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Message-ID: <9178e19f5c0e141772b61b759abaa0d176f902b6.camel@ndufresne.ca>
Subject: Re: DMA-buf and uncached system memory
From:   Nicolas Dufresne <nicolas@ndufresne.ca>
To:     Daniel Vetter <daniel@ffwll.ch>,
        Christian =?ISO-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>
Cc:     linux-media <linux-media@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        linaro-mm-sig@lists.linaro.org,
        lkml <linux-kernel@vger.kernel.org>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        "Sharma, Shashank" <Shashank.Sharma@amd.com>
Date:   Wed, 22 Jun 2022 15:39:36 -0400
In-Reply-To: <YCuPhOT4GhY3RR/6@phenom.ffwll.local>
References: <91ff0bbb-ea3a-2663-3453-dea96ccd6dd8@amd.com>
         <YCuPhOT4GhY3RR/6@phenom.ffwll.local>
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Le mardi 16 f=C3=A9vrier 2021 =C3=A0 10:25 +0100, Daniel Vetter a =C3=A9cri=
t=C2=A0:
> So I think if AMD also guarantees to drop clean cachelines just do the
> same thing we do right now for intel integrated + discrete amd, but in
> reserve. It's fragile, but it does work.

Sorry to disrupt, but if you pass V4L2 vmalloc data to Intel display driver=
, you
also get nice dirt on the screen. If you have a UVC webcam that produces a =
pixel
format compatible with your display, you can reproduce the issue quite easi=
ly
with:

  gst-launch-1.0 v4l2src device=3D/dev/video0 ! kmssink

p.s. some frame-rate are less likely to exhibit the issue, make sure you cr=
eate
movement to see it.

The only solution I could think of (not implemented) was to detect in the
attach() call what the importers can do (with dev->coherent_dma_mask if I
recall), and otherwise flush the cache immediately and start flushing the c=
ache
from now on signalling it for DQBUF (in vb2 workqueue or dqbuf ioctl, I don=
't
have an idea yet). I bet this idea is inapplicable to were you have fences,=
 we
don't have that in v4l2.

This idea was hinted by Robert Becket (now in CC), but perhaps I picked it =
up
wrong, explaining it wrong, etc. I'm no expert, just noticed there wasn't r=
eally
a good plan for that, so one needs to make one up. I'm not aware oh an impo=
rter
could know how the memory was allocated by the exporter, and worst, how an
importer could figure-out that the export is going to produce buffer with h=
ot
CPU cache (UVC driver does memcpy from USB chunks of variable size to produ=
ce a
fixed size image).

Nicolas