Re: [RFC][PATCH v2 2/2] dma-heap: Add a system-uncached heap

[John Stultz]

On Fri, Aug 14, 2020 at 2:17 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
>
> On Fri, Aug 14, 2020 at 06:24:58AM +0000, John Stultz wrote:
> > This adds a heap that allocates non-contiguous buffers that are
> > marked as writecombined, so they are not cached by the CPU.
> >
> > This is useful, as most graphics buffers are usually not touched
> > by the CPU or only written into once by the CPU. So when mapping
> > the buffer over and over between devices, we can skip the CPU
> > syncing, which saves a lot of cache management overhead, greatly
> > improving performance.
> >
> > For folk using ION, there was a ION_FLAG_CACHED flag, which
> > signaled if the returned buffer should be CPU cacheable or not.
> > With DMA-BUF heaps, we have no such flag, and by default the
> > current heaps (system and cma) produce CPU cachable buffers.
> > So for folks transitioning from ION to DMA-BUF Heaps, this fills
> > in some of that missing functionality.
> >
> > This does have a few "ugly" bits that were required to get
> > the buffer properly flushed out initially which I'd like to
> > improve. So feedback would be very welcome!
> >
> > Many thanks to Liam Mark for his help to get this working.
> >
> > Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
> > Cc: Andrew F. Davis <afd@xxxxxx>
> > Cc: Benjamin Gaignard <benjamin.gaignard@xxxxxxxxxx>
> > Cc: Liam Mark <lmark@xxxxxxxxxxxxxx>
> > Cc: Laura Abbott <labbott@xxxxxxxxxx>
> > Cc: Brian Starkey <Brian.Starkey@xxxxxxx>
> > Cc: Hridya Valsaraju <hridya@xxxxxxxxxx>
> > Cc: Robin Murphy <robin.murphy@xxxxxxx>
> > Cc: linux-media@xxxxxxxxxxxxxxx
> > Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
> > ---
> > v2:
> > * Fix build issue on sh reported-by: kernel test robot <lkp@xxxxxxxxx>
> > * Rework to use for_each_sgtable_sg(), dma_map_sgtable(), and
> >   for_each_sg_page() along with numerous other cleanups suggested
> >   by Robin Murphy
>
> Mildly annoying me, but where's the userspace for this? Do we have a
> gralloc somewhere that works with open driver stacks and uses this?

So this is still early RFC, but I've added support to the HiKey960
gralloc, and have pending patches (following DRM rules) I pushed here:
  https://android-review.googlesource.com/c/device/linaro/hikey/+/1399519

And avoiding the cache syncing overhead improves performance nicely on
that board.

There is also work in progress to change the codec2 implementation in
AOSP (which uses ion directly), to move over to DMA BUF heaps and as
it used the !ION_FLAG_CACHED case there this heap would be useful to
match ION's functionality.

The latest patches for that are here:
https://android-review.googlesource.com/c/platform/frameworks/av/+/1360640
(though I'm expecting a deeper rework on those)

thanks
-john