[PATCH v2 0/2] Add generic serial MIDI driver using serial bus API

[Daniel Kaehn]

Changes in v2:
- Fix 'snd_serial_generic_write_wakeup' missing static keyword as 
  reported by the kernel test robot
- Correct 3.125 kBaud -> 31.25 kBaud and 3.84 kBaud -> 38.4 kBaud
  in documentation for MIDI
Reported-by: kernel test robot <lkp at intel.com>

Background as included in the initial cover letter:


Generic serial MIDI driver adding support for using serial devices
compatible with the serial bus as raw MIDI devices, allowing using
additional serial devices not compatible with the existing
serial-u16550 driver. Supports only setting standard serial baudrates on
the underlying serial device; however, the underlying serial device can
be configured so that a requested 38.4 kBaud is actually the standard MIDI
31.25 kBaud. Supports DeviceTree configuration.

The need for this driver arose from a project using a Raspberry Pi4 which
needed to receive and send raw MIDI with low latency. The pl011 UART
used is not compatible with the existing serial MIDI driver made for
u16550-style devices. Using a userspace program such as ttymidi to feed
input from the TTY device to a virtual ALSA MIDI device was functional,
but not ideal.

I am not sure if a MIDI driver needing the mentioned 'hack' to clock
38.4 kBaud down to the standard MIDI baud is permissible in the mainline
kernel, but am submitting nevertheless in case it is useful. To my knowledge,
it doesn't seem that there would be any way for this driver to manually
configure a serial port to 31.25 kBaud using the serial bus API (please 
correct me f I'm wrong). In my use case, I am actually configuring one port
to run at 115.2 kBaud for faster communication with a custom onboard MIDI controller.

Daniel Kaehn (2):
  dt-bindings: sound: Add generic serial MIDI device
  Add generic serial MIDI driver using serial bus API

 .../devicetree/bindings/sound/serialmidi.yaml |  41 +++
 sound/drivers/Kconfig                         |  17 +
 sound/drivers/Makefile                        |   2 +
 sound/drivers/serial-generic.c                | 344 ++++++++++++++++++
 4 files changed, 404 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/sound/serialmidi.yaml
 create mode 100644 sound/drivers/serial-generic.c


base-commit: b253435746d9a4a701b5f09211b9c14d3370d0da
-- 
2.35.1