Re: [alsa-devel] [PATCH] ALSA: pcm: fix buffer_bytes max constrained by preallocated bytes issue

[Takashi Iwai <tiwai@suse.de>]

On Thu, 16 Jan 2020 16:31:02 +0100,
Jie, Yang wrote:
> 
> > -----Original Message-----
> > From: Jie, Yang
> > Sent: Thursday, January 16, 2020 10:14 PM
> > To: 'Takashi Iwai' <tiwai@suse.de>; Keyon Jie <yang.jie@linux.intel.com>
> > Cc: alsa-devel@alsa-project.org
> > Subject: RE: [alsa-devel] [PATCH] ALSA: pcm: fix buffer_bytes max
> > constrained by preallocated bytes issue
> > 
> > > -----Original Message-----
> > > From: Alsa-devel <alsa-devel-bounces@alsa-project.org> On Behalf Of
> > > Takashi Iwai
> > > Sent: Thursday, January 16, 2020 7:51 PM
> > > To: Keyon Jie <yang.jie@linux.intel.com>
> > > Cc: alsa-devel@alsa-project.org
> > > Subject: Re: [alsa-devel] [PATCH] ALSA: pcm: fix buffer_bytes max
> > > constrained by preallocated bytes issue
> > >
> > > On Thu, 16 Jan 2020 12:25:38 +0100,
> > > Keyon Jie wrote:
> > > >
> > > > On Thu, 2020-01-16 at 11:27 +0100, Takashi Iwai wrote:
> > > > > On Thu, 16 Jan 2020 10:50:33 +0100,
> > > > >
> > > > > Oh, you're right, and I completely misread the patch.
> > > > >
> > > > > Now I took a coffee and can tell you the story behind the scene.
> > > > >
> > > > > I believe the current code is intentionally limiting the size to
> > > > > the preallocated size.  This limitation was brought for not trying
> > > > > to allocate a larger buffer when the buffer has been preallocated.
> > > > > In the past, most hardware allocated the continuous pages for a
> > > > > buffer and the allocation of a large buffer fails quite likely.
> > > > > This was the reason of the buffer preallocation.  So, the driver
> > > > > wanted to tell the user-space the limit.  If user needs to have an
> > > > > extra large buffer, they are supposed to fiddle with prealloc
> > > > > procfs (either setting zero to clear the preallocation or setting
> > > > > a large enough buffer beforehand).
> > > >
> > > > Thank you for the sharing, it is interesting and knowledge learned
> > > > to me.
> > > >
> > > > >
> > > > > For SG-buffers, though, limitation makes less sense than
> > > > > continuous pages.  e.g. a patch below removes the limitation for SG-
> > buffers.
> > > > > But changing this would definitely cause the behavior difference,
> > > > > and I don't know whether it's a reasonable move -- I'm afraid that
> > > > > apps would start hogging too much memory if the limitation is gone.
> > > >
> > > > I just went through all invoking to
> > > > snd_pcm_lib_preallocate_pages*(), for those SNDRV_DMA_TYPE_DEV,
> > some
> > > > of them set the *size* equal to
> > > the
> > > > *max*, some set the *max* several times to the *size*, IMHO, the
> > > > *max*s are matched to those hardware's limiatation, comparing to the
> > > > *size*s, aren't they?
> > > >
> > > > In this case, I still think my patch hanle all
> > > > TYPE_DEV/SNDRV_DMA_TYPE_DEV/TYPE_SG/SNDRV_DMA_TYPE_DEV
> > > cases more
> > > > gracefully, we will still take the limitation from the specific
> > > > driver set, from the *max* param, and the test results looks very
> > > > nice here, we will take what the user space wanted for buffer-bytes
> > > > via aply exactly, as long as it is suitable for the interval and constraints.
> > >
> > > Well, I have a mixed feeling.  Certainly we'd need some better way to
> > > allow a larger buffer allocation, especially for HDA.  OTOH, if the
> > > buffer was preallocated, it's meant to be used actually.  That's the
> > > point of the hw_constraint setup.
> > 
> > So if the buffer was preallocated, it won't be re-allocated at hw_params()
> > stage, is this conflict with the re-allocate logic in hw_params()?
> > 
> > >
> > > And now thinking again after another cup of coffee, I wonder why we do
> > > preallocate for HDA at all.  For HD-audio, the allocation of any large
> > > buffer would succeed very likely because of SG-buffer.
> > >
> > > So, just setting 0 to the preallocation size (but keeping else) would work,
> > e.g.
> > > something like below?  The help text needs adjustment, but you can see
> > > the rough idea.
> > 
> > So, do you suggest not doing preallocation(or calling it with 0 size) for all
> > driver with TYPE_SG? I am fine if this is the recommended method, I can try
> > this on SOF I2S platform to see if it can work as we required for very large
> > buffer size.
> 
> Tried and found setting 0 size for preallocation doesn't work for me, I have
> even tried to setting the size as big as the max(which the user space may
>  require for buffer-bytes), it still doesn't work for me.

How did you test it?  I quickly checked now on my machine, and it
seems working...

# echo 1024 > /proc/asound/card0/pcm0p/sub0/prealloc
# aplay -Dhw:0 -v --buffer-size=1048576 foo.wav
Hardware PCM card 0 'HDA Intel PCH' device 0 subdevice 0
Its setup is:
  stream       : PLAYBACK
  ....
  buffer_size  : 262144

# echo 0 > /proc/asound/card0/pcm0p/sub0/prealloc
# aplay -Dhw:0 -v --buffer-size=1048576 foo.wav
Hardware PCM card 0 'HDA Intel PCH' device 0 subdevice 0
Its setup is:
  stream       : PLAYBACK
  ....
  buffer_size  : 1048576


Takashi
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