On Wed, Oct 12, 2011 at 03:34:54PM +0100, Dave Airlie wrote: > On Wed, Oct 12, 2011 at 3:24 PM, Rob Clark <robdclark@xxxxxxxxx> wrote: > > On Wed, Oct 12, 2011 at 9:01 AM, Dave Airlie <airlied@xxxxxxxxx> wrote: > >>> But then we'd need a different set of accessors for every different > >>> drm/v4l/etc driver, wouldn't we? > >> > >> Not any more different than you need for this, you just have a new > >> interface that you request a sw object from, > >> then mmap that object, and underneath it knows who owns it in the kernel. > > > > oh, ok, so you are talking about a kernel level interface, rather than > > userspace.. > > > > but I guess in this case I don't quite see the difference. It amounts > > to which fd you call mmap (or ioctl[*]) on.. If you use the dmabuf fd > > directly then you don't have to pass around a 2nd fd. > > > > [*] there is nothing stopping defining some dmabuf ioctls (such as for > > synchronization).. although the thinking was to keep it simple for > > first version of dmabuf > > > > Yes a separate kernel level interface. > > Well I'd like to keep it even simpler. dmabuf is a buffer sharing API, > shoehorning in a sw mapping API isn't making it simpler. > > The problem I have with implementing mmap on the sharing fd, is that > nothing says this should be purely optional and userspace shouldn't > rely on it. > > In the Intel GEM space alone you have two types of mapping, one direct > to shmem one via GTT, the GTT could be even be a linear view. The > intel guys initially did GEM mmaps direct to the shmem pages because > it seemed simple, up until they > had to do step two which was do mmaps on the GTT copy and ended up > having two separate mmap methods. I think the problem here is it seems > deceptively simple to add this to the API now because the API is > simple, however I think in the future it'll become a burden that we'll > have to workaround. Yeah, that's my feeling, too. Adding mmap sounds like a neat, simple idea, that could simplify things for simple devices like v4l. But as soon as you're dealing with a real gpu, nothing is simple. Those who don't believe this, just take a look at the data upload/download paths in the open-source i915,nouveau,radeon drivers. Making this fast (and for gpus, it needs to be fast) requires tons of tricks, special-cases and jumping through loops. You absolutely want the device-specific ioctls to do that. Adding a generic mmap just makes matters worse, especially if userspace expects this to work synchronized with everything else that is going on. Cheers, Daniel -- Daniel Vetter Mail: daniel@xxxxxxxx Mobile: +41 (0)79 365 57 48 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>