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

[Oswald Buddenhagen <oswald.buddenhagen@gmx.de>]

On Sat, Apr 08, 2023 at 01:58:21AM +0200, Jaroslav Kysela wrote:
>On 05. 04. 23 22:12, Oswald Buddenhagen wrote:
>> Draining will always playback somewhat beyond the end of the filled
>> buffer. This would produce artifacts if the user did not set up the
>> auto-silencing machinery. This patch makes it work out of the box.
>> 
>I think that it was really bad decision to apply this patch without a 
>broader discussion.

>When we designed the API, we knew about described problems and we 
>decided to keep this up to applications.
>
i ran into no documentation of either the problems nor the decisions and 
their implications for the user.

>The silencing may not help in all cases where the PCM samples ends with 
>a high volume.
>
that would just create a slight crack, which isn't any different from a 
"regular" sudden stop.

> A volume ramping should be used and it's an application job.
>
imo, that entirely misses the point - the volume is most likely already 
zero at the end of the buffer. that doesn't mean that it's ok to play 
the samples again where the volume might not be *quite* zero yet.

>Also, silencing touches the DMA buffer which may not be desired.
>
hypothetically, yes. but practically? why would anyone want to play the 
same samples after draining? draining is most likely followed by closing 
the device. and even if not, in most cases (esp. where draining would 
actually make sense) one wouldn't play a fixed pattern that could be 
just re-used, so one would have to re-fill the buffer prior to starting 
again anyway. never mind the effort necessary to track the state of the 
buffer instead of just re-filling it. so for all practical purposes, 
already played samples can be considered undefined data and thus safe to 
overwrite.

>And lastly drivers can handle draining correctly (stop at the exact 
>position - see substream->ops->trigger with SNDRV_PCM_TRIGGER_DRAIN 
>argument).
>
yeah. hypothetically. afaict, there is exactly one driver which supports 
this. most (older) hardware wouldn't even have the capability to do such 
precise timing without external help.

On Sat, Apr 08, 2023 at 07:55:48AM +0200, Takashi Iwai wrote:
>Applying the silencing blindly might be an overkill, indeed, although
>this could be seen as an easy solution.  Let's see.
>
i don't think that "overkill" is right here. someone has to do the 
silencing for draining to be useful at all, and so the question is only 
who that should be. my argument is that not auto-silencing is 
*extremely* unexpected, and thus just bad api. i'm pretty certain that 
about 99% of the usages of DRAIN start out missing this, and most never 
get fixed.

imo, if any api is added, it should be to opt *out* of auto-silencing.  
but i don't think this makes any sense; there would be ~zero users of 
this ever.

regards