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

[Takashi Iwai]

On Fri, 17 Jan 2020 11:43:24 +0100,
Keyon Jie wrote:
> 
> 
> 
> On 2020/1/17 下午4:00, Takashi Iwai wrote:
> > On Fri, 17 Jan 2020 06:37:16 +0100,
> > Keyon Jie wrote:
> >>
> >> On 2020/1/17 上午1:40, Pierre-Louis Bossart wrote:
> >>>
> >>>>>>> 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.
> >>>>>
> >>>>> Keyon, for the rest of us to follow this patch, would you mind
> >>>>> clarifying what drives the need for a 'very large buffer size',
> >>>>> and what order of magnitude this very large size would be.
> >>>>>
> >>>>> FWIW, we've measured consistently on different Windows/Linux
> >>>>> platforms, maybe 10 years ago, that once you reach a buffer of 1s
> >>>>> (384 kB) the benefits from increasing that buffer size further are
> >>>>> marginal in terms of power consumption, and generate all kinds of
> >>>>> issues with volume updates and deferred routing changes.
> >>>>>
> >>>> We need bigger buffer on host side to compensate the wake up time
> >>>> from d0ix to d0 which takes ~2 seconds on my setup. So, wiith
> >>>> smaller buffer sizes like < 2 seconds we overwrite data since FW
> >>>> keeps copping while host doesn't read until its up and running
> >>>> again.
> >>>
> >>> Right, that's a valid case, but that's 256 kB, not 'very large' or
> >>> likely to ever trigger an OOM case.
> >>
> >> For S24_LE, it is 512KB, the point is that if we can't re-allocate
> >> buffer at hw_params() stage, then we need follow a BKM that we have to
> >> preallocate the largest DMA buffer that we claim to support at
> >> pcm_new(), I think this is actually another kind of wast with these
> >> largest pinned buffer that can't be swapped out...
> >
> > Well, that's the case you'd need a larger preallocation.
> > I guess many distros already set it to a higher value for PulseAudio.
> > The default 64kB is just from historical and compatibility reason, and
> > we may extend it to 1MB or so now.
> 
> 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.


Takashi