ALSA: documentation: Add description for USB MIDI 2.0 gadget driver

The USB MIDI 2.0 gadget driver is now supported for 6.6 kernel, and
here we show a brief instruction how to enable and use it.

Link: https://lore.kernel.org/r/20230824075108.29958-5-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/designs/midi-2.0.rst

@@ -376,3 +376,191 @@ Sequencer API Extensions
   name and attributes accordingly, and notifies the changes via the
   announcement to the ALSA sequencer system port, similarly like the
   normal port change notification.
+
+
+MIDI2 USB Gadget Function Driver
+================================
+
+The latest kernel contains the support for USB MIDI 2.0 gadget
+function driver, which can be used for prototyping and debugging MIDI
+2.0 features.
+
+`CONFIG_USB_GADGET`, `CONFIG_USB_CONFIGFS` and
+`CONFIG_USB_CONFIGFS_F_MIDI2` need to be enabled for the MIDI2 gadget
+driver.
+
+In addition, for using a gadget driver, you need a working UDC driver.
+In the example below, we use `dummy_hcd` driver (enabled via
+`CONFIG_USB_DUMMY_HCD`) that is available on PC and VM for debugging
+purpose.  There are other UDC drivers depending on the platform, and
+those can be used for a real device, instead, too.
+
+At first, on a system to run the gadget, load `libcomposite` module::
+
+  % modprobe libcomposite
+
+and you'll have `usb_gadget` subdirectory under configfs space
+(typically `/sys/kernel/config` on modern OS).  Then create a gadget
+instance and add configurations there, for example::
+
+  % cd /sys/kernel/config
+  % mkdir usb_gadget/g1
+
+  % cd usb_gadget/g1
+  % mkdir configs/c.1
+  % mkdir functions/midi2.usb0
+
+  % echo 0x0004 > idProduct
+  % echo 0x17b3 > idVendor
+  % mkdir strings/0x409
+  % echo "ACME Enterprises" > strings/0x409/manufacturer
+  % echo "ACMESynth" > strings/0x409/product
+  % echo "ABCD12345" > strings/0x409/serialnumber
+
+  % mkdir configs/c.1/strings/0x409
+  % echo "Monosynth" > configs/c.1/strings/0x409/configuration
+  % echo 120 > configs/c.1/MaxPower
+
+At this point, there must be a subdirectory `ep.0`, and that is the
+configuration for a UMP Endpoint.  You can fill the Endpoint
+information like::
+
+  % echo "ACMESynth" > functions/midi2.usb0/iface_name
+  % echo "ACMESynth" > functions/midi2.usb0/ep.0/ep_name
+  % echo "ABCD12345" > functions/midi2.usb0/ep.0/product_id
+  % echo 0x0123 > functions/midi2.usb0/ep.0/family
+  % echo 0x4567 > functions/midi2.usb0/ep.0/model
+  % echo 0x123456 > functions/midi2.usb0/ep.0/manufacturer
+  % echo 0x12345678 > functions/midi2.usb0/ep.0/sw_revision
+
+The default MIDI protocol can be set either 1 or 2::
+
+  % echo 2 > functions/midi2.usb0/ep.0/protocol
+
+And, you can find a subdirectory `block.0` under this Endpoint
+subdirectory.  This defines the Function Block information::
+
+  % echo "Monosynth" > functions/midi2.usb0/ep.0/block.0/name
+  % echo 0 > functions/midi2.usb0/ep.0/block.0/first_group
+  % echo 1 > functions/midi2.usb0/ep.0/block.0/num_groups
+
+Finally, link the configuration and enable it::
+
+  % ln -s functions/midi2.usb0 configs/c.1
+  % echo dummy_udc.0 > UDC
+
+where `dummy_udc.0` is an example case and it differs depending on the
+system.  You can find the UDC instances in `/sys/class/udc` and pass
+the found name instead::
+
+  % ls /sys/class/udc
+  dummy_udc.0
+
+Now, the MIDI 2.0 gadget device is enabled, and the gadget host
+creates a new sound card instance containing a UMP rawmidi device by
+`f_midi2` driver::
+
+  % cat /proc/asound/cards
+  ....
+  1 [Gadget         ]: f_midi2 - MIDI 2.0 Gadget
+                       MIDI 2.0 Gadget
+
+And on the connected host, a similar card should appear, too, but with
+the card and device names given in the configfs above::
+
+  % cat /proc/asound/cards
+  ....
