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

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Date:   Wed, 6 May 2020 12:04:12 -0400
From:   "Andrew F. Davis" <afd@...com>
To:     Brian Starkey <brian.starkey@....com>,
        John Stultz <john.stultz@...aro.org>
CC:     lkml <linux-kernel@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Liam Mark <lmark@...eaurora.org>,
        Pratik Patel <pratikp@...eaurora.org>,
        Laura Abbott <labbott@...hat.com>,
        Chenbo Feng <fengc@...gle.com>,
        Alistair Strachan <astrachan@...gle.com>,
        Sandeep Patil <sspatil@...gle.com>,
        Hridya Valsaraju <hridya@...gle.com>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-mm <linux-mm@...ck.org>, nd <nd@....com>
Subject: Re: [RFC][PATCH 1/4] devicetree: bindings: Add linux,cma-heap tag for
 reserved memory

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@....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@...nel.org>
>>>> Cc: Sumit Semwal <sumit.semwal@...aro.org>
>>>> Cc: "Andrew F. Davis" <afd@...com>
>>>> Cc: Benjamin Gaignard <benjamin.gaignard@...aro.org>
>>>> Cc: Liam Mark <lmark@...eaurora.org>
>>>> Cc: Pratik Patel <pratikp@...eaurora.org>
>>>> Cc: Laura Abbott <labbott@...hat.com>
>>>> Cc: Brian Starkey <Brian.Starkey@....com>
>>>> Cc: Chenbo Feng <fengc@...gle.com>
>>>> Cc: Alistair Strachan <astrachan@...gle.com>
>>>> Cc: Sandeep Patil <sspatil@...gle.com>
>>>> Cc: Hridya Valsaraju <hridya@...gle.com>
>>>> Cc: Christoph Hellwig <hch@....de>
>>>> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
>>>> Cc: Robin Murphy <robin.murphy@....com>
>>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>>> Cc: devicetree@...r.kernel.org
>>>> Cc: dri-devel@...ts.freedesktop.org
>>>> Cc: linux-mm@...ck.org
>>>> Signed-off-by: John Stultz <john.stultz@...aro.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.


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@...00000";

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@...00000/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