[PATCH v3 1/2] drm/gem: drm_gem_dumb_map_offset(): reject dma-buf

[Eric Anholt]

Daniel Vetter <daniel at ffwll.ch> writes:

> On Fri, Aug 18, 2017 at 10:41:21AM -0700, Eric Anholt wrote:
>> Noralf Trønnes <noralf at tronnes.org> writes:
>> > Den 18.08.2017 09.46, skrev Daniel Vetter:
>> >> On Thu, Aug 17, 2017 at 06:21:30PM +0200, Noralf Trønnes wrote:
>> >>> Reject mapping an imported dma-buf since is's an invalid use-case.
>> >>>
>> >>> Cc: Philipp Zabel <p.zabel at pengutronix.de>
>> >>> Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
>> >>> Cc: Sean Paul <seanpaul at chromium.org>
>> >>> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
>> >>> Signed-off-by: Noralf Trønnes <noralf at tronnes.org>
>> >> I think acks from someone using mali would be good too. amdgpu already has
>> >> such checks, so I think on the desktop side we're ok.
>> >>
>> >> Acked-by: Daniel Vetter <daniel.vetter at ffwll.ch>
>> >>
>> >> But I think this one here definitely needs a few more acks. I could break
>> >> uabi if we're unlucky, so let's not rush it.
>> >
>> > Ok, I've CC'ed the affected parties to increase the odds that they look
>> > at this. These are the drivers using drm_gem_dumb_map_offset()
>> > (hopefully I got the list right):
>> 
>> If I understand the affected path right, this would break the PL111+VC4
>> combination: PL111 makes (dumb) buffers for scanout, and VC4 imports
>> them and uses them for rendering.  A vc4 glReadPixels of the window
>> system buffer would map it and fail.
>
> It only rejects the map call on dumb buffers, and mmap on imported dma-buf
> tends to not really work well, or at least break a few abstractions. Are
> you sure this works currently?

OK, that's right -- vc4 would be doing its "native" map call (the same
code), not dumb map.

Furthermore, I had it backwards (I had written things both ways at
different points, iirc).  We have VC4 making the buffers and PL111
dma-buf importing them.  I don't see X11 mapping those buffers if glamor
is enabled, so this should be OK for vc4.

GBM's dumb mapping looks safe to me.  X11 does some dumb maps, but I
don't think any of those would be on imports.
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