.html

On the device:

$ ./pnxmit -a 0x6c -r

On the host:

$ ./pnxmit -a 0x10 -s 0x6c

As a result some data should be sent from host to device. Then the other way round:

On the host:

$ ./pnxmit -a 0x10 -r

On the device:

$ ./pnxmit -a 0x6c -s 0x10

13. RNDIS function¶

The function is provided by usb_f_rndis.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “rndis”. The RNDIS function provides these attributes in its function directory:

ifname

network device interface name associated with this function instance

qmult

queue length multiplier for high and super speed

host_addr

MAC address of host’s end of this Ethernet over USB link

dev_addr

MAC address of device’s end of this Ethernet over USB link

and after creating the functions/rndis.<instance name> they contain default values: qmult is 5, dev_addr and host_addr are randomly selected. The ifname can be written to if the function is not bound. A write must be an interface pattern such as “usb%d”, which will cause the net core to choose the next free usbX interface. By default, it is set to “usb%d”.

Testing the RNDIS function¶

Configure IP addresses of the device and the host. Then:

On the device:

ping <host's IP>

On the host:

ping <device's IP>

14. SERIAL function¶

The function is provided by usb_f_gser.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “gser”. The SERIAL function provides just one attribute in its function directory:

port_num

The attribute is read-only.

There can be at most 4 ACM/generic serial/OBEX ports in the system.

Testing the SERIAL function¶

On host:

insmod usbserial
echo VID PID >/sys/bus/usb-serial/drivers/generic/new_id

On host:

cat > /dev/ttyUSB<X>

On target:

cat /dev/ttyGS<Y>

then the other way round

On target:

cat > /dev/ttyGS<Y>

On host:

cat /dev/ttyUSB<X>

15. SOURCESINK function¶

The function is provided by usb_f_ss_lb.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “SourceSink”. The SOURCESINK function provides these attributes in its function directory:

pattern

0 (all zeros), 1 (mod63), 2 (none)

isoc_interval

1..16

isoc_maxpacket

0 - 1023 (fs), 0 - 1024 (hs/ss)

isoc_mult

0..2 (hs/ss only)

isoc_maxburst

0..15 (ss only)

bulk_buflen

buffer length

bulk_qlen

depth of queue for bulk

iso_qlen

depth of queue for iso

Testing the SOURCESINK function¶

device: run the gadget

host: test-usb (tools/usb/testusb.c)

16. UAC1 function (legacy implementation)¶

The function is provided by usb_f_uac1_legacy.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “uac1_legacy”. The uac1 function provides these attributes in its function directory:

audio_buf_size

audio buffer size

fn_cap

capture pcm device file name

fn_cntl

control device file name

fn_play

playback pcm device file name

req_buf_size

ISO OUT endpoint request buffer size

req_count

ISO OUT endpoint request count

The attributes have sane default values.

Testing the UAC1 function¶

device: run the gadget

host:

aplay -l # should list our USB Audio Gadget

17. UAC2 function¶

The function is provided by usb_f_uac2.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “uac2”. The uac2 function provides these attributes in its function directory:

c_chmask

capture channel mask

c_srate

list of capture sampling rates (comma-separated)

c_ssize

capture sample size (bytes)

c_sync

capture synchronization type (async/adaptive)

c_mute_present

capture mute control enable

c_volume_present

capture volume control enable

c_volume_min

capture volume control min value (in 1/256 dB)

c_volume_max

capture volume control max value (in 1/256 dB)

c_volume_res

capture volume control resolution (in 1/256 dB)

c_hs_bint

capture bInterval for HS/SS (1-4: fixed, 0: auto)

fb_max

maximum extra bandwidth in async mode

p_chmask

playback channel mask

p_srate

list of playback sampling rates (comma-separated)

p_ssize

playback sample size (bytes)

p_mute_present

playback mute control enable

p_volume_present

playback volume control enable

p_volume_min

playback volume control min value (in 1/256 dB)

p_volume_max

playback volume control max value (in 1/256 dB)

p_volume_res

playback volume control resolution (in 1/256 dB)

p_hs_bint

playback bInterval for HS/SS (1-4: fixed, 0: auto)

req_number

the number of pre-allocated request for both capture and playback

function_name

name of the interface

c_terminal_type

code of the capture terminal type

p_terminal_type

code of the playback terminal type

The attributes have sane default values.

