Re: linux-6.4 alsa sound broken

[Takashi Iwai <tiwai@suse.de>]

On Wed, 03 May 2023 22:00:37 +0200,
Oswald Buddenhagen wrote:
> 
> On Wed, May 03, 2023 at 09:32:02PM +0200, Jaroslav Kysela wrote:
> > On 03. 05. 23 18:10, Takashi Iwai wrote:
> >> The dmix uses the silence_size=boundary as a fill-all operation, and
> >> it's a free-wheel mode, so supposedly something was overlooked in your
> >> code refactoring.
> >> 
> >> Could you check it and address quickly?  I'd like to fix it before
> >> 6.4-rc1 release, so if no fix comes up in a couple of days, I'll have
> >> to revert the change for 6.4-rc1.
> > 
> > I would revert this patch.
> 
> > It seems that this "do silence right after the playback is finished"
> > mechanism is not handled in the updated code (and I overlooked that,
> > too):
> > 
> no, there is nothing wrong with the code _per se_.
>
> what's happening is that the dmix plugin doesn't update the
> application pointer, and somehow gets away with it.
> 
> that means that it would have never worked with thresholded silencing
> mode, either, but, well, it uses top-up mode.

Well, the code made just a wrong interpretation for the behavior with
silence_size == boundary.  This mode is actually a kind of tailored
operation for dmix.

In the description of alsa-lib snd_pcm_sw_params_set_silence_size(),
you can find it:

/**
 * \brief Set silence size inside a software configuration container
 * \param pcm PCM handle
 * \param params Software configuration container
 * \param val Silence size in frames (0 for disabled)
 * \return 0 otherwise a negative error code
 *
 * A portion of playback buffer is overwritten with silence when playback
 * underrun is nearer than silence threshold (see 
 * #snd_pcm_sw_params_set_silence_threshold)
 *
 * The special case is when silence size value is equal or greater than
 * boundary. The unused portion of the ring buffer (initial written samples
 * are untouched) is filled with silence at start. Later, only just processed
 * sample area is filled with silence. Note: silence_threshold must be set to zero.
 */

So, the "top-up" silencing happens only at start, but not after that.
In the code path of hw_ptr update, it doesn't check the appl_ptr any
longer, but fills the processed area by the hw_ptr update with
silence.  That's the intended behavior for use cases of free-wheel
mode without appl_ptr updates like dmix.


Takashi