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

[Jie, Yang]

> -----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.

Thanks,
~Keyon

> 
> 
> thanks,
> 
> Takashi
> 
> --- a/sound/hda/Kconfig
> +++ b/sound/hda/Kconfig
> @@ -21,9 +21,10 @@ config SND_HDA_EXT_CORE
>         select SND_HDA_CORE
> 
>  config SND_HDA_PREALLOC_SIZE
> -	int "Pre-allocated buffer size for HD-audio driver"
> +	int "Pre-allocated buffer size for HD-audio driver"
> if !SND_DMA_SGBUF
>  	range 0 32768
> -	default 64
> +	default 64 if !SND_DMA_SGBUF
> +	default 0 if SND_DMA_SGBUF
>  	help
>  	  Specifies the default pre-allocated buffer-size in kB for the
>  	  HD-audio driver.  A larger buffer (e.g. 2048) is preferred
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