[PATCH 0/3] drm: panfrost: Coherency support

[Steven Price]

On 16/09/2020 18:46, Rob Herring wrote:
> On Wed, Sep 16, 2020 at 11:04 AM Alyssa Rosenzweig
> <alyssa.rosenzweig at collabora.com> wrote:
>>
>>> So I get a performance regression with the dma-coherent approach, even if it's
>>> clearly the cleaner.
>>
>> That's bizarre -- this should really be the faster of the two.
> 
> Coherency may not be free. CortexA9 had something like 4x slower
> memcpy if SMP was enabled as an example. I don't know if there's
> anything going on like that specifically here. If there's never any
> CPU accesses mixed in with kmscube, then there would be no benefit to
> coherency.

The DDK blob has the ability to mark only certain areas of memory as 
coherent for performance reasons. For simple things like kmscube I would 
expect that it's basically write-only from the CPU and almost all memory 
the GPU touches isn't touched by the CPU. I.e. coherency isn't helping 
and the coherency traffic is probably expensive. Whether the complexity 
is worth it for "real" content I don't know - it may just be silly 
benchmarks that benefit.

Steve