Re: [RFC][PATCH 1/4] devicetree: bindings: Add linux,cma-heap tag for reserved memory

[Brian Starkey <brian.starkey@arm.com>]

On Fri, May 01, 2020 at 11:40:16AM -0700, John Stultz wrote:
> On Fri, May 1, 2020 at 3:42 AM Brian Starkey <brian.starkey@arm.com> wrote:
> >
> > Hi,
> >
> > On Fri, May 01, 2020 at 07:39:46AM +0000, John Stultz wrote:
> > > This patch adds a linux,cma-heap property for CMA reserved memory
> > > regions, which will be used to allow the region to be exposed via
> > > the DMA-BUF Heaps interface
> > >
> > > Cc: Rob Herring <robh+dt@kernel.org>
> > > Cc: Sumit Semwal <sumit.semwal@linaro.org>
> > > Cc: "Andrew F. Davis" <afd@ti.com>
> > > Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
> > > Cc: Liam Mark <lmark@codeaurora.org>
> > > Cc: Pratik Patel <pratikp@codeaurora.org>
> > > Cc: Laura Abbott <labbott@redhat.com>
> > > Cc: Brian Starkey <Brian.Starkey@arm.com>
> > > Cc: Chenbo Feng <fengc@google.com>
> > > Cc: Alistair Strachan <astrachan@google.com>
> > > Cc: Sandeep Patil <sspatil@google.com>
> > > Cc: Hridya Valsaraju <hridya@google.com>
> > > Cc: Christoph Hellwig <hch@lst.de>
> > > Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> > > Cc: Robin Murphy <robin.murphy@arm.com>
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Cc: devicetree@vger.kernel.org
> > > Cc: dri-devel@lists.freedesktop.org
> > > Cc: linux-mm@kvack.org
> > > Signed-off-by: John Stultz <john.stultz@linaro.org>
> > > ---
> > >  .../devicetree/bindings/reserved-memory/reserved-memory.txt    | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> > > index bac4afa3b197..e97b6a4c3bc0 100644
> > > --- a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> > > +++ b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> > > @@ -68,6 +68,9 @@ Linux implementation note:
> > >  - If a "linux,cma-default" property is present, then Linux will use the
> > >    region for the default pool of the contiguous memory allocator.
> > >
> > > +- If a "linux,cma-heap" property is present, then Linux will expose the
> > > +  the CMA region via the DMA-BUF Heaps interface.
> > > +
> >
> > Would it be useful or even possible to give some indication of what
> > the heap will end up being called? I'm afraid I don't remember what if
> > any conclusions came out of previous discussions on UAPI for heap
> > enumeration.
> 
> So the name we expose is the CMA name itself. So with dt it will be
> the name of the reserved memory node that the flag property is added
> to.
> 

Yeah I'm just wondering if that's "stable" so we can say "the heap
will use the node name", or if saying that would cause us a headache
in the future.

> > I suppose CMA names haven't been relevant to userspace before, but
> > they perhaps would be with this change.
> >
> > Alternatively, leaving it effectively undefined doesn't tie us down,
> > and something like links in sysfs can be added as a richer API in the
> > future.
> 
> Hrm. Mind expanding on what you're thinking here?

Super hand-wavy, something like:

/sys/devices/blah/display@2f000000/cma_region is a symlink to
	/sys/class/dma_heaps/heap_display

I think danvet had some thoughts in this vein.

Cheers,
-Brian

> 
> thanks
> -john