Testing the UAC2 function¶

device: run the gadget host: aplay -l # should list our USB Audio Gadget

This function does not require real hardware support, it just sends a stream of audio data to/from the host. In order to actually hear something at the device side, a command similar to this must be used at the device side:

$ arecord -f dat -t wav -D hw:2,0 | aplay -D hw:0,0 &

e.g.:

$ arecord -f dat -t wav -D hw:CARD=UAC2Gadget,DEV=0 | \
  aplay -D default:CARD=OdroidU3

18. UVC function¶

The function is provided by usb_f_uvc.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “uvc”. The uvc function provides these attributes in its function directory:

streaming_interval

interval for polling endpoint for data transfers

streaming_maxburst

bMaxBurst for super speed companion descriptor

streaming_maxpacket

maximum packet size this endpoint is capable of sending or receiving when this configuration is selected

function_name

name of the interface

There are also “control” and “streaming” subdirectories, each of which contain a number of their subdirectories. There are some sane defaults provided, but the user must provide the following:

control header

create in control/header, link from control/class/fs and/or control/class/ss

streaming header

create in streaming/header, link from streaming/class/fs and/or streaming/class/hs and/or streaming/class/ss

format description

create in streaming/mjpeg and/or streaming/uncompressed

frame description

create in streaming/mjpeg/<format> and/or in streaming/uncompressed/<format>

Each frame description contains frame interval specification, and each such specification consists of a number of lines with an interval value in each line. The rules stated above are best illustrated with an example:

# mkdir functions/uvc.usb0/control/header/h
# cd functions/uvc.usb0/control/
# ln -s header/h class/fs
# ln -s header/h class/ss
# mkdir -p functions/uvc.usb0/streaming/uncompressed/u/360p
# cat <<EOF > functions/uvc.usb0/streaming/uncompressed/u/360p/dwFrameInterval
666666
1000000
5000000
EOF
# cd $GADGET_CONFIGFS_ROOT
# mkdir functions/uvc.usb0/streaming/header/h
# cd functions/uvc.usb0/streaming/header/h
# ln -s ../../uncompressed/u
# cd ../../class/fs
# ln -s ../../header/h
# cd ../../class/hs
# ln -s ../../header/h
# cd ../../class/ss
# ln -s ../../header/h

Testing the UVC function¶

device: run the gadget, modprobe vivid:

# uvc-gadget -u /dev/video<uvc video node #> -v /dev/video<vivid video node #>

where uvc-gadget is this program:: http://git.ideasonboard.org/uvc-gadget.git

with these patches:

http://www.spinics.net/lists/linux-usb/msg99220.html

host:

luvcview -f yuv

19. PRINTER function¶

The function is provided by usb_f_printer.ko module.

Function-specific configfs interface¶

The function name to use when creating the function directory is “printer”. The printer function provides these attributes in its function directory:

pnp_string

Data to be passed to the host in pnp string

q_len

Number of requests per endpoint

Testing the PRINTER function¶

The most basic testing:

device: run the gadget:

# ls -l /devices/virtual/usb_printer_gadget/

should show g_printer<number>.

If udev is active, then /dev/g_printer<number> should appear automatically.

host:

If udev is active, then e.g. /dev/usb/lp0 should appear.

host->device transmission:

device:

# cat /dev/g_printer<number>

host:

# cat > /dev/usb/lp0

device->host transmission:

# cat > /dev/g_printer<number>

host:

# cat /dev/usb/lp0

More advanced testing can be done with the prn_example described in Linux USB Printer Gadget Driver.

20. UAC1 function (virtual ALSA card, using u_audio API)¶

The function is provided by usb_f_uac1.ko module. It will create a virtual ALSA card and the audio streams are simply sinked to and sourced from it.

