[PATCH RFC wayland-protocols] unstable/linux-dmabuf: add wp_linux_dmabuf_device_hint

Simon Ser contact at emersion.fr
Sun Nov 18 14:08:57 UTC 2018

On Thursday, November 15, 2018 6:11 PM, Philipp Zabel <p.zabel at pengutronix.de> wrote:
> Hi Pekka,
> thank you for the explanation.


Thanks Pekka for clarifying.

> On Wed, 2018-11-14 at 11:03 +0200, Pekka Paalanen wrote:
> [...]
> > The hints protocol we are discussing here is a subset of what
> > https://github.com/cubanismo/allocator aims to achieve. Originally we
> > only concentrated on getting the format and modifier more optimal, but
> > the question of where and how to allocate the buffers is valid too. Is
> > it in scope for this extension is the big question below.
> My guess is: probably not. Either way, I'd prefer the protocol docs to
> be explicit about this.
> > Ideally, the protocol would do something like this:
> >
> > - Tell the client which device and for which use case the device must
> >   be able to access the buffer at minimum and always.
> >
> > - Tell the client that if it could make the buffer suitable also for a
> >   secondary device and a secondary use case, the compositor could do a
> >   more optimal job (e.g. putting the buffer in direct scanout,
> >   bypassing composition, or a hardware video encoder in case the output
> >   is going to be streamed).
> >
> > We don't have the vocabulary for use cases and there are tons of
> > different details to be taken into account, which is the whole point of
> > the allocator project. So we cannot do the complete solution here and
> > now, but we can do an approximate solution by negotiating pixel
> > formats and modifiers.
> >
> > The primary device is what the compositor uses for the fallback path,
> > which is compositing with a GPU.
> >
> >  Therefore at very minimum, clients
> > need to allocate buffers that can be used with the primary device. We
> > guarantee this in the zwp_linux_dmabuf protocol by having the
> > compositor test the buffer import into EGL (or equivalent) before it
> > accepts that the buffer even exists. The client does not absolutely
> > necessarily need the primary device for this, but it will have much
> > better chances of making usable buffers if it uses it for allocation at
> > least.
> So the client must provide buffers that the primary device can import
> and sample a texture from, ideally directly.
> Can something like this be added to the interface description, to make
> it clear what the primary device actually is supposed to be in this
> context?

This seems sensible, I'll do that.

> > The primary device also has another very different meaning: the
> > compositor will likely be using the primary device anyway so it is kept
> > active and if clients use the same device instead of some other device,
> > it probably results in considerable power savings. IOW, the primary
> > device is the preferred rendering device as well. Or so I assume, these
> > two concepts could be decoupled as well.
> And the client should default to using the same primary device for
> rendering for power savings.

Will be in the next version, but with "can" instead of "should", because some
clients (games with DRI_PRIME) might want to use another device to get better

> > A secondary device is optional. In system where the GPU and display
> > devices are separate DRM devices, the GPU will be the primary device,
> > and the display device would be the secondary device. So there seems to
> > be a use case for sending the secondary device (or devices?) in
> > addition to the primary device.
> >
> > AFAIK, the unix device memory allocator project does not yet have
> > anything we should be encoding as a Wayland extension, so all we seem
> > to be able to do is to deliver the device file descriptors and the
> > format+modifier sets.
> Ok.
> > Now the design question: do we want to communicate the secondary
> > devices in this extension? Quite likely we need a different extension
> > to be used with the allocator project.
> As long as the use case is not clear, I'd say leave it out.
> A "secondary_device" event may be added later with a version update if
> needed.

Yes, I agree, I'd prefer not having this in the protocol for now.

> > My current opinion is that if there is no generic way for an
> > application to benefit from the secondary device fd, then we should not
> > add secondary devices in this extension yet.
> I agree.


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