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

From: Alyssa Rosenzweig <[email-masked]>
Date: Thu, 17 Sep 2020 08:03:37 -0400

> 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.

Right, Panfrost userspace specifically assumes GPU reads to be expensive
and treats GPU memory as write-only *except* for a few special cases
(compute-like workloads, glReadPixels, some blits, etc).

The vast majority of the GPU memory - everything used in kmscube - will be
write-only to the CPU and fed directly into the display zero-copy (or
read by the GPU later as a dmabuf).
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