Re: [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@xxxxxxxx>

> thanks,
>
> Takashi


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