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

[Takashi Iwai]

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.

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.


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