[Mesa-dev] [PATCH EGL/MESA] EGL/mesa: Initial writeup for MESA_query_renderer

[Adam Jackson]

On Thu, 2018-11-08 at 02:42 +0530, Veluri Mithun wrote:
> Hi all,
> Thanks Adam for your interest in this.
> 
> > > +New Tokens
> > > +
> > > +    Accepted as an <attribute> in EGLQueryRendererIntegerMESA and
> > > +    EGLQueryCurrentRendererIntegerMESA:
> > > +
> > > +        EGL_RENDERER_VENDOR_ID_MESA                      0xXXXX
> > > +        EGL_RENDERER_DEVICE_ID_MESA                      0xXXXX
> 
>  
> except the above 2 tokens we aren't using any tokens in adriconf(
> https://github.com/jlHertel/adriconf/blob/master/DRIQuery.cpp#L46) If
> we don't need all the below ones, I'll remove them.

adriconf is not the only user of GLX_MESA_query_renderer. glxinfo uses
it and prints everything it can query, eglinfo would probably want to
do likewise.

> > The corresponding eglQueryCurrentRendererIntegerMESA is not documented.
> > I'm not entirely sure it should even exist, to be honest; I'd prefer if
> > these attributes were instead newly legal values to pass to
> > eglQueryContext. Too late to make that change for GLX I suppose.
>  
> What is the difference between EGLContext and EGLSurface?

Surfaces are window-system objects you can draw to. Contexts are
objects that describe how you'll draw (shader pipeline, blend modes,
etc.) The "current renderer" is the renderer associated with the
current context. Which means a call like:

    eglQueryCurrentRendererIntegerMESA(EGL_RENDERER_VERSION_MESA,
                                       &value);

Would be equivalent to:

    eglQueryContext(eglGetCurrentDisplay(),
                    eglGetCurrentContext(),
                    EGL_RENDERER_VERSION_MESA,
                    &value);

Which is why I said QueryCurrentRendererInteger is redundant. The other
three are not redundant.

> If you look at the usage of query fun in adriocnf(
> https://github.com/jlHertel/adriconf/blob/master/DRIQuery.cpp#L43).
> we are iterating with all the available screens. In Wayland do we
> need to iterate through contexts??

No; the question doesn't really make sense. GLX has screens only
because X11 has screens, they're sub-objects of the display. An
EGLDisplay corresponds to an X11 display+screen combination, so there's
nothing to iterate over.

Contexts aren't pre-existing objects like screens, they're things the
application explicitly creates. Notice that adriconf does not create a
context at all. This is sort of the point of GLX_MESA_query_renderer:
most of the information it supplies _could_ be discovered by querying
an existing context, but you'd rather know what kind of context you
_can_ create before you do it.
 
> @Jean Hertel In the above provided link, why are we passing 3rd
> argument(renderer) as 0 for all the screens?

As I said in my previous email: the GLX extension spec claims to allow
multiple renderers for a given display (and even to force a binding to
a particular one at creation time), but Mesa does not in fact implement
that, and it would be a difficult feature to use in any case because
there's no way to query how many renderers a display+screen actually
has.

- ajax