[Mesa-dev] [PATCHES] surfaceless EGL: new DRI extension

[Kristian Høgsberg]

On Sun, Aug 29, 2010 at 1:54 PM, nobled <nobled at dreamwidth.org> wrote:
> The current egl_dri2 code for enabling the EGL_KHR_surfaceless_*
> extensions is a bit broken right now. First, the code in dri_util.c
> and drisw_util.c doesn't check if it gets passed a NULL __DRIconfig*,
> leading to segfaults when egl_dri2 does exactly that when creating a
> surfaceless context.

Which entrypoints don't check for NULL?  The DRI1 (legacy) isn't
supposed to work with surfaceless, it just doesn't make sense.

> Second, egl_dri2's logic for deciding if it *can* create a surfaceless
> context doesn't really do that:
>
>   if (dri2_dpy->dri2->base.version >= 2) {
>      disp->Extensions.KHR_surfaceless_gles1 = EGL_TRUE;
>      disp->Extensions.KHR_surfaceless_gles2 = EGL_TRUE;
>      disp->Extensions.KHR_surfaceless_opengl = EGL_TRUE;
>   }
>
> But the dri2 extension's version 2 doesn't actually mean "this driver
> can create and bind contexts without drawables"--and right now only
> the intel drivers can.

No, that is actually one of the implied features of version 2 of the
DRI2 extension: it gives you multi-api support and surfaceless
contexts.  We may be advertising that more than we should, and it
looks like we should add a driDRI2v1Extension to dri_util.c for
drivers that can't advertise v2 yet.  Of course, I should have
documented this in dri_interface.h.

> There should be a new extension to advertise
> that capability, like the __DRI_READ_DRAWABLE extension.*
>
> This patch series a) adds an extension for just that:
> __DRI_NO_DRAWABLE, b) advertises it in the intel driver, and c)
> implements it in swrast and all gallium drivers through st/dri.
>
> http://github.com/nobled/mesa/commits/surfaceless
> 1. dri_interface.h: Add __DRI_NO_DRAWABLE extension
> 2. egl_dri2: Require __DRI_NO_DRAWABLE extension for surfaceless EGL
> 3. dri: Really enable EGL_KHR_surfaceless_* extensions in dri2 (fixes
> the segfaults)
> 4. dri: Enable surfaceless extensions on DRI legacy
> 5. dri: Provide __DRI_NO_DRAWABLE extension in dri_util
> 6. intel: Advertise __DRI_NO_DRAWABLE extension
> 7. dri: Allow EGL_KHR_surfaceless_* in software drivers
> 8. dri: Provide __DRI_NO_DRAWABLE extension in drisw_util
> 9. swrast: Implement __DRI_NO_DRAWABLE extension (this patch I'm least
> sure of; review?)
> 10. st/dri: Implement __DRI_NO_DRAWABLE extension (builds on Chia-I
> Wu's 41c095bf31cedf4f463e315e8dbad8a007985464 patch)
>
> *BTW, what exactly does __DRI_READ_DRAWABLE require/do? Only GLX's
> dri1 code checks it, to enable "GLX_SGI_make_current_read" (which
> dri2_glx enables unconditionally), and it's in all the hardware
> drivers but not in the swrast driver for some reason.

__DRI_READ_DRAWALBE is only useful for DRI1 drivers, where support for
GLX_SGI_make_current_read is optional.  If you're a DRI2 driver,
support for __DRI_READ_DRAWABLE is implied (similar to how
__DRI_NO_DRAWABLE is implied for DRI2v2).

Kristian