[PATCH] ALSA: pcm: fix incorrect hw_base increase

[Jaroslav Kysela]

Dne 15. 05. 20 v 12:39 Takashi Iwai napsal(a):
> On Fri, 15 May 2020 11:30:54 +0200,
> Jaroslav Kysela wrote:
>>
>> Dne 15. 05. 20 v 11:04 Lu, Brent napsal(a):
>>>>
>>>> Is this a bugfix needed for older kernels as well?  When did this issue show
>>>> up?
>>>>
>>>> thanks,
>>>>
>>>> greg k-h
>>>
>>> It happens when DMA stop moving data from host to DSP/DAI for a long time
>>> (> half of buffer time). I know host driver should do something about it. But if
>>> not, the HWSYNC will keep increasing the hw_base and hw_ptr and confuses
>>> user space program.
>>
>> I'm afraid, but with this code, you turn off the hw_ptr jiffies
>> code. It would be better to fix the driver in this case (return the
>> updated / estimated DMA pointer, increase DMA buffer size etc.). This
>> "lag" is unacceptable.
> 
> The problem is obviously in the driver's side and it's best to be
> addressed there.  But, I think it's still worth to apply this change.
> 
> The hw_ptr jiffies check is performed basically in two places: one is
> snd_pcm_period_elapsed() call from ISR, and another is with the
> no_period_wakeup flag.  In both cases, it calculates the diff of
> jiffies from the previous update, and corrects the hw_ptr_base if that
> exceeds the threshold.
> 
> And the bug here is that the "previous" jiffies is kept as long as the
> hwptr itself is updated.  What we need is the correction of the base
> when it really has processed the period size; i.e. hwptr got the same
> value (with no_period_wakeup) and yet the jiffies diff is big.  For
> this check, it's correct to update hw_ptr_jiffies at each call no
> matter whether hwptr moved or not; we need to evaluate from the
> previous update, after all.
> 
> But I might overlook something.  Jaroslav, could you check it again?
> The jiffies check code is your black magic :)

I tried to imagine a negative impact for this hw_ptr_jiffies update when the 
DMA position is not updated from the driver and I haven't found any so far.

Let's apply this and we'll see in future :-)

And yes, the patch description should be improved (DMA ptr is not updated / 
streaming is inactive).

Reviewed-by: Jaroslav Kysela <perex at perex.cz>

> 
> 
> thanks,
> 
> Takashi
> 


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