[PATCH] media: mediatek: vcodec: Alloc DMA memory with DMA_ATTR_ALLOC_SINGLE_PAGES

[Doug Anderson]

Hi,

On Mon, Apr 22, 2024 at 11:27 AM Nicolas Dufresne
<nicolas.dufresne at collabora.com> wrote:
>
> Hi,
>
> Le lundi 22 avril 2024 à 10:03 -0700, Douglas Anderson a écrit :
> > As talked about in commit 14d3ae2efeed ("ARM: 8507/1: dma-mapping: Use
> > DMA_ATTR_ALLOC_SINGLE_PAGES hint to optimize alloc"), it doesn't
> > really make sense to try to allocate contiguous chunks of memory for
> > video encoding/decoding. Let's switch the Mediatek vcodec driver to
> > pass DMA_ATTR_ALLOC_SINGLE_PAGES and take some of the stress off the
> > memory subsystem.
> >
> > Signed-off-by: Douglas Anderson <dianders at chromium.org>
> > ---
> > NOTE: I haven't personally done massive amounts of testing with this
> > change, but I originally added the DMA_ATTR_ALLOC_SINGLE_PAGES flag
> > specifically for the video encoding / decoding cases and I know it
> > helped avoid memory problems in the past on other systems. Colleagues
> > of mine have told me that with this change memory problems are harder
> > to reproduce, so it seems like we should consider doing it.
>
> One thing to improve in your patch submission is to avoid abstracting the
> problems. Patch review and pulling is based on a technical rational and very
> rarely on the trust that it helps someone somewhere in some unknown context.
> What kind of memory issues are you facing ? What is the technical advantage of
> using DMA_ATTR_ALLOC_SINGLE_PAGES over the current approach that helps fixing
> the issue? I do expect this to be documented in the commit message itselfé.

Right. The problem here is that I'm not _directly_ facing any problems
here and I also haven't done massive amounts of analysis of the
Mediatek video codec. I know that some of my colleagues have run into
issues on Mediatek devices where the system starts getting
unresponsive when lots of videos are decoded in parallel. That
reminded me of the old problem I debugged in 2015 on Rockchip
platforms and is talked about a bunch in the referenced commit
14d3ae2efeed ("ARM: 8507/1: dma-mapping: Use
DMA_ATTR_ALLOC_SINGLE_PAGES hint to optimize alloc") so I wrote up
this patch. The referenced commit contains quite a bit of details
about the problems faced back in 2015.

When I asked, my colleagues said that my patch seemed to help, though
it was more of a qualitative statement than a quantitative one.

I wasn't 100% sure if it was worth sending the patch up at this point,
but logically, I think it makes sense. There aren't great reasons to
hog all the large chunks of memory for video decoding.

-Doug