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

Pekka Paalanen ppaalanen at gmail.com
Mon Nov 12 09:18:16 UTC 2018

On Sat, 10 Nov 2018 13:34:31 +0000
Simon Ser <contact at emersion.fr> wrote:

> On Monday, November 5, 2018 9:57 AM, Pekka Paalanen <ppaalanen at gmail.com> wrote:
> > > > Yeah. Another option is to send a wl_array of modifiers per format and
> > > > tranch.  
> > >
> > > True. Any reason why this hasn't been done in the global?  
> >
> > For formats? Well, it is simpler without a wl_array, and there might be
> > a lot of formats.
> >
> > Could there be a lot of modifiers per format? Would a wl_array make
> > anything easier? Just a thought.  
> It's true that for this list of formats sorted by preference, we'll probably
> need to split modifiers anyway so I don't think we'd benefit a lot from this
> approach.

Hi Simon,

just to be clear, I was thinking of something like:

event(uint format, wl_array(modifiers))

But I definitely do not insist on it if you don't see any obvious
benefits with it.

It seems you and I made very different assumptions on how the hints
would be sent, I only realized it just now. More about that below.

> > > > I suppose it will be enough to send tranches for just the currently
> > > > used format? Otherwise it could be "a lot" of data.  
> > >
> > > What do you mean by "the currently used format"?  
> >
> > This interface is used to send clients hints after they are already
> > presenting, which means they already have a format chosen and probably
> > want to stick with it, just changing the modifiers to be more optimal.  
> If we only send the modifiers for the current format, how do clients tell the
> difference between the initial hints (which don't have a "currently used
> format") and the subsequent hints?

I'm not sure I understand why they would need to see the difference.
But yes, I was short-sighted here and didn't consider the
initialization when a surface is not mapped yet. I didn't expect that
hints can be calculated if the surface is not mapped, but of course a
compositor can provide some defaults. I suppose the initial default
hints would boil down to what is most efficient to composite.

> > > I expect clients to bind to this interface and create a surface hints object
> > > before the surface is mapped. In this case there's no "currently used format".  
> >
> > Right, that's another use case.
> >  
> > > It will be a fair amount of data, yes. However it's just a list of integers.
> > > When we send strings over the protocol (e.g. toplevel title in xdg-shell) it's
> > > about the same amount of data I guess.  
> >
> > If the EGLConfig or GLXFBConfig or GLX visual lists are of any
> > indication... yes, they account for depth, stencil, aux, etc. but then
> > we will have modifiers.
> >
> > We already advertise the list of everything supported of format+modifer
> > in the linux_dmabuf extension. Could we somehow minimize the number of
> > recommended format+modifiers in hints? Or maybe that's not a concern
> > for the protocol spec?  
> I'm not sure.
> After this patch, I'm not even sure how the formats+modifiers advertised by the
> global work. Are these formats+modifiers supported on the GPU the compositor
> uses for rendering? Intersection or union of formats+modifiers supported on all
> GPUs?

The format+modifier advertised by the global before this patch are the
ones that can work at all, or the compositor is willing to make them
work at least in the worst fallback case. This patch must not change
that meaning. These formats also must always work regardless of which
GPU a client decides to use, but that is already implied by the
compositor being able to import a dmabuf. The compositor does not need
to try to factor in what other GPUs on the system might be able to
render or not, that is for the client to figure out when it knows the
formats the compositor can accept and is choosing a GPU to render with.
It is theoretically possible that a client tries to use a GPU that
cannot render any formats the compositor can use, but that is the
client's responsibility to figure out.

So clearly the formats from the global can be used by a client at any
time. The hint formats OTOH has no reason to list absolutely
everything the compositor supports, but a compositor can choose on its
own judgement to send only a sub-set it would prefer.

However, after a client has picked a format and used it, then there
should be hints with that format, at least if they can make any

I'm not sure. Not listing everything always was my intuitive
assumption, and I believe you perhaps assumed the opposite so that a
client has absolutely all the information to e.g. optimize the modifier
of a format that the compositor would not prefer at all even though it
does work.

It would be simpler to always send everything, but that will be much
more protocol traffic. Would it be too much? I don't know, could you
calculate some examples of how many bytes a typical hints update would
be if sending everything always?

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