[v5,0/2] Add generic serial MIDI driver using serial bus API

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.

Changes in v5:
- Reword and add to description in dt-binding
- Change 'speed' dt property to 'current-speed'
- Move MIDI output loop onto workqueue (since this could loop quite a while,
    if ALSA provides a continuous stream of bytes)
- Add tx_state bit flags to snd_serial_generic struct
- Safegard critical section in tx_work with atomic bit ops on tx_state
- Switch operations on `filemode` to use atomic bit ops

Changes in v4:
- Fix regressed typo - Correct 3.84 kBaud -> 38.4 kBaud in DT & Kconfig
  (sorry about spam - noticed after sending v3 and didn't want to let
  the error sit around for too long)

Changes in v3:
- Replace use of snd_printk() with dev_* alternatives
- Removed unnecessary initialization of err variables
- Replaced instances of `== SERIAL_MODE_NOT_OPENED` with zero check
- Loop on output_write to completely fill output buffer if data available
- Depend on CONFIG_OF in Kconfig
- Replace use of devm_kzalloc() with extra_size allocation in snd_devm_card_new()
- Use module_serdev_device_driver() instead of module_init() and module_exit(0)

Changes in v2:
- Fix 'snd_serial_generic_write_wakeup' missing static keyword 
- Correct 3.125 kBaud > 31.25 kBaud in documentation for MIDI         


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 |  46 +++
 sound/drivers/Kconfig                         |  18 +
 sound/drivers/Makefile                        |   2 +
 sound/drivers/serial-generic.c                | 377 ++++++++++++++++++
 4 files changed, 443 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/sound/serialmidi.yaml
 create mode 100644 sound/drivers/serial-generic.c


base-commit: 3e71713c9e75c34fc03f55ea86b381856ca952ee

On Mon, May 02, 2022 at 10:04:03AM -0500, Daniel Kaehn wrote:
> Adds dt-binding for a Generic MIDI Interface using a serial device.
> 
> Signed-off-by: Daniel Kaehn <kaehndan@gmail.com>
> ---
>  .../devicetree/bindings/sound/serialmidi.yaml | 46 +++++++++++++++++++
>  1 file changed, 46 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/sound/serialmidi.yaml
> 
> diff --git a/Documentation/devicetree/bindings/sound/serialmidi.yaml b/Documentation/devicetree/bindings/sound/serialmidi.yaml
> new file mode 100644
> index 000000000000..06a894e1b91d
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/sound/serialmidi.yaml
> @@ -0,0 +1,46 @@
> +# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
> +
> +%YAML 1.2
> +---
> +$id: http://devicetree.org/schemas/sound/serialmidi.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> +
> +title: Generic Serial MIDI Interface
> +
> +maintainers:
> +  - Daniel Kaehn <kaehndan@gmail.com>
> +
> +description: 
> +  Generic MIDI interface using a serial device. This denotes that a serial device is
> +  dedicated to MIDI communication, either to an external MIDI device through a DIN5
> +  or other connector, or to a known hardwired MIDI controller. This device must be a
> +  child node of a serial node.
> +
> +  Can only be set to use standard baud rates corresponding to supported rates of the
> +  parent serial device. If the standard MIDI baud of 31.25 kBaud is needed 
> +  (as would be the case if interfacing with arbitrary external MIDI devices),
> +  configure the clocks of the parent serial device so that a requested baud of 38.4 kBaud
> +  resuts in the standard MIDI baud rate, and set the 'current-speed' property to 38400.

s/resuts/results/

> +
> +properties:
> +  compatible:
> +    const: serialmidi

serial-midi would be a bit more readable. (And then align the filename 
with that.)

> +
> +  current-speed:
> +    $ref: /schemas/types.yaml#/definitions/uint32

Already has a type applied by serial.yaml, so you can drop.

> +    description: Baudrate to set the serial port to when this MIDI device is opened.
> +      If not specified, the parent serial device is allowed to use its default baud.
> +
> +required:
> +  - compatible
> +
> +additionalProperties: false
> +
> +examples:
> +  - |
> +    serial {
> +        midi {
> +            compatible = "serialmidi";
> +            current-speed = <38400>;
> +        };
> +    };
> -- 
> 2.33.0
> 
>