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

[Andrew F. Davis]

On 5/4/20 4:50 AM, Brian Starkey wrote:
> 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@xxxxxxx> 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@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>
>>>> ---
>>>>  .../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.


The issue is going to be this causes the node name in DT to become a
kind of ABI. Right now until we have some userspace lib that enumerates
the heaps in a stable way programs will hard-code the full heap name,
which right now would look like:

char *heap = "/dev/dma_heap/dma_heap_mem@89000000";

Yuk.. we might want to look into exporting heap properties to make them
searchable based on something other than name here soon. Or this will be
a mess to cleanup in the future.

Andrew


> 
>>> 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