On Fri, Nov 1, 2013 at 11:03 AM, Lucas Stach l.stach@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