[RFC v2 1/2] dma-buf: Introduce dma buffer sharing mechanism

Daniel Vetter daniel at ffwll.ch
Mon Dec 5 17:33:36 EST 2011

On Mon, Dec 05, 2011 at 04:11:46PM -0600, Rob Clark wrote:
> On Mon, Dec 5, 2011 at 3:23 PM, Daniel Vetter <daniel at ffwll.ch> wrote:
> > On Mon, Dec 05, 2011 at 02:46:47PM -0600, Rob Clark wrote:
> >> I sort of preferred having the DMABUF shim because that lets you pass
> >> a buffer around userspace without the receiving code knowing about a
> >> device specific API.  But the problem I eventually came around to: if
> >> your GL stack (or some other userspace component) is batching up
> >> commands before submission to kernel, the buffers you need to wait for
> >> completion might not even be submitted yet.  So from kernel
> >> perspective they are "ready" for cpu access.  Even though in fact they
> >> are not in a consistent state from rendering perspective.  I don't
> >> really know a sane way to deal with that.  Maybe the approach instead
> >> should be a userspace level API (in libkms/libdrm?) to provide
> >> abstraction for userspace access to buffers rather than dealing with
> >> this at the kernel level.
> >
> > Well, there's a reason GL has an explicit flush and extensions for sync
> > objects. It's to support such scenarios where the driver batches up gpu
> > commands before actually submitting them.
> Hmm.. what about other non-GL APIs..  maybe vaapi/vdpau or similar?
> (Or something that I haven't thought of.)

They generally all have a concept of when they've actually commited the
rendering to an X pixmap or egl image. Usually it's rather implicit, e.g.
the driver will submit any outstanding batches before returning from any
Daniel Vetter
Mail: daniel at ffwll.ch
Mobile: +41 (0)79 365 57 48

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