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

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).