[alsa-devel] Driver code with mpc5200 pointer problem.

Jaroslav Kysela perex at perex.cz
Tue Apr 28 13:30:09 CEST 2009

On Tue, 28 Apr 2009, Takashi Iwai wrote:

> At Tue, 28 Apr 2009 12:15:25 +0200 (CEST),
> Jaroslav Kysela wrote:
>> On Tue, 28 Apr 2009, Takashi Iwai wrote:
>>> Well, I think it'd be better, at this moment for 2.6.30, to allow
>>> problematic drivers to skip the jiffies check.  Adding the fundamental
>>> change at this late stage is bad, especially if the change wasn't
>>> tested much by many others and has an influence on user-side API.
>>> So, as I proposed, we can simply add the pcm info check, and add
>>> BATCH flag to needed drivers.  Of course, it still requires some
>>> more testing, but basically it'll take back to the old behavior and
>>> should be safer.
>>> Meanwhile, applying the delay account patch now for 2.6.31 should be
>>> good (ealier is better).  It doesn't conflict with the info flag
>>> check, so we can work parallel.
>> Could we just add the runtime->delay variable without touching other code
>> (pcm status), to correct the jiffies based check now?
> That's possible, of course.
>> It seems like a good
>> step forward to me and the lowlevel drivers will be more prepared for next
>> PCM midlevel code changes.
> Well, but I guess providing the proper FIFO size isn't trivial to each
> driver.  There are many devices, as Mark suggested, which may be
> broken right now, and we can't determine all h/w specs and test them.
> OTOH, adding INFO_BATCH is fairly easy, because it can be found from
> the pointer callback implementation of each driver.  On these devices,
> the jiffies check doesn't help much anyway because it cannot update
> the hwptr any other than the period size.  So, skipping jiffies check
> is logically correct as a regression fix.

I don't see much difference between adding INFO_BATCH and adding 
runtime->delay = (runtime->period_size)-1 to avoid jiffies check for one 
period to these drivers (with some notice to author of the driver to 
correct this value).

INFO_BATCH is currently in:

/usb/usbaudio.c (which works with the current jiffies check at least for
 		 44100+ rates)

Another (much cleaner) possibility is to make jiffies check conditional to 
xrun_check() for 2.6.30 and do not bother with the INFO_BATCH flag.

Jiffies check helped me to debug HDA/intel8x0 pointer problems but cannot 
be triggered until driver is really buggy - the HDA/intel8x0 driver should 
be fixed now (I know about one intel8x0 pointer problem to this time which 
triggers this check, but there is still something wrong with returned 
hardware pointer because audio clicks anyway - no regression from previous 


Jaroslav Kysela <perex at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.

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