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

[Brian Starkey]

On Fri, May 01, 2020 at 12:01:40PM -0700, John Stultz wrote:
> On Fri, May 1, 2020 at 4:08 AM Robin Murphy <robin.murphy@xxxxxxx> 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@xxxxxxxxxx>
> > >> Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
> > >> Cc: "Andrew F. Davis" <afd@xxxxxx>
> > >> Cc: Benjamin Gaignard <benjamin.gaignard@xxxxxxxxxx>
> > >> Cc: Liam Mark <lmark@xxxxxxxxxxxxxx>
> > >> Cc: Pratik Patel <pratikp@xxxxxxxxxxxxxx>
> > >> Cc: Laura Abbott <labbott@xxxxxxxxxx>
> > >> Cc: Brian Starkey <Brian.Starkey@xxxxxxx>
> > >> Cc: Chenbo Feng <fengc@xxxxxxxxxx>
> > >> Cc: Alistair Strachan <astrachan@xxxxxxxxxx>
> > >> Cc: Sandeep Patil <sspatil@xxxxxxxxxx>
> > >> Cc: Hridya Valsaraju <hridya@xxxxxxxxxx>
> > >> Cc: Christoph Hellwig <hch@xxxxxx>
> > >> Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> > >> Cc: Robin Murphy <robin.murphy@xxxxxxx>
> > >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > >> Cc: devicetree@xxxxxxxxxxxxxxx
> > >> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > >> Cc: linux-mm@xxxxxxxxx
> > >> Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
> > >> ---
> > >>   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?

My thinking was more like there would already be a reserved-memory
node in DT for the chunk of memory, appropriately tagged so that it
gets added as a CMA region. 

The device's node would have "memory-region=<&blah>;" and would use
of_reserved_mem_device_init() to link up dev->cma_area to the
corresponding cma region.

So far, that's all in-place already. The bit that's missing is
exposing that dev->cma_area to userspace as a dma_heap - so we could
just have "int cma_heap_add(struct cma *cma)" or "int
cma_heap_dev_add(struct device *dev)" or something exported for
drivers to expose their device-assigned cma region if they wanted to.

I don't think this runs into the lifetime problems of generalised
heaps-as-modules either, because the CMA region will never go away
even if the driver does.

Alongside that, I do think the completely DT-driven approach can be
useful too - because there may be regions which aren't associated with
any specific device driver, that we want exported as heaps.

Hope that makes sense,
-Brian

> 
> thanks
> -john