Re: [PATCH alsa-lib 1/4] pcm: hw: setup explicit silencing for snd_pcm_drain by default

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

On Wed, May 03, 2023 at 01:20:37PM +0200, Oswald Buddenhagen wrote:
>On Tue, May 02, 2023 at 01:50:07PM +0200, Jaroslav Kysela wrote:
>>+		 * or the next period wake up)
>>+		 */
>>+		sw_params.silence_threshold = pcm->buffer_size;
>>+		sw_params.silence_size = silence_size;
>>
>so at this point i got the thought "huh, that can exceed the buffer 
>size. is that ok?" ...
>and yes, it is. but ...
>
>the kernel doesn't check silence_threshold, so user space can trigger 
>the snd_BUG_ON() in snd_pcm_playback_silence(). whoops.
>(yes, this predates my patch.)
>i'm not sure what the best way to deal with this is. anyway, different 
>tree, different patch.

actually, that analysis is garbage, because i didn't look at enough 
context. :}

the kernel _does_ check the values in snd_pcm_sw_params(), which means 
that silence_size exceeding silence_threshold would lead to EINVAL, and 
therefore silencing being broken. this will inevitably happen with small 
buffer sizes, where the 1/10th sec extension dominates.

as snd_pcm_sw_params() checks the parameters (and snd_pcm_hw_params() 
resets the sw params to defaults, so inverse calling order cannot bypass 
it), the concern about the crash is invalid. phew.

regards