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

Jaroslav Kysela perex at perex.cz
Tue Nov 2 10:02:49 CET 2010

On Tue, 2 Nov 2010, Takashi Iwai wrote:

> 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.

I don't see any wakeup. It's just about to compare the current jiffies 
with last hw_ptr update jiffies. The multiple buffer size crossing can be 

> So, basically we need to decide whether
> - to check via jiffies as a bottom-line; then no big merit over period=1

No, the difference is that no interrupts are really required.


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

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