[alsa-devel] [PATCH] ALSA: add support for disabling period irq

Takashi Iwai tiwai at suse.de
Tue Nov 2 09:41:58 CET 2010

At Tue, 2 Nov 2010 09:26:48 +0100 (CET),
Jaroslav Kysela wrote:
> On Tue, 2 Nov 2010, Takashi Iwai wrote:
> > At Tue, 2 Nov 2010 09:17:33 +0100 (CET),
> > Jaroslav Kysela wrote:
> >>
> >> On Tue, 2 Nov 2010, Takashi Iwai wrote:
> >>
> >>> At Mon,  1 Nov 2010 17:12:53 -0500,
> >>> Pierre-Louis Bossart wrote:
> >>>>
> >>>> Merged and cleaned patch based on earlier patches posted
> >>>> on alsa-devel by Clemens Ladisch <clemens at ladisch.de> and
> >>>> Pierre-Louis Bossart <pierre-louis.bossart at intel.com>
> >>>>
> >>>> This patch disables period interrupts which are not
> >>>> needed when the application relies on a system timer
> >>>> to wake-up and refill the ring buffer. The behavior of
> >>>> the driver is left unchanged, and interrupts are only
> >>>> disabled if the application requests this configuration.
> >>>> The behavior in case of underruns is slightly different,
> >>>> instead of being detected during the period interrupts the
> >>>> underruns are detected when the application calls
> >>>> snd_pcm_update_avail, which in turns forces a refresh of the
> >>>> hw pointer and shows the buffer is empty.
> >>>
> >>> So, this silently assumes that the applications do call
> >>> snd_pcm_update_avail() appropriately at the right timing?
> >>> If so, any sense to check XRUN in the driver at all...?
> >>>
> >>> And, even more, any sense to report the incremental position by this
> >>> approach?  The only reliable information in this case is the offset in
> >>> the ring buffer.  The linear position as the current ALSA API provides
> >>> isn't guaranteed without the period irq.
> >>
> >> We can detect the buffer size crossing using jiffies, but I agree, it's
> >> something which should be added to the patch to not break hw_ptr in
> >> case when the application does not call the update function in time.
> >> We have both values in runtime->hw_ptr_jiffies and
> >> runtime->hw_ptr_buffer_jiffies.
> >
> > But I thought this patch also disabled the jiffies check?
> These values are not related only to the debug jiffies check. I'm using 
> these values also to detect the double irq acks which were discovered
> recently.

Ah, OK, I overlooked it.

> > I feel that some bottom-line check is needed if we keep the linear
> > position over the buffer size even without the period irq.
> > If the app doesn't care, OK fine, the driver shouldn't care, too.
> > But then it doesn't make sense to keep the linear position, either.
> But if we can keep the linear position using the light-weight jiffies 
> check, it's probably OK to keep it rather than changing the hw_ptr 
> behaviour.

Well, but the point of the patch is to avoid wakeups as much as
possible.  The jiffies-check adds an unconditional wakeup for each
buffer size, thus it's against the purpose of the patch.

So, basically we need to decide whether
 - to check via jiffies as a bottom-line; then no big merit over period=1
 - let the driver not to care anything, apps are responsible for any
   wrong values; in this case, any sense to keep the linear position?

Maybe keeping the linear position would make sense only for the code
compatibility reason.  Many codes are written based on the linear
position behavior.  But then I feel somehow uneasy around this...



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