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

[Takashi Iwai <tiwai@suse.de>]

On Wed, 12 Apr 2023 21:59:28 +0200,
Jaroslav Kysela wrote:
> 
> On 12. 04. 23 12:37, Takashi Iwai wrote:
> > On Wed, 12 Apr 2023 10:04:31 +0200,
> > Oswald Buddenhagen wrote:
> >> 
> >> On Wed, Apr 12, 2023 at 09:54:54AM +0200, Takashi Iwai wrote:
> >>> I'm thinking whether we need to change anything in the kernel side for
> >>> this at all.  Can't it be changed rather in alsa-lib side instead?
> >>> 
> >> it could, but it would be a lot uglier. user space would have to do a
> >> "man-in-the-middle attack" on the data, while in the kernel we can
> >> just slightly modify the consumer. this would be particularly obvious
> >> in the case of write() access.
> > 
> > But basically it'd be like fiddling sw_params temporarily for
> > draining, I suppose?  And the "attack" here can be taken too
> > seriously; the whole PCM operation can be somehow interfered if a
> > process may have the access to the PCM device, and changing sw_params
> > itself must not introduce too much trouble.
> 
> 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.  Also, we may skip the
workaround for applications accessing directly via mmap as default.
Also we may provide a flag in asoundrc config, if any, too.
The implementation in alsa-lib is more flexible in this regard.

> The other not-yet-discussed option is to just print an warning in
> alsa-lib that the residue samples may be played (when no silencing /
> period size align is used). Then introduce a new helper function to
> setup silencing for the drivers without new
> SNDRV_PCM_INFO_PERFECT_DRAIN flag set.

Hm, I don't think this would be useful.  Spewing warnings are rather
annoying or confusing for end users, and I bet the old applications
wouldn't be fixed even with such annoyance.


thanks,

Takashi