[PATCH v5 0/2] ALSA: usb-audio: the Topping M62's vendor controls

[Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>]

v5 takes three more points from the automated review, all in 1/2, and
declines two that are a repeat of v3's.

Taken:

  - Allocation on the resume path now asks for no I/O. topping_send()
    is reached from topping_resume(), where reclaim can wait on a block
    device that has not woken yet, and the URB submitted there wanted
    the same treatment. This is what snd_usb_mixer_activate() does a
    few hundred lines away in mixer.c, so it is the file's own habit
    rather than a novelty.

  - A resubmit that fails with -EPERM is no longer reported. That is a
    URB usb_kill_urb() has already refused, which is the ordinary sound
    of an unplug rather than a fault. The line has been wrong since v1;
    nobody noticed until now.

Declined, with the same reasoning the v4 cover letter gave, since the
finding came back unchanged: a control callback cannot dereference a
freed private structure during disconnect. snd_ctl_elem_read() and
snd_ctl_elem_write() take snd_power_ref_and_wait(card) around the
callback; snd_card_disconnect() ends with snd_power_sync_ref(card),
which waits until every such reference is dropped; and in usb-audio's
disconnect, snd_card_disconnect() runs before
usb_audio_disconnect_components() reaches snd_usb_mixer_disconnect()
and hence this driver's private_free(). No callback can be inside the
driver when the structure is freed, and none can enter afterwards --
snd_power_ref_and_wait() sees card->shutdown and returns -ENODEV.

Nothing else changed since v4. 2/2 is unchanged. The questions from the
v2 cover letter still stand: whether snd-usb-audio registering the
hid_driver itself would be a better shape than either road posted, and
whether there is a convention for a control that can be written but not
read.

Tested on the hardware as before, including a suspend and resume cycle
and unbind and bind again, on a KASAN and lockdep kernel; 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 | 744 ++++++++++++++++++++++++++++++++++++++
 sound/usb/mixer_topping.h |   7 +
 sound/usb/usbaudio.h      |   4 +
 9 files changed, 791 insertions(+)
 create mode 100644 sound/usb/mixer_topping.c
 create mode 100644 sound/usb/mixer_topping.h


base-commit: 2709dd5ae32f0828f386327c76bba9f39f63a1c6
-- 
2.55.0