[Mesa-dev] [PATCH RFC 0/2] GBM API extension to support fusing KMS and render devices

[Daniel Vetter]

On Fri, Mar 04, 2016 at 06:34:37PM +0000, Emil Velikov wrote:
> On 4 March 2016 at 17:38, Lucas Stach <l.stach at pengutronix.de> wrote:
> > Am Freitag, den 04.03.2016, 17:20 +0000 schrieb Daniel Stone:
> >> Hi,
> >>
> >> On 4 March 2016 at 16:08, Lucas Stach <l.stach at pengutronix.de> wrote:
> >> > Am Freitag, den 04.03.2016, 15:09 +0000 schrieb Daniel Stone:
> >> >> Thanks for taking this on, it looks really good! I just have the one
> >> >> question though - did you look at the EGLDevice extension? Using that
> >> >> to enumerate the GPUs, we could create the gbm_device using the KMS
> >> >> device and pass that in to the EGLDisplay, with an additional attrib
> >> >> to pass in an EGLDevice handle to eglGetPlatformDisplay. This could
> >> >> possibly be better since it is more independent of DRM as the API, and
> >> >> also allows people to share device enumeration/selection code with
> >> >> other platforms (e.g. choosing between multiple GPUs when using a
> >> >> winsys like Wayland or X11).
> >> >>
> >> > I have not looked at this in detail yet, but I think it's just an
> >> > extension to the interface outlined by this series.
> >> >
> >> > If we require the KMS device to have a DRI2/Gallium driver it should be
> >> > easy to hook up the EGLDevice discovery for them.
> >> > Passing in a second device handle for the KMS device is then just the
> >> > EGL implementation calling gbm_device_set_kms_provider() on the render
> >> > GBM device, instead of the application doing it manually.
> >>
> >> It turns the API backwards a bit though ...
> >>
> >> Right now, what we require is that the GBM device passed in is the KMS
> >> device, not the GPU device; what you're suggesting is that we discover
> >> the GPU device and then add the KMS device.
> >>
> >> So, with your proposal:
> >> gbm_gpu = gbm_device_create("/dev/dri/renderD128");
> >> egl_dpy = eglGetDisplay(gbm_gpu);
> >> gbm_kms = gbm_device_create("/dev/dri/card0");
> >> gbm_device_set_kms_provider(gbm_gpu, gbm_kms);
> >>
> >> i.e. the device the user creates first is the GPU device.
> >>
> >> With EGLDevice, we would have:
> >> gbm_kms = gbm_device_create("/dev/dri/card0");
> >> egl_gpus = eglGetDevicesEXT();
> >> egl_dpy = eglGetPlatformDisplay(gbm_kms, { EGL_TARGET_DEVICE, egl_gpus[0] });
> >>
> >> So, the first/main device the user deals with is the KMS device - same
> >> as today. This makes sense, since GBM is the allocation API for KMS,
> >> and EGL should be the one dealing with the GPU ...
> >>
> > Right, my API design was from my view of GBM being the API to bootstrap
> > EGL rendering, but defining it as the KMS allocation API makes a lot
> > more sense, when you think about it.
> >
> >> Maybe it would make sense to reverse the API, so rather than creating
> >> a GBM device for the GPU and then linking that to the KMS device -
> >> requiring users to make different calls, e.g. gbm_bo_get_kms_bo(),
> >> which makes it harder to use and means we need to port current users -
> >> we create a GBM device for KMS and then link that to a GPU device.
> >> This would then mean that eglGetPlatformDisplay could do the linkage
> >> internally, and then existing users using gbm_bo_get_handle() etc
> >> would still work without needing any different codepaths.
> >
> > Yes, this will make the implementation inside GBM a bit more involved,
> > but it seems more natural this way around when thinking about hooking it
> > up to EGLDevice. I'