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

[Jie, Yang]

> -----Original Message-----
> From: Jie, Yang
> Sent: Thursday, January 16, 2020 10:14 PM
> To: 'Takashi Iwai' <tiwai at suse.de>; Keyon Jie <yang.jie at linux.intel.com>
> Cc: alsa-devel at 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 at alsa-project.org> On Behalf Of
> > Takashi Iwai
> > Sent: Thursday, January 16, 2020 7:51 PM
> > To: Keyon Jie <yang.jie at linux.intel.com>
> > Cc: alsa-devel at 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.

Thanks,
~Keyon

> 
> Thanks,
> ~Keyon
>