Re: [RFC][PATCH 3/4] dma-buf: cma_heap: Extend logic to export CMA regions tagged with "linux,cma-heap"

[John Stultz <john.stultz@linaro.org>]

On Fri, May 1, 2020 at 4:08 AM Robin Murphy <robin.murphy@arm.com> wrote:
>
> On 2020-05-01 11:21 am, Brian Starkey wrote:
> > Hi John,
> >
> > On Fri, May 01, 2020 at 07:39:48AM +0000, John Stultz wrote:
> >> This patch reworks the cma_heap initialization so that
> >> we expose both the default CMA region and any CMA regions
> >> tagged with "linux,cma-heap" in the device-tree.
> >>
> >> 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>
> >> ---
> >>   drivers/dma-buf/heaps/cma_heap.c | 18 +++++++++---------
> >>   1 file changed, 9 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/drivers/dma-buf/heaps/cma_heap.c b/drivers/dma-buf/heaps/cma_heap.c
> >> index 626cf7fd033a..dd154e2db101 100644
> >> --- a/drivers/dma-buf/heaps/cma_heap.c
> >> +++ b/drivers/dma-buf/heaps/cma_heap.c
> >> @@ -141,6 +141,11 @@ static int __add_cma_heap(struct cma *cma, void *data)
> >>   {
> >>      struct cma_heap *cma_heap;
> >>      struct dma_heap_export_info exp_info;
> >> +    struct cma *default_cma = dev_get_cma_area(NULL);
> >> +
> >> +    /* We only add the default heap and explicitly tagged heaps */
> >> +    if (cma != default_cma && !cma_dma_heap_enabled(cma))
> >> +            return 0;
> >
> > Thinking about the pl111 thread[1], I'm wondering if we should also
> > let drivers call this directly to expose their CMA pools, even if they
> > aren't tagged for dma-heaps in DT. But perhaps that's too close to
> > policy.
>
> That sounds much like what my first thoughts were - apologies if I'm
> wildly off-base here, but as far as I understand:
>
> - Device drivers know whether they have their own "memory-region" or not.
> - Device drivers already have to do *something* to participate in dma-buf.
> - Device drivers know best how they make use of both the above.
> - Therefore couldn't it be left to drivers to choose whether to register
> their CMA regions as heaps, without having to mess with DT at all?

I guess I'm not opposed to this. But I guess I'd like to see some more
details? You're thinking the pl111 driver would add the
"memory-region" node itself?

Assuming that's the case, my only worry is what if that memory-region
node isn't a CMA area, but instead something like a carveout? Does the
driver need to parse enough of the dt to figure out where to register
the region as a heap?

thanks
-john