Re: usb:gadget:f_uac2: EP OUT is adaptive instead of async

[Ruslan Bilovol <ruslan.bilovol@gmail.com>]

On Fri, Apr 24, 2020 at 2:21 PM Pavel Hofman <pavel.hofman@ivitera.com> wrote:
>
>
>
> Dne 11. 02. 20 v 20:02 Pavel Hofman napsal(a):
> > Hi Ruslan,
> >
> > Dne 11. 02. 20 v 17:10 Ruslan Bilovol napsal(a):
> >> On Thu, Feb 6, 2020 at 3:35 PM Pavel Hofman <pavel.hofman@ivitera.com> wrote:
> >>>
> >>> .
> >>
> >> Are you working on async feedback EP implementation? I'm interested in that
> >> feature and I can implement it soon but do not want to do double work
> >> if somebody
> >> is already working on it and will send to the community soon
> >
> > I would be happy if you focused on the feedback. I want to solve the
> > g_audio usability somehow first
> > https://lore.kernel.org/linux-usb/df2eeff0-ca9c-35f9-2e72-8426b2cf72c9@ivitera.com/
> > as it would allow easy usage of the existing adaptive gadget version.
> >
> > The feedback - I have been shown a simple implementation which is not
> > public and is not using the g_audio alsa device on the other side.
> >
> > IMO the key issue is designing the async feedback to accept feedback
> > values from userspace as well as from any third-party kernel module. Why
> > userspace? The stream provided by the g_audio capture device  can be
> > output to a real master-clock alsa device (e.g. after synchronous
> > resampling), be sent by network to some master-clock device, many other
> > options possible. Any master-clock output device/ userspace sink should
> > be able to provide data for calculating proper up-to-date feedback value
> > for the slaved UAC2 gadget.
> >
> > I have done a few trials with master alsa output device -
> >
> > https://www.diyaudio.com/forums/pc-based/342070-linux-usb-audio-gadget-rpi4-otg.html#post5909816
> >
> >
> > https://www.diyaudio.com/forums/pc-based/342070-linux-usb-audio-gadget-rpi4-otg.html#post5910911
> >
> > Details for alsa-lib are discussed in
> > https://www.spinics.net/lists/alsa-devel/msg96781.html
> >
> >
> > This is a solution I need - syncing the UAC2 gadget to master clock of
> > real alsa soundcard . But again - I think the solution should be
> > flexible to support any source of feedback information, be it in kernel
> > or from userspace.
> >
>
> Hi, please can we resume this discussion about the feedback endpoint?
>
> Meanwhile a simple method described in
> https://www.aktives-hoeren.de/viewtopic.php?p=137829&sid=0d6cd50e0f58618da33621c62e412ada#p137829
> for obtaining required rate shift from /proc/asound/.../status to keep
> the master side buffer optimally filled was tested. That could be one
> source for the rate shift, to be passed to the driver. Perhaps a control
> element like the "PCM Rate Shift" of the snd-aloop driver could be used.

I worked on this during last moths and implemented feedback endpoint
and other improvements to UAC2 driver.
Currently it's under testing, I do expect to submit patches to community soon.

I used the same idea with "PCM Rate Shift" control to make it work with
existing alsaloop tool, but the in this case I do think it's better to expose
frequency directly to the control

Anyway, let'l look at the patches

Thanks,
Ruslan