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

[Daniel Vetter]

On Thu, Nov 30, 2017 at 08:56:42PM +0000, Chris Wilson wrote:
> 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>

Pushed to drm-misc-next, thx for patch&review.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch