[alsa-devel] [ALSA patch] [PATCH - 1/1] pcm: direct: Round down of slave_hw_ptr when buffer size is two period size

[Takashi Iwai]

On Thu, 06 Feb 2020 09:44:58 +0100,
Takashi Iwai wrote:
> 
> On Thu, 06 Feb 2020 09:14:42 +0100,
> Channaiah Vanitha (RBEI/ECF3) wrote:
> > 
> > Hello Takashi-san,
> > 
> > This patch is regarding the fix for rounding down/up of slave pointers.
> > For buffer_size >= 2*period_size, round down of slave pointers and
> > for buffer_size < 2*period_size, round up of slave pointers will avoid xruns.
> > which otherwise causes snd_pcm_wait() to block for more than expected snd_pcm_period_elapsed() which leads to xruns.
> > 
> > We had similar discussion for same issue in below link:
> > https://mailman.alsa-project.org/pipermail/alsa-devel/2019-June/151169.html
> 
> Yes, and I still don't get why this must be required.
> Doesn't this imply that you drop the samples instead?

Actually, in such an extreme situation, there is no perfect solution
with dmix.  That's the reason we introduced the explicit
hw_ptr_alignment option to specify the behavior.  The default value is
to keep the current behavior for compatibility; i.e. unless you are
100% sure that this change won't break any existing usage, there is no
big reason to change the behavior, too.


thanks,

Takashi

> 
> 
> Takashi
> 
> > 
> > Regards,
> > Vanitha
> > 
> > 
> > -----Original Message-----
> > From: Channaiah Vanitha (RBEI/ECF3) <Vanitha.Channaiah at in.bosch.com> 
> > Sent: Thursday, February 6, 2020 1:43 PM
> > To: Channaiah Vanitha (RBEI/ECF3) <Vanitha.Channaiah at in.bosch.com>; patch at alsa-project.org
> > Cc: alsa-devel at alsa-project.org; Wischer Timo (ADITG/ESS) <twischer at de.adit-jv.com>
> > Subject: [PATCH - 1/1] pcm: direct: Round down of slave_hw_ptr when buffer size is two period size
> > 
> > From: Vanitha Channaiah <vanitha.channaiah at in.bosch.com>
> > 
> > For buffer size equal to two period size, the start position of slave_hw_ptr is rounded down in order to avoid xruns
> > 
> > For e.g.:
> > Considering below parameters and its values:
> > Period size = 96 (0x60)
> > Buffer size = 192 (0xC0)
> > Timer ticks = 1
> > avail_min = 0x60
> > slave_hw_ptr = unaligned value during dmix_start()
> > 
> > Issue:
> > - Initial, app_ptr = hw_ptr = 0
> > - Application fills buffer size of data. so app_ptr = 0xC0, hw_ptr = 0
> > - During dmix_start(), current slave_hw_ptr is not rounded down. current slave_hw_ptr would be 0x66
> > - slave_hw_ptr is keep on updating at the hardware 0x67, 0x68, 0x69
> > - The diff calculation between old_slave_hw_ptr(0x66) and new_slave_hw_ptr(0x69)
> >   results in avail = 0x6
> > - After 1 snd_pcm_period_elapsed(), slave_hw_ptr = 0xC0
> > - The result of avail = 0x5A which is less than avail_min(0x60)
> > - Here, xruns will be observed
> > 
> > Fix:
> > - Initial, app_ptr = hw_ptr = 0
> > - Application fills buffer size of data. so app_ptr = 0xC0, hw_ptr = 0
> > - Round down of slave_hw_ptr during dmix_start() leads to below calculation:
> > - During dmix_start(), slave_hw_ptr rounded to 0x60 (old slave_hw_ptr)
> > - The diff calculation between old_slave_hw_ptr(0x60) and new_slave_hw_ptr(0x69)
> >   results in avail = 0x9
> > - After 1 snd_pcm_period_elapsed(), slave_hw_ptr = 0xC0
> > - The result of avail = 0x60 which is avail_min(0x60)
> > - Here, xruns can be avoided
> > 
> > Signed-off-by: Vanitha Channaiah <vanitha.channaiah at in.bosch.com>
> > 
> > diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c index 54d9900..a201fa3 100644
> > --- a/src/pcm/pcm_direct.c
> > +++ b/src/pcm/pcm_direct.c
> > @@ -2043,10 +2043,14 @@ int snd_pcm_direct_parse_open_conf(snd_config_t *root, snd_config_t *conf,
> >  
> >  void snd_pcm_direct_reset_slave_ptr(snd_pcm_t *pcm, snd_pcm_direct_t *dmix)  {
> > +    /*
> > +     * For buffer size equal to two period size, slave_hw_ptr is rounded down
> > +     * to avoid xruns
> > +     */
> >  
> >  	if (dmix->hw_ptr_alignment == SND_PCM_HW_PTR_ALIGNMENT_ROUNDUP ||
> >  		(dmix->hw_ptr_alignment == SND_PCM_HW_PTR_ALIGNMENT_AUTO &&
> > -		pcm->buffer_size <= pcm->period_size * 2))
> > +		pcm->buffer_size < pcm->period_size * 2))
> >  		dmix->slave_appl_ptr =
> >  			((dmix->slave_appl_ptr + dmix->slave_period_size - 1) /
> >  			dmix->slave_period_size) * dmix->slave_period_size;
> > --
> > 2.7.4
> > 
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> > 
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