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

From: Eric Anholt <eric@anholt.net>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: narmstrong@baylibre.com, daniel.vetter@ffwll.ch,
	liviu.dudau@arm.com, dri-devel@lists.freedesktop.org,
	thierry.reding@gmail.com, laurent.pinchart@ideasonboard.com,
	daniel.vetter@intel.com, marex@denx.de,
	boris.brezillon@free-electrons.com, abrodkin@synopsys.com,
	linux@armlinux.org.uk, z.liuxinliang@hisilicon.com,
	kong.kongxinwei@hisilicon.com, tomi.valkeinen@ti.com,
	airlied@redhat.com, puck.chen@hisilicon.com, jsarha@ti.com,
	vincent.abriou@st.com, alison.wang@freescale.com,
	sw0312.kim@samsung.com, philippe.cornu@st.com,
	yannick.fertre@st.com, kyungmin.park@samsung.com,
	zourongrong@gmail.com, maxime.ripard@free-electrons.com,
	shawnguo@kernel.org, kraxel@redhat.com
Subject: Re: [PATCH v3 1/2] drm/gem: drm_gem_dumb_map_offset(): reject dma-buf
Date: Fri, 18 Aug 2017 13:34:45 -0700	[thread overview]
Message-ID: <87shgopqca.fsf@eliezer.anholt.net> (raw)
In-Reply-To: <20170818201730.62slltrw4fwlj5qe@phenom.ffwll.local>

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Daniel Vetter <daniel@ffwll.ch> writes:

> On Fri, Aug 18, 2017 at 10:41:21AM -0700, Eric Anholt wrote:
>> Noralf Trønnes <noralf@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@pengutronix.de>
>> >>> Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
>> >>> Cc: Sean Paul <seanpaul@chromium.org>
>> >>> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
>> >>> Signed-off-by: Noralf Trønnes <noralf@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@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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