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

[Tomeu Vizoso <tomeu.vizoso@collabora.com>]

On 9/17/20 12:38 PM, Steven Price wrote:
> On 16/09/2020 18:46, Rob Herring wrote:
>> On Wed, Sep 16, 2020 at 11:04 AM Alyssa Rosenzweig
>> <alyssa.rosenzweig@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.

Or maybe it's only a problem for applications that do silly things? I 
don't think kmscube was ever optimized for performance.

Regards,

Tomeu
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