Function-specific configfs interface¶

The function name to use when creating the function directory is “uac1”. The uac1 function provides these attributes in its function directory:

c_chmask

capture channel mask

c_srate

list of capture sampling rates (comma-separated)

c_ssize

capture sample size (bytes)

c_mute_present

capture mute control enable

c_volume_present

capture volume control enable

c_volume_min

capture volume control min value (in 1/256 dB)

c_volume_max

capture volume control max value (in 1/256 dB)

c_volume_res

capture volume control resolution (in 1/256 dB)

p_chmask

playback channel mask

p_srate

list of playback sampling rates (comma-separated)

p_ssize

playback sample size (bytes)

p_mute_present

playback mute control enable

p_volume_present

playback volume control enable

p_volume_min

playback volume control min value (in 1/256 dB)

p_volume_max

playback volume control max value (in 1/256 dB)

p_volume_res

playback volume control resolution (in 1/256 dB)

req_number

the number of pre-allocated requests for both capture and playback

function_name

name of the interface

The attributes have sane default values.

Testing the UAC1 function¶

device: run the gadget host: aplay -l # should list our USB Audio Gadget

This function does not require real hardware support, it just sends a stream of audio data to/from the host. In order to actually hear something at the device side, a command similar to this must be used at the device side:

$ arecord -f dat -t wav -D hw:2,0 | aplay -D hw:0,0 &

e.g.:

$ arecord -f dat -t wav -D hw:CARD=UAC1Gadget,DEV=0 | \
  aplay -D default:CARD=OdroidU3

21. MIDI2 function¶

The function is provided by usb_f_midi2.ko module. It will create a virtual ALSA card containing a UMP rawmidi device where the UMP packet is looped back. In addition, a legacy rawmidi device is created. The UMP rawmidi is bound with ALSA sequencer clients, too.

Function-specific configfs interface¶

The function name to use when creating the function directory is “midi2”. The midi2 function provides these attributes in its function directory as the card top-level information:

process_ump

Bool flag to process UMP Stream messages (0 or 1)

static_block

Bool flag for static blocks (0 or 1)

iface_name

Optional interface name string

The directory contains a subdirectory “ep.0”, and this provides the attributes for a UMP Endpoint (which is a pair of USB MIDI Endpoints):

protocol_caps

MIDI protocol capabilities; 1: MIDI 1.0, 2: MIDI 2.0, or 3: both protocols

protocol

Default MIDI protocol (either 1 or 2)

ep_name

UMP Endpoint name string

product_id

Product ID string

manufacturer

Manufacture ID number (24 bit)

family

Device family ID number (16 bit)

model

Device model ID number (16 bit)

sw_revision

Software revision (32 bit)

Each Endpoint subdirectory contains a subdirectory “block.0”, which represents the Function Block for Block 0 information. Its attributes are:

name

Function Block name string

direction

Direction of this FB 1: input, 2: output, or 3: bidirectional

first_group

The first UMP Group number (0-15)

num_groups

The number of groups in this FB (1-16)

midi1_first_group

The first UMP Group number for MIDI 1.0 (0-15)

midi1_num_groups

The number of groups for MIDI 1.0 (0-16)

ui_hint

UI-hint of this FB 0: unknown, 1: receiver, 2: sender, 3: both

midi_ci_verison

Supported MIDI-CI version number (8 bit)

is_midi1

Legacy MIDI 1.0 device (0-2) 0: MIDI 2.0 device, 1: MIDI 1.0 without restriction, or 2: MIDI 1.0 with low speed

sysex8_streams

Max number of SysEx8 streams (8 bit)

active

Bool flag for FB activity (0 or 1)

If multiple Function Blocks are required, you can add more Function Blocks by creating subdirectories “block.<num>” with the corresponding Function Block number (1, 2, ....). The FB subdirectories can be dynamically removed, too. Note that the Function Block numbers must be continuous.

Similarly, if you multiple UMP Endpoints are required, you can add more Endpoints by creating subdirectories “ep.<num>”. The number must be continuous.

For emulating the old MIDI 2.0 device without UMP v1.1 support, pass 0 to process_ump flag. Then the whole UMP v1.1 requests are ignored.

Testing the MIDI2 function¶

On the device: run the gadget, and running:

$ cat /proc/asound/cards

will show a new sound card containing a MIDI2 device.

OTOH, on the host:

$ cat /proc/asound/cards

will show a new sound card containing either MIDI1 or MIDI2 device, depending on the USB audio driver configuration.

On both, when ALSA sequencer is enabled on the host, you can find the UMP MIDI client such as “MIDI 2.0 Gadget”.

As the driver simply loops back the data, there is no need for a real device just for testing.

For testing a MIDI input from the gadget to the host (e.g. emulating a MIDI keyboard), you can send a MIDI stream like the following.

On the gadget:

$ aconnect -o
....
client 20: 'MIDI 2.0 Gadget' [type=kernel,card=1]
    0 'MIDI 2.0        '
    1 'Group 1 (MIDI 2.0 Gadget I/O)'
$ aplaymidi -p 20:1 to_host.mid

On the host:

$ aconnect -i
....
client 24: 'MIDI 2.0 Gadget' [type=kernel,card=2]
    0 'MIDI 2.0        '
    1 'Group 1 (MIDI 2.0 Gadget I/O)'
$ arecordmidi -p 24:1 from_gadget.mid

If you have a UMP-capable application, you can use the UMP port to send/receive the raw UMP packets, too. For example, aseqdump program with UMP support can receive from UMP port. On the host:

$ aseqdump -u 2 -p 24:1
Waiting for data. Press Ctrl+C to end.
Source  Group    Event                  Ch  Data
 24:1   Group  0, Program change          0, program 0, Bank select 0:0
 24:1   Group  0, Channel pressure        0, value 0x80000000

For testing a MIDI output to the gadget to the host (e.g. emulating a MIDI synth), it’ll be just other way round.

On the gadget:

$ arecordmidi -p 20:1 from_host.mid

On the host:

$ aplaymidi -p 24:1 to_gadget.mid

The access to MIDI 1.0 on altset 0 on the host is supported, and it’s translated from/to UMP packets on the gadget. It’s bound to only Function Block 0.

The current operation mode can be observed in ALSA control element “Operation Mode” for SND_CTL_IFACE_RAWMIDI. For example:

$ amixer -c1 contents
numid=1,iface=RAWMIDI,name='Operation Mode'
  ; type=INTEGER,access=r--v----,values=1,min=0,max=2,step=0
  : values=2

where 0 = unused, 1 = MIDI 1.0 (altset 0), 2 = MIDI 2.0 (altset 1). The example above shows it’s running in 2, i.e. MIDI 2.0.

ifname	network device interface name associated with this function instance
qmult	queue length multiplier for high and super speed
host_addr	MAC address of host’s end of this Ethernet over USB link
dev_addr	MAC address of device’s end of this Ethernet over USB link

protocol	HID protocol to use
report_desc	data to be used in HID reports, except data passed with /dev/hidg<X>
report_length	HID report length
subclass	HID subclass to use

stall	Set to permit function to halt bulk endpoints. Disabled on some USB devices known not to work correctly. You should set it to true.
num_buffers	Number of pipeline buffers. Valid numbers are 2..4. Available only if CONFIG_USB_GADGET_DEBUG_FILES is set.

file	The path to the backing file for the LUN. Required if LUN is not marked as removable.
ro	Flag specifying access to the LUN shall be read-only. This is implied if CD-ROM emulation is enabled as well as when it was impossible to open “filename” in R/W mode.
removable	Flag specifying that LUN shall be indicated as being removable.
cdrom	Flag specifying that LUN shall be reported as being a CD-ROM.
nofua	Flag specifying that FUA flag in SCSI WRITE(10,12)
forced_eject	This write-only file is useful only when the function is active. It causes the backing file to be forcibly detached from the LUN, regardless of whether the host has allowed it. Any non-zero number of bytes written will result in ejection.

buflen	MIDI buffer length
id	ID string for the USB MIDI adapter
in_ports	number of MIDI input ports
index	index value for the USB MIDI adapter
out_ports	number of MIDI output ports
qlen	USB read request queue length

pattern	0 (all zeros), 1 (mod63), 2 (none)
isoc_interval	1..16
isoc_maxpacket	0 - 1023 (fs), 0 - 1024 (hs/ss)
isoc_mult	0..2 (hs/ss only)
isoc_maxburst	0..15 (ss only)
bulk_buflen	buffer length
bulk_qlen	depth of queue for bulk
iso_qlen	depth of queue for iso

audio_buf_size	audio buffer size
fn_cap	capture pcm device file name
fn_cntl	control device file name
fn_play	playback pcm device file name
req_buf_size	ISO OUT endpoint request buffer size
req_count	ISO OUT endpoint request count

c_chmask	capture channel mask
c_srate	list of capture sampling rates (comma-separated)
c_ssize	capture sample size (bytes)
c_sync	capture synchronization type (async/adaptive)
c_mute_present	capture mute control enable
c_volume_present	capture volume control enable
c_volume_min	capture volume control min value (in 1/256 dB)
c_volume_max	capture volume control max value (in 1/256 dB)
c_volume_res	capture volume control resolution (in 1/256 dB)
c_hs_bint	capture bInterval for HS/SS (1-4: fixed, 0: auto)
fb_max	maximum extra bandwidth in async mode
p_chmask	playback channel mask
p_srate	list of playback sampling rates (comma-separated)
p_ssize	playback sample size (bytes)
p_mute_present	playback mute control enable
p_volume_present	playback volume control enable
p_volume_min	playback volume control min value (in 1/256 dB)
p_volume_max	playback volume control max value (in 1/256 dB)
p_volume_res	playback volume control resolution (in 1/256 dB)
p_hs_bint	playback bInterval for HS/SS (1-4: fixed, 0: auto)
req_number	the number of pre-allocated request for both capture and playback
function_name	name of the interface
c_terminal_type	code of the capture terminal type
p_terminal_type	code of the playback terminal type

streaming_interval	interval for polling endpoint for data transfers
streaming_maxburst	bMaxBurst for super speed companion descriptor
streaming_maxpacket	maximum packet size this endpoint is capable of sending or receiving when this configuration is selected
function_name	name of the interface

control header	create in control/header, link from control/class/fs and/or control/class/ss
streaming header	create in streaming/header, link from streaming/class/fs and/or streaming/class/hs and/or streaming/class/ss
format description	create in streaming/mjpeg and/or streaming/uncompressed
frame description	create in streaming/mjpeg/<format> and/or in streaming/uncompressed/<format>

pnp_string	Data to be passed to the host in pnp string
q_len	Number of requests per endpoint

process_ump	Bool flag to process UMP Stream messages (0 or 1)
static_block	Bool flag for static blocks (0 or 1)
iface_name	Optional interface name string

protocol_caps	MIDI protocol capabilities; 1: MIDI 1.0, 2: MIDI 2.0, or 3: both protocols
protocol	Default MIDI protocol (either 1 or 2)
ep_name	UMP Endpoint name string
product_id	Product ID string
manufacturer	Manufacture ID number (24 bit)
family	Device family ID number (16 bit)
model	Device model ID number (16 bit)
sw_revision	Software revision (32 bit)

name	Function Block name string
direction	Direction of this FB 1: input, 2: output, or 3: bidirectional
first_group	The first UMP Group number (0-15)
num_groups	The number of groups in this FB (1-16)
midi1_first_group	The first UMP Group number for MIDI 1.0 (0-15)
midi1_num_groups	The number of groups for MIDI 1.0 (0-16)
ui_hint	UI-hint of this FB 0: unknown, 1: receiver, 2: sender, 3: both
midi_ci_verison	Supported MIDI-CI version number (8 bit)
is_midi1	Legacy MIDI 1.0 device (0-2) 0: MIDI 2.0 device, 1: MIDI 1.0 without restriction, or 2: MIDI 1.0 with low speed
sysex8_streams	Max number of SysEx8 streams (8 bit)
active	Bool flag for FB activity (0 or 1)