[PATCH] drm/prime: Fix mmap fake offset for drm_gem_object_funcs.mmap

[Rob Herring]

On Thu, Oct 24, 2019 at 7:32 AM Daniel Vetter <daniel at ffwll.ch> wrote:
>
> On Thu, Oct 24, 2019 at 11:02:40AM +0200, Gerd Hoffmann wrote:
> > On Wed, Oct 23, 2019 at 05:22:26PM -0500, Rob Herring wrote:
> > > Commit c40069cb7bd6 ("drm: add mmap() to drm_gem_object_funcs")
> > > introduced a GEM object mmap() hook which is expected to subtract the
> > > fake offset from vm_pgoff.
> >
> > Long-term it is probably a good idea to just remove the fake offset
> > handling from drivers.  But that'll only work once all drivers switched
> > away from custom fops->mmap handlers so we can handle the offset -> obj
> > lookup in the drm core for everybody.
> >
> > So let's go this way for now.
> >
> > Acked-by: Gerd Hoffmann <kraxel at redhat.com>
>
> Uh this sounds like doubling down on rather horrible semantics. Can we at
> least stop the mess instead of baking it in for real? The hook is very
> very new after all. I.e.
> - Document that obj->funcs->mmap will have 0 offset in the kerneldoc.
> - Remove the subtracting from the shmem helper
> - In ttm_bo_mmap_obj re-add the offset with a huge FIXME comment.
> - Adjust drm_gem_mmap_obj to do that same for obj->funcs->mmap and also
>   document the expectation there too.

Okay.

> This feels like very much going the wrong direction ...
>
> Also I guess Gerd didn't really test this prime mmap support?

Perhaps because at least parts of the IGT "vgem" tests really have
nothing specific for "vgem" and there doesn't seem to be another test
case that does run doing the same thing. And none of the IGT prime
tests run without an Intel driver. Looking at IGT always makes me sad,
and then I move on to other things... </rant>

BTW, are there IGT test results for vgem/vkms somewhere? I didn't have
any luck finding anything.

Rob