On 16/09/2020 18:46, Rob Herring wrote:
On Wed, Sep 16, 2020 at 11:04 AM Alyssa Rosenzweig
<alyssa.rosenzweig@xxxxxxxxxxxxx> 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
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