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

[Takashi Iwai]

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...


Takashi