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

[Takashi Iwai]

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.


Takashi