Re: [alsa-devel] [PATCH 7/8] ALSA: pcm: Add card sync_irq field

["Jie, Yang" <yang.jie@intel.com>]

>-----Original Message-----
>From: Takashi Iwai <tiwai@suse.de>
>Sent: Thursday, November 21, 2019 1:13 PM
>To: Sridharan, Ranjani <ranjani.sridharan@intel.com>
>Cc: Jie, Yang <yang.jie@intel.com>; Ranjani Sridharan
><ranjani.sridharan@linux.intel.com>; Linux-ALSA <alsa-devel@alsa-project.org>;
>Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
>Subject: Re: [alsa-devel] [PATCH 7/8] ALSA: pcm: Add card sync_irq field
>
>On Thu, 21 Nov 2019 21:46:17 +0100,
>Sridharan, Ranjani wrote:
>>
>>     >
>>     > Hi Takashi,
>>     >
>>     > Sorry the stress tests took a while.
>>     > As we discussed earlier, adding the sync_stop() op didnt quite help the
>>     SOF
>>     > driver in removing the delayed work for
>> snd_pcm_period_elapsed().
>>
>>     Yeah, that's understandable.  If the stop operation itself needs some
>>     serialization, sync_stop() won't influence at all.
>>
>>     However, now after these discussions, I have some concerns in the
>>     current code:
>>
>>     - The async work started by schedule_work() may be executed
>>       (literally) immediately.  So if the timing or the serialization
>>       matters, it doesn't guarantee at all.  The same level of concurrency
>>       can happen at any time.
>>
>>     - The period_elapsed work might be pending at prepare or other
>>       operation;
>>       the async work means also that it doesn't guarantee its execution in
>>       time, and it might be delayed much, and the PCM core might go to
>>       prepare or other state even before the work is executed.
>>
>>     The second point can be fixed easily now with sync_stop.  You can just
>>     put flush_work() in sync_stop in addition to synchronize_irq().
>>
>>     But the first point is still unclear.  More exactly, which operation
>>     does it conflict?  Does it the playback drain?  Then it might take
>>     very long (up to seconds) to block the next operation?
>>
>> Hi Takashi,
>>
>> As I understand the original intention for adding the
>> period_elapsed_work() was  that snd_pcm_period_elapsed() could cause a
>> STOP trigger while the current IPC interrupt is still being handled.
>> In this case, the STOP trigger generates an IPC to the DSP but the
>> host never misses the IPC response from the DSP because it is still
>> handling the previous interrupt.
>
>OK, that makes sense.  So the issue is that the trigger stop itself requires the ack
>via the interrupt and it can't be caught because it's being called from the irq
>handler itself.
>
>In that case, though, another solution would be to make the trigger- stop an
>async work (but conditionally) while processing the normal period_elapsed in the
>irq handler.  That is, set some flag before calling snd_pcm_period_elapsed(), and
>in the trigger-stop, check the flag.  If the flag is set, schedule the work and return.
>And, you'll sync this async work with sync_stop().  In that way, the period handling
>is processed without any delay more lightly.

Yes, this was actually the method @Uimonen, Jaska suggested on April, since we have
sync_stop() to flush the potential un-finished async trigger_stop task now, it's time
to switch to use this solution now.

Thanks,
~Keyon

>
>
>thanks,
>
>Takashi
>
>> Adding Keyon who added this change to add more and clarify your concerns.
>>
>> Thanks,
>> Ranjani
>>
>>     thanks,
>>
>>     Takashi
>>
>>
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