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

[Eric Anholt <eric@anholt.net>]

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Daniel Vetter <daniel@ffwll.ch> writes:

> On Fri, May 31, 2019 at 08:46:58AM -0700, Eric Anholt wrote:
>> 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.
>
> Since I missed that obviously: Where do the shmem helpers set write
> combined mode for userspace mmap?

That was the trick when I asked the question, too.  drm_gem.c is what
sets WC by default.

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