+  2 [ACMESynth      ]: USB-Audio - ACMESynth
+                       ACME Enterprises ACMESynth at usb-dummy_hcd.0-1, high speed
+
+You can play a MIDI file on the gadget side::
+
+  % aplaymidi -p 20:1 to_host.mid
+
+and this will appear as an input from a MIDI device on the connected
+host::
+
+  % aseqdump -p 20:0 -u 2
+
+Vice versa, a playback on the connected host will work as an input on
+the gadget, too.
+
+Each Function Block may have different direction and UI-hint,
+specified via `direction` and `ui_hint` attributes.
+Passing `1` is for input-only, `2` for out-only and `3` for
+bidirectional (the default value).  For example::
+
+  % echo 2 > functions/midi2.usb0/ep.0/block.0/direction
+  % echo 2 > functions/midi2.usb0/ep.0/block.0/ui_hint
+
+When you need more than one Function Blocks, you can create
+subdirectories `block.1`, `block.2`, etc dynamically, and configure
+them in the configuration procedure above before linking.
+For example, to create a second Function Block for a keyboard::
+
+  % mkdir functions/midi2.usb0/ep.0/block.1
+  % echo "Keyboard" > functions/midi2.usb0/ep.0/block.1/name
+  % echo 1 > functions/midi2.usb0/ep.0/block.1/first_group
+  % echo 1 > functions/midi2.usb0/ep.0/block.1/num_groups
+  % echo 1 > functions/midi2.usb0/ep.0/block.1/direction
+  % echo 1 > functions/midi2.usb0/ep.0/block.1/ui_hint
+
+The `block.*` subdirectories can be removed dynamically, too (except
+for `block.0` which is persistent).
+
+For assigning a Function Block for MIDI 1.0 I/O, set up in `is_midi1`
+attribute.  1 is for MIDI 1.0, and 2 is for MIDI 1.0 with low speed
+connection::
+
+  % echo 2 > functions/midi2.usb0/ep.0/block.1/is_midi1
+
+For disabling the processing of UMP Stream messages in the gadget
+driver, pass `0` to `process_ump attribute in the top-level config::
+
+  % echo 0 > functions/midi2.usb0/process_ump
+
+The MIDI 1.0 interface at altset 0 is supported by the gadget driver,
+too.  When MIDI 1.0 interface is selected by the connected host, the
+UMP I/O on the gadget is translated from/to USB MIDI 1.0 packets
+accordingly while the gadget driver keeps communicating with the
+user-space over UMP rawmidi.
+
+MIDI 1.0 ports are set up from the config in each Function Block.
+For example::
+
+  % echo 0 > functions/midi2.usb0/ep.0/block.0/midi1_first_group
+  % echo 1 > functions/midi2.usb0/ep.0/block.0/midi1_num_groups
+
+The configuration above will enable the Group 1 (the index 0) for MIDI
+1.0 interface.  Note that those groups must be in the groups defined
+for the Function Block itself.
+
+The gadget driver supports more than one UMP Endpoints, too.
+Similarly like the Function Blocks, you can create a new subdirectory
+`ep.1` (but under the card top-level config) to enable a new Endpoint::
+
+  % mkdir functions/midi2.usb0/ep.1
+
+and create a new Function Block there.  For example, to create 4
+Groups for the Function Block of this new Endpoint::
+
+  % mkdir functions/midi2.usb0/ep.1/block.0
+  % echo 4 > functions/midi2.usb0/ep.1/block.0/num_groups
+
+Now, you'll have 4 rawmidi devices in total: the first two are UMP
+rawmidi devices for Endpoint 0 and Endpoint 1, and other two for the
+legacy MIDI 1.0 rawmidi devices corresponding to both EP 0 and EP 1.
+
+The current altsetting on the gadget can be informed via a control
+element "Operation Mode" with `RAWMIDI` iface.  e.g. you can read it
+via `amixer` program running on the gadget host like::
+
+  % amixer -c1 cget iface=RAWMIDI,name='Operation Mode'
+  ; type=INTEGER,access=r--v----,values=1,min=0,max=2,step=0
+  : values=2
+
+The value (shown in the second returned line with `: values=`)
+indicates 1 for MIDI 1.0 (altset 0), 2 for MIDI 2.0 (altset 1) and 0
+for unset.
+
+As of now, the configurations can't be changed after binding.