[PATCH v7 0/2] the Topping M62's vendor controls, on the component framework

[Mikhail Gavrilov]

The Topping M62 keeps its analogue input gains, output volumes and
output source selectors behind a vendor protocol on a HID interface,
outside the USB Audio Class. On Linux they can only be set by hand on
the front panel; the one capture gain snd-usb-audio exposes is a
digital trim after the converter.

This series makes them ordinary ALSA controls on the card
snd-usb-audio already creates. 1/2 is a HID driver that speaks the
protocol. 2/2 makes the M62's mixer quirk a component master, which
hands the HID driver its struct snd_card at bind time and takes the
controls away at unbind; neither driver needs to know about the
other's disconnect.

This is the first posting without RFC. The split was Takashi's
suggestion in the thread of an earlier series that did the same job
as a mixer quirk claiming the HID interface. After RFC v6 he wrote,
for the audio side:

  If the implementation with the component works actually, I'm for
  it. So, from the audio side, feel free to submit patches for
  upstreaming without RFC.

  https://lore.kernel.org/all/87mrsy50gy.wl-tiwai@xxxxxxx/

and left the question of which tree takes it until the HID side is
settled. So this posting is mainly for Jiri and Benjamin.

The one HID question the RFC carried is gone. RFC v6 cleared
intf->needs_remote_wakeup after hid_hw_open(), so that the card could
runtime-suspend although it has no remote wakeup. That was wrong for
this driver. The card reports every turn of a front-panel knob, and a
suspended card without remote wakeup cannot deliver one; after resume
it re-reports the gains of connected inputs but not the output
volumes, so a headphone volume changed while it slept would stay stale
in its control. v7 leaves the flag as usbhid_open() sets it, which is
what every other driver in drivers/hid does -- none of them touches
it -- and runtime autosuspend of the card is refused while the driver
is bound.

On the tree question: the two patches depend on each other neither at
build time nor at run time. 1/2 alone binds, speaks to the card and
creates no controls; 2/2 alone registers a master that never matches.
They can go through one tree or each through its own.

Changes since RFC v6
(https://lore.kernel.org/all/20260904165848.3940603-1-mikhail.v.gavrilov@xxxxxxxxx/):

  - the needs_remote_wakeup clear in probe is gone, with the comment
    that carried it, as above. Nothing else changed.

What the card reports

After a subscribe the card reports itself in two waves: jacks at about
0.9 s, then at about 5.2 s the jacks again, the output mutes and the
gain of each input whose jack is present. Every turn of a front-panel
knob is reported as it happens. A source selector is never reported:
the card reports events, and a selector has no front-panel control.
So the controls are published at bind and carry whatever alsactl
restores, and the selectors start at an "Unknown" item until something
writes them. 1/2's commit message has the details.

One problem outside the kernel, now fixed there: removing controls
from a live card -- which is what unbinding this driver does --
corrupted the heap of any process holding the card through alsa-lib's
ctl remap plugin, and UCM profiles that use MixerRemap put WirePlumber
in exactly that position. It crashed some time later, inside unrelated
free() calls. The cause is a one-line slip in
remap_forget_numid_child(), present since alsa-lib 1.2.14; the fix is
posted:

  https://lore.kernel.org/alsa-devel/20260930212624.30308-1-mikhail.v.gavrilov@xxxxxxxxx/

Tested

Fedora, 7.3.0-rc5-551c722f4080 plus this series, with KASAN (generic),
lockdep and UBSAN; M62 firmware V87.05.45.48.27. The loaded module was
matched against the installed file by build ID, so everything below
ran on the code posted here.

All nine controls present, and front-panel knobs reported to ALSA as
they turn.

Fifty cycles of module unload and load against a live card with
PipeWire running: fifty binds and nothing in the kernel log. With stock
alsa-lib WirePlumber crashes during this, for the reason above; with
the fix, ten cycles under valgrind run clean.

System suspend to RAM and back: the headphone source selector kept its
value across it, and the front panel still reports afterwards.

The cable pulled out twice while a control was being written in a
loop: no write errors logged, and the card binds again when plugged
back in.

Two M62s on one host, on different hub ports, one in Pro Audio mode
and one in Mobile Mode: each binds to its own HID device and carries
its own nine controls, a knob on the second card's panel produces
events on that card only, and unplugging the first leaves the second's
controls in place.

With no UCM profile involved -- the card in Mobile Mode, where none
applies -- PipeWire's stock analog-output-headphones path finds
"Headphone Playback Volume" by name, and the desktop volume drives the
card's analogue headphone stage through the driver.

Not tested:

  - an audio-side unbind and rebind with the HID driver left bound;
  - hibernation, and with it .reset_resume, which shares
    topping_resume() with .resume: this machine powers off instead of
    saving an image, for reasons unrelated to this series;
  - kmemleak, compiled in here but disabled at boot;
  - a card whose battery has run down;
  - a big-endian host.

Mikhail Gavrilov (2):
  HID: topping-m62: driver for the M62's vendor controls
  ALSA: usb-audio: bind the Topping M62's vendor controls

 MAINTAINERS                   |   9 +
 drivers/hid/Kconfig           |  19 +
 drivers/hid/Makefile          |   1 +
 drivers/hid/hid-ids.h         |   3 +
 drivers/hid/hid-quirks.c      |   3 +
 drivers/hid/hid-topping-m62.c | 911 ++++++++++++++++++++++++++++++++++
 sound/usb/Makefile            |   1 +
 sound/usb/mixer_quirks.c      |   5 +
 sound/usb/mixer_topping.c     | 236 +++++++++
 sound/usb/mixer_topping.h     |   7 +
 10 files changed, 1195 insertions(+)
 create mode 100644 drivers/hid/hid-topping-m62.c
 create mode 100644 sound/usb/mixer_topping.c
 create mode 100644 sound/usb/mixer_topping.h

-- 
2.55.0