[alsa-devel] hw_ptr_interrupt removal broke interrupt pointer updates

Jaroslav Kysela perex at perex.cz
Wed Jan 27 08:06:29 CET 2010

On Wed, 27 Jan 2010, Raymond Yau wrote:

> 2010/1/27 Jaroslav Kysela <perex at perex.cz>
>> On Tue, 26 Jan 2010, Clemens Ladisch wrote:
>>> Commit "cleanup & merge hw_ptr update functions" says:
>>>> The main change is hw_ptr_interrupt variable removal to simplify code
>>>> logic. This variable can be computed directly from hw_ptr.
>>> The hw_ptr_interrupt variable was needed to differentiate between the
>>> position at the last normal pointer update and the position of the last
>>> signaled period boundary.
>>>       if (in_interrupt) {
>>>               /* we know that one period was processed */
>>>               /* delta = "expected next hw_ptr" for in_interrupt != 0 */
>>>               delta = old_hw_ptr - (old_hw_ptr % runtime->period_size)
>>>                       + runtime->period_size;
>>>               if (delta > new_hw_ptr) {
>>>                       hw_base += runtime->buffer_size;
>>> It is possible for the status/delay ioctls to be called when the sound
>>> card's pointer register alreay shows a position at the beginning of the
>>> new period, but immediately before the interrupt is actually executed.
>>> (This happens regularly on a SMP machine with mplayer.)  When that
>>> happens, the code thinks that the position must be at least one period
>>> ahead of the current position and drops an entire buffer of data.
>> Clements, thank you for nice explanation how I was wrong. I returned
>> hw_ptr_interrupt variable back. I am testing this patch now:
>> http://git.alsa-project.org/?p=alsa-kernel.git;a=commitdiff;h=04d64a69fcb9fd182d73d6f1a8de55b2f527a1de
>> A review is always welcome. Thanks.
>>                                                Jaroslav
> do snd_pcm_period_elapsed really handle the case when more than one period
> are elasped ?
> For au88x0 , each substream have four sets of hardware registers ,  it seem
> that the driver can recover lost interrupt with no underrun when using very
> small period size
> http://www.alsa-project.org/~tiwai/writing-an-alsa-driver/ch05s07.html#pcm-interface-interrupt-handler-boundary
> On calling snd_pcm_period_elapsed()
> In both cases, even if more than one period are elapsed, you don't have to
> call snd_pcm_period_elapsed() many times. Call only once. And the pcm layer
> will check the current hardware pointer and update to the latest status.

Yes, the new code handles more alapsed periods, too. The in_interrupt 
check is mainly for situations when period count == 1 and it compares 
expected next hw_ptr for interrupt with hw_ptr computed from the hw_base 
and actual position in the ring buffer.


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

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