Wayland generic dmabuf protocol

[Rob Clark]

On Wed, Jun 11, 2014 at 2:26 AM, Thomas Hellstrom <thellstrom at vmware.com> wrote:
> On 06/09/2014 01:23 PM, Daniel Stone wrote:
>> Hi,
>>
>> 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
>>     <https://urldefense.proofpoint.com/v1/url?u=https://bugzilla.gnome.org/show_bug.cgi?id%3D711155&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=l5Ago9ekmVFZ3c4M6eauqrJWGwjf6fTb%2BP3CxbBFkVM%3D%0A&m=NHSSDNdXVt2oCxexwhcqQAQSZ3K6Xa8Zvuv0jMYfS2c%3D%0A&s=f6d8c24d7fa4ccdacae79aeaa8221773d892350a191f96cb3bab4f542fb3ad1c>)
>>     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.
>>
>>     Right.
>>
>>     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
>>     design
>>     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:
>> https://lwn.net/Articles/565469/
>> <https://urldefense.proofpoint.com/v1/url?u=https://lwn.net/Articles/565469/&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=l5Ago9ekmVFZ3c4M6eauqrJWGwjf6fTb%2BP3CxbBFkVM%3D%0A&m=NHSSDNdXVt2oCxexwhcqQAQSZ3K6Xa8Zvuv0jMYfS2c%3D%0A&s=36b71b58f7fc6f3d594529cc674364a23718e5cbb38a439e4a9c628848a13b3a>
>>
>> See the 'possible solutions' part for a way for people to agree on
>> restrictions wrt tiling, stride, contiguousness, etc.
>
> Hi!
>
> 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?

well, central standalone allocator device would need to know about all
possible devices.  Works ok in a cell phone.  I don't really see it
scaling well to desktop, so it isn't really an option.

BR,
-R

> /Thomas
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>> Cheers,
>> Daniel
>>
>>
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