Re: [PATCH 0/3] drm: panfrost: Coherency support
From: Steven Price <steven.price@arm.com>
Date: 2020-09-17 10:39:52
Also in:
dri-devel, linux-amlogic, linux-iommu
On 16/09/2020 18:46, Rob Herring wrote:
On Wed, Sep 16, 2020 at 11:04 AM Alyssa Rosenzweig [off-list ref] wrote:quoted
quoted
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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel