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
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