dma-buf non-coherent mmap

[Daniel Vetter]

On Fri, Nov 1, 2013 at 11:03 AM, Lucas Stach <l.stach at pengutronix.de> wrote:
> GStreamer needs _some_ way of accessing the buffer contents with the
> CPU, this doesn't necessarily have to be the dumb mmap we have right
> now.
> DMA-BUF support in GSt is not really finished (I know we have a lot of
> patches internally to make it actually work with anything, which are
> trickling upstream right now), so this might be a good time to hammer
> out how it should be done, so we can adapt GSt before people start to
> rely on the dma-buf fallback mmap.
>
> I would think the bracketed mmap idea that was thrown around by Rob some
> time ago when the mmap topic first surfaced is simple enough that we
> don't need additional userspace helpers and should be enough to make the
> coherency issue disappear.

Yeah, if we add a BEGIN_MMAP/END_MMAP ioctl or so to the dma-buf mmap
stuff and simply demand that userspace calls that we'd have something
half-sane at least. Since we currently don't really have any real
users we could still do this abi change I think.

One open issue is whether the BEGIN_MMAP ioctl should also synchronize
with any outstanding gpu access. Imo it should, since that's pretty
much how all the current drm drivers work, but maybe we should reserve
space for a few flags so that this can be extended later on - Android
seems to be pretty insistent on using explicit syncpoints everywhere
instead of implicit synchronization. Or maybe we should have the flag
already, but reject implicit syncing until Maarten's dma_fence stuff
is in and enabled, dunno. Adding him to cc.

The clear thing otoh is that these two ioctls should only serve as
barriers, not as locks as has been proposed in some other RFCs
floating around (iirc the one from exynos was borked in this fashion).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch