[PATCH] drm/prime: skip CPU sync in map/unmap dma_buf

[Chris Wilson]

Quoting Lucas Stach (2017-11-30 17:34:28)
> Dma-bufs should already be device coherent, as they are only pulled in the
> CPU domain via the begin/end cpu_access calls. As we cache the mapping set
> up by dma_map_sg a CPU sync at this point will not actually guarantee proper
> coherency on non-coherent architectures, so we can as well stop pretending.

That matches my understanding of the dma-buf API, device coherent with
explicit cpu coherency managed by ioctl.

> This is an important performance fix for architectures which need explicit
> cache synchronization and userspace doing lots of dma-buf imports.
> Improves Weston on Etnaviv performance 5x, where before this patch > 90%
> of Weston CPU time was spent synchronizing caches for buffers which are
> already device coherent.
> 
> Signed-off-by: Lucas Stach <l.stach at pengutronix.de>

Sent an equivalent patch through i915's CI, which didn't show any
problems.

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris