Re: [alsa-devel] [PATCH] ASoC: soc-pcm: Use delay set in pointer function

From: Takashi Iwai
Date: Tue Jul 31 2018 - 01:30:23 EST

On Tue, 31 Jul 2018 03:25:06 +0200,
Agrawal, Akshu wrote:
> On 7/30/2018 9:20 PM, Mark Brown wrote:
> > On Mon, Jul 30, 2018 at 05:32:21PM +0200, Takashi Iwai wrote:
> >
> >> That said, if delay callback of CPU dai provides the additional delay,
> >> the patch does correct thing. OTOH, if CPU dai provides the base
> >> delay instead, we need to clarify that it's rather a must; the delay
> >> calculation in pointer callback becomes bogus in this scenario.
> >
> > Part of the theory here is that every component might have a delay
> > independently of the rest and we need to add them all together to figure
> > out what the system as a whole will see. Personally I'd rather just
> > have everything use a callack consistently to avoid confusion.
> >
> For consistency we can add a delay callback in snd_pcm_ops and modify
> the drivers which directly assigning runtime->delay to use the callback.

No, ALSA PCM ops definition is fine. The delay calculation is
basically tied with the position, hence it has to be set together, and
that's the pointer callback.

Judging from the call pattern, the current design of ASoC delay
callback implies that the return value is more or less constant, which
can be accumulated on top of the base value. So your patch is natural
from that POV.

OTOH, if the CPU dai can really provide a dynamic value that is
strictly tied with pointer, CPU dai itself should provide the pointer
callback that covers both the pointer and the base delay, and it
should be used instead of component pointer callback.

> Apart from the 2 drivers mentioned in commit message I also found
> sound/usb to be doing the same and its delay getting lost.

The USB driver hasn't been used in ASoC, no?