Re: [PATCH 2/2] ALSA: pcm: auto-fill buffer with silence when draining playback

[Takashi Iwai <tiwai@suse.de>]

On Thu, 13 Apr 2023 12:16:04 +0200,
Oswald Buddenhagen wrote:
> 
> On Thu, Apr 13, 2023 at 07:42:06AM +0200, Takashi Iwai wrote:
> > On Wed, 12 Apr 2023 21:59:28 +0200,
> > Jaroslav Kysela wrote:
> >> This looks like a sane proposal, but some drivers does not require
> >> the
> >> silencing at all, so we can probably skip this step for them (new
> >> SNDRV_PCM_INFO_PERFECT_DRAIN flag?).
> > 
> > Sure, we should apply it only conditionally.
> > 
> i don't think the extra complexity is justified. with my improved
> proposal, we're talking about a cost of filling two periods per
> draining op, which aren't exactly super-frequent.

I'm not much fan of introducing a new flag for that behavior, either.

> > Also, we may skip the
> > workaround for applications accessing directly via mmap as default.
> > 
> no, because one may easily miss that more than one period is required.
> also, i think that forgetting it entirely is an easy mistake to make,
> even if it's harder with mmap than with write.

I don't agree with that point -- if application wants the access only
via mmap (without any write actions via alsa-lib functions), the
buffer and data management relies fully on the application itself.
Manipulating the data *silently* is no good action for such
applications.  For them, the drain simply means to stop at the certain
point.  OTOH, for the explicit write, basically alsa-lib / kernel is
responsible for the buffer / data management, and the workaround can
be applied.

That's I mentioned to "apply conditionally".  There are cases where we
shouldn't touch the data at all.  For the usual cases with the
standard write, we may apply it.


thanks,

Takashi