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

[Takashi Iwai]

On Fri, 17 Jan 2020 06:30:18 +0100,
Keyon Jie wrote:
> 
> On 2020/1/17 上午4:37, Takashi Iwai wrote:
> > On Thu, 16 Jan 2020 18:40:26 +0100,
> > 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.
> >
> > That size shouldn't matter, and would work even with the
> > preallocation.
> >
> > My concern is that removing the limitation would allow the allocation
> > of too large sizes.  Even with dma_max limit, it can go up to 32MB
> > physical pages per stream for HDA.  Depending on the hardware setup,
> > there can be a lot of streams assignment (e.g. HDMI codecs) and
> > multiple codecs / controllers, and imagine that all those allocated
> > pages are pinned and can't be swapped out...
> 
> Hi Takashi, I get your concern here, but if we switch to use dma_max
> limit, we won't change the preallocated buffer, it will be still 64KB
> for each stream, user space can ask for re-allocate buffer for each
> stream up to 32MB, but those pinned and can't be swapped out ones are
> the 64KB preallocated ones only, am I wrong?

No, in general, all sound hardware buffers are pinned.


Takashi