[PATCH 1/2] drm/shmem: Use cached mappings by default

[Gerd Hoffmann]

On Mon, May 18, 2020 at 10:50:15AM +0200, Thomas Zimmermann wrote:
> Hi Gerd
> 
> Am 18.05.20 um 10:23 schrieb Gerd Hoffmann:
> >>> $ git grep drm_gem_shmem_mmap
> >>>
> >>> We also need correct access from userspace, otherwise the gpu is going to
> >>> be sad.
> >>
> >> I've been thinking about this, and I think it means that we can never
> >> have cached mappings anywhere. Even if shmem supports it internally for
> >> most drivers, as soon as the page are exported, the importer could
> >> expect uncached memory.
> > 
> > The importer should not expect anything but call dma-buf ops so the
> > exporter has a chance to handle this correctly.
> 
> I have the following case in mind: Suppose the exporter maps cached
> pages and the importer expects uncached pages for DMA. There is
> map_dma_buf/unmap_dma_buf, which can implement a cache flush for the
> cached pages. Is it guaranteed that the importer calls this around each
> DMA operation?

I think the importer is supposed to do that, but I wouldn't surprised if
there are cases in tree where this isn't implemented correctly ...

take care,
  Gerd