[PATCH v3 0/2] ALSA: usb-audio: the Topping M62's vendor controls

[Mikhail Gavrilov]

v3 answers the automated review of v2. Four of its five findings were
real, and one of them was a bug a user would have met on every boot.

  - The enum refused "Unknown", which is the value it reports until a
    hand has chosen. alsactl stores and restores that value like any
    other, so the refusal failed a restore of the driver's own report
    -- observed here as "Cannot write control ... Invalid argument"
    from alsactl. Writing "Unknown" is now a quiet no-op rather than an
    error, since it is a report and not a choice either way.

  - The hardware is reached under snd_usb_lock_shutdown(), the way the
    rest of this directory reaches it. Without it nothing made the
    teardown wait for a control callback already in flight.

  - Suspend and resume are handled rather than survived: the URB does
    not outlive a system sleep, so notifications stopped for good after
    the first one. The resume path resubmits, subscribes again and asks
    for the state, which also refreshes a cache that may have gone
    stale while the panel was reachable and the driver was not.

  - The claimed interface is released, on the error path and at
    teardown, so unbind and bind again works instead of failing at the
    claim. That needed a release helper beside snd_usb_claim_iface(),
    for the same reason the claim needed one.

  - A mutex spans each write from the comparison to the cache update.
    The review called two writers reaching the device in one order and
    the cache in the other a race, and it is one, though a narrow one.

Nothing else changed since v2; the questions in that cover letter about
the third form of the driver and about a control that can be written
but not read still stand.

Tested on the hardware as before: values arrive by themselves after
probe, a front panel knob still reaches the driver ten minutes later, a
write reaches the analogue stage -- one source recorded at gain 30 and
at gain 60 differs by 29.7 dB against the 30.0 dB the taper table
predicts -- and for 2/2 the audible test, where pointing an output away
from the bus being played silences it and pointing it back returns the
sound. On a KASAN and lockdep kernel, including unplug while a stream
was running, and now also across a suspend and resume cycle; no
reports.

Mikhail Gavrilov (2):
  ALSA: usb-audio: expose the Topping M62's analogue gains as mixer
    controls
  ALSA: usb-audio: let the M62's outputs say what they listen to

 MAINTAINERS               |   6 +
 drivers/hid/hid-ids.h     |   3 +
 drivers/hid/hid-quirks.c  |   2 +
 sound/usb/Makefile        |   1 +
 sound/usb/card.c          |  19 +
 sound/usb/mixer_quirks.c  |   5 +
 sound/usb/mixer_topping.c | 709 ++++++++++++++++++++++++++++++++++++++
 sound/usb/mixer_topping.h |   7 +
 sound/usb/usbaudio.h      |   4 +
 9 files changed, 756 insertions(+)
 create mode 100644 sound/usb/mixer_topping.c
 create mode 100644 sound/usb/mixer_topping.h


base-commit: 2709dd5ae32f0828f386327c76bba9f39f63a1c6
-- 
2.55.0