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

[Takashi Iwai]

On Sun, 19 Jan 2020 04:52:55 +0100,
Keyon Jie wrote:
> 
> 
> On 2020/1/17 下午7:12, Takashi Iwai wrote:
> > On Fri, 17 Jan 2020 11:43:24 +0100,
> > Keyon Jie wrote:
> >>
> >> In SOF driver, we don't use kernel config item like
> >> CONFIG_SND_HDA_PREALLOC_SIZE for HDA, the code for it is:
> >>
> >> 	snd_pcm_lib_preallocate_pages(pcm->streams[stream].substream,
> >> 				      SNDRV_DMA_TYPE_DEV_SG, sdev->dev,
> >> 				le32_to_cpu(caps->buffer_size_min),
> >> 				le32_to_cpu(caps->buffer_size_max));
> >>
> >> So the preallocated size is configured via topology file, that is
> >> caps->buffer_size_min, no chance for PulseAudio to reconfigure it.
> >>
> >> So, it looks like we have to change it to this if we don't change the
> >> ALSA core:
> >>
> >> 	snd_pcm_lib_preallocate_pages(pcm->streams[stream].substream,
> >> 				      SNDRV_DMA_TYPE_DEV_SG, sdev->dev,
> >> -				le32_to_cpu(caps->buffer_size_min),
> >> +				le32_to_cpu(caps->buffer_size_max),
> >> 				le32_to_cpu(caps->buffer_size_max));
> >
> > Yes, passing buffer_size_min for the preallocation sounds already
> > bad.  The default value should be sufficient for usual operations, not
> > the cost-cutting minimum.  Otherwise there is no merit of
> > preallocation.
> >
> > Alternatively, we may pass 0 there, indicating no limitation, too.
> > But, this would need a bit other adjustment, e.g. snd_pcm_hardware
> > should have lower buffer_bytes_max.
> 
> Thank you Takashi, then let's follow it to pre-allocate with
> caps->buffer_size_max, as we don't specify any limitations in
> snd_pcm_hardware today, we want to leave it configurable to each
> specific topology file for different machines.

How big is caps->buffer_size_max?  Passing the value there means
actually trying to allocate the given size as default, and it'd be a
lot of waste if a too large value (e.g. 32MB) is passed there.

I think we can go for passing zero as default, which means skipping
preallocation.  In addition, we may add an upper limit of the total
amount of allocation per card, controlled in pcm_memory.c, for
example.  This logic can be applied to the legacy HDA, too.

This should be relatively easy, and I'll provide the patch in the next
week.


Takashi