Re: [Linaro-mm-sig] [RFC 1/2] dma-buf: Introduce dma buffer sharing mechanism

[Daniel Vetter <daniel@ffwll.ch>]

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@gmail.com> wrote:
> > On Wed, Oct 12, 2011 at 9:01 AM, Dave Airlie <airlied@gmail.com> 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@ffwll.ch
Mobile: +41 (0)79 365 57 48

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