[alsa-devel] [PATCH]: No sound when using software based mmap support and dmix plugin

[Takashi Iwai]

At Tue, 19 May 2015 11:14:13 -0700,
Yogesh Soni wrote:
> 
> Hi Takashi,
> 
> Thanks for your reply. Please consider the following snippet from
> pcm-indirect.h.  When using other plugins (like softvol), appl_ptr was
> updated correctly and below code worked well. In case of dmix, The samples
> are never copied to substream because
>    appl_ptr = rec->appl_ptr = diff = 0  --> rec->sw_ready = 0 .
> 
> *snd_pcm_sframes_t diff = appl_ptr - rec->appl_ptr;*
> .....
> if (*diff*) {
> .......
> rec->sw_ready += (int)frames_to_bytes(runtime, diff);
> .......
> }
>  while (rec->hw_ready < qsize &&* rec->sw_ready > 0*) {
>  *copy(substream, rec, bytes);*
>          }

I see.  But, the free wheeling mode doesn't mean that appl_ptr is
always invalid.  It means nothing but we don't do any xrun checks and
keeps the stream running.

So, instead of checking only the stop_threshold, how about to check if
it's in xrun, and then use it as diff?


thanks,

Takashi


> 
> Regards
> Yogesh
> 
> On Tue, May 19, 2015 at 4:47 AM, Takashi Iwai <tiwai at suse.de> wrote:
> 
> > At Wed, 13 May 2015 17:12:33 -0700,
> > Yogesh Soni wrote:
> > >
> > > Hi all,
> > >
> > > I am trying to have dmix plugin work ( I succeeded) with the audio card
> > > that does not have hardware mmap support and uses pcm_indirect method to
> > > map software buffer for mmap feature.
> > >
> > > Most of the plugins were working quite well with mmap/pcm_indirect combo.
> > > However, dmix plugin did not work (no sound). I debugged and realized
> > that
> > > appl_ptr in pcm_direct.h(snd_pcm_indirect_playback_transfer()) was
> > always 0
> > > and hence nothing got copied to slave pcm buffer. I then discovered that
> > > dmix operates in freewheeling-mode..
> > >
> > > I have made a small workaround patch (I believe it can be cleaned up) in
> > > case of free-wheeling mode, basically using sw_ready to hold bytes that
> > > need to be copied. I used rec->sw_ready because it looked safest to use
> > for
> > > this purpose. As a result of this patch, normally period_size bytes are
> > > copied to pcm buffer every period and sound is playing smooth.
> > >
> > > I am a noob in alsa and linux in general. So, I would like your opinion
> > on
> > > the patch. I would much prefer if I could write a patch that is generic
> > and
> > > clean enough, so that it could be used by others. patch is attached.
> >
> > Thanks for reporting and the patch.
> >
> > So, what actually prevents working with the current code in the free
> > wheeling mode?
> >
> >
> > Takashi
> >
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