Re: RFC: usb: gadget: u_audio: Notifying gadget that host started playback/capture?

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Dne 21. 09. 23 v 3:30 Arun Raghavan napsal(a):
Hi folks,

On Fri, 1 Oct 2021, at 8:38 AM, Pavel Hofman wrote:
Hi,

Dne 08. 09. 21 v 10:21 Pavel Hofman napsal(a):
Hi,

The current audio gadget has no way to inform the gadget side that the
host side has started playback/capture and that gadget-side alsa
processes should be started.

Playback/capture processes on the host side do not get stuck without the
gadget side consuming/producing data (OUT requests are ignored in
u_audio_iso_complete, IN ones send initial zeros in their req->buf).

However, playback/capture processes on the gadget side get stuck without
the host side sending playback OUT packets or capture IN requests and
time out with error. If there was a way to inform the gadget side that
playback/capture has started on the host side, the gadget clients could
react accordingly.


I drafted a simple patch for u_audio.c which defines read-only boolean
ctl elems "Capture Requested" and "Playback Requested". Their values are
set/reset in methods u_audio_start_capture/playback and
u_audio_stop_capture/playback, i.e. at changes of respective altsettings
from 0 to 1 and back. Every ctl elem value change sends notification via
snd_ctl_notify. The principle works OK for capture/playback start/stop
on the host, as monitored by alsactl:

pi@raspberrypi:~ $ alsactl monitor hw:UAC2Gadget
node hw:UAC2Gadget, #4 (3,0,0,Capture Requested,0) VALUE
node hw:UAC2Gadget, #4 (3,0,0,Capture Requested,0) VALUE
node hw:UAC2Gadget, #3 (3,0,0,Playback Requested,0) VALUE
node hw:UAC2Gadget, #3 (3,0,0,Playback Requested,0) VALUE

However at enumeration the USB host switches both playback and capture
altsettings repeatedly, generating "fake" events from the gadget side
POW. The host even sends regular-sized EP-OUT packets filled with zeros
during enumeration (tested on linux only for now).

Please is there any way to "detect" the enumeration stage to mask out
the "fake" playback/capture start/stop events?

The attached patch does not apply cleanly to mainline u_audio.c because
it's rebased on other patches not submitted yet but it's only a
discussion inducer for now.

Resurrecting this one -- is there any input on how we want to deal wit letting UAC gadgets know when the host is sending/receiving data?

The current version uses the Playback/Capture Rate alsa ctls with notifications https://lore.kernel.org/all/20220121155308.48794-8-pavel.hofman@xxxxxxxxxxx/

Example of handling is e.g. https://github.com/pavhofman/gaudio_ctl , the controller is being used in a number of projects, mostly DIY.

Recently Qualcomm devs have submitted patches for alternative approach using uevents https://lore.kernel.org/lkml/2023050801-handshake-refusing-0367@gregkh/T/#mcd6b346f3ddab6ab34792be0141633bb362d168f and later versions. The detection is identical, monitoring change in altsetting from 0 to non zero and back (methods u_audio_[start/stop]_[capture/playback]), just a different means of communicating the events to userspace.

Both methods (using the same principle) suffer from not knowing what's going on the host side and cannot differentiate between player really starting playback vs. UAC2 host driver or Pulseaudio shortly checking device availability. That's why the gaudio_ctl controller can debounce the playback/capture start https://github.com/pavhofman/gaudio_ctl#debouncing . But that is just an ugly workaround...

With regards,

Pavel.



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