Wayland generic dmabuf protocol
thellstrom at vmware.com
Tue Jun 10 23:26:47 PDT 2014
On 06/09/2014 01:23 PM, Daniel Stone wrote:
> On 9 June 2014 12:06, Pekka Paalanen <pekka.paalanen at collabora.co.uk
> <mailto:pekka.paalanen at collabora.co.uk>> wrote:
> On Mon, 9 Jun 2014 11:00:04 +0200
> Benjamin Gaignard <benjamin.gaignard at linaro.org
> <mailto:benjamin.gaignard at linaro.org>> wrote:
> > One of the main comment on the latest patches was that wl_dmabuf use
> > DRM for buffer allocation.
> > This appear to be an issue since wayland doesn't want to rely on one
> > specific framework (DRM, or V4L2) for buffer allocation, so we have
> > start working on a "central dmabuf allocation" on kernel side. The
> > goal is provide some as generic as possible to make it acceptable by
> > wayland.
> Why would Wayland need a central allocator for dmabuf?
> I think you've just answered your own question further below:
> > On my hardware the patches you have (+ this one on gstwaylandsink
> > https://bugzilla.gnome.org/show_bug.cgi?id=711155
> allow me to do zero
> > copy between the hardware video decoder and the display engine. I
> > don't have implemented GPU yet because my hardware is able to do
> > compose few video overlays planes and it was enough for my tests.
> What I have been thinking is, that the compositor must be able to use
> the new wl_buffer and we need to guarantee that before-hand. If the
> compositor fails to use a wl_buffer when the client has already
> attached it to a wl_surface and it is time to repaint, it is too late
> and the user will see a glitch. Recovering from that requires asking
> the client to provide a new wl_buffer of a different kind, which might
> take time. Or a very rude compositor would just send a protocol error,
> and then we'd get bug reports like "the video player just disappears
> when I try to play (and ps. I have an old kernel that doesn't support
> importing whatever)".
> I believe we must allow the compositor to test the wl_buffer before it
> is usable for the client. That is the reason for the roundtrippy
> of the below proposal.
> A central allocator would solve these issues, by having everyone agree
> on the restrictions upfront, instead of working out which of the media
> decode engine, camera, GPU, or display controller is the lowest common
> denominator, and forcing all allocations through there.
> One such solution was discussed a while back WRT ION:
> See the 'possible solutions' part for a way for people to agree on
> restrictions wrt tiling, stride, contiguousness, etc.
I think before deciding on something like this, one needs also to
account for the virtual drivers like vmwgfx.
Here, a dma-buf internally holds an opaque handle to an object on the
host / hypervisor, and the actual memory buffer is only temporarily
allocated for dma-buf operations that strictly need it. Not to hold the
data while transferring it between devices or applications.
Let's say you'd want to use a USB display controller in a virtual
machine with the vmwgfx exported prime objects, for example. There's no
common denominator. The vmwgfx driver would need to read the dma-buf
data from the host object at sg-table export (dma-buf map) time.
Whereas if you just want to share data between a wayland server and
client, no pages are ever allocated and the only thing passed
around is in effect the opaque handle to the host / hypervisor object.
I'm currently having trouble seeing how a central allocator would be
able to deal with this?
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