Re: [PATCH] drm/gem_shmem: Use a writecombine mapping for ->vaddr

[Eric Anholt <eric@anholt.net>]

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Boris Brezillon <boris.brezillon@collabora.com> writes:

> Right now, the BO is mapped as a cached region when ->vmap() is called
> and the underlying object is not a dmabuf.
> Doing that makes cache management a bit more complicated (you'd need
> to call dma_map/unmap_sg() on the ->sgt field everytime the BO is about
> to be passed to the GPU/CPU), so let's map the BO with writecombine
> attributes instead (as done in most drivers).
>
> Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> ---
> Found this issue while working on panfrost perfcnt where the GPU dumps
> perf counter values in memory and the CPU reads them back in
> kernel-space. This patch seems to solve the unpredictable behavior I
> had.
>
> I can also go for the other option (call dma_map/unmap/_sg() when
> needed) if you think that's more appropriate.

writecombined was the intent, and this makes kernel vmap match the
userspace mmap path.

Reviewed-by: Eric Anholt <eric@anholt.net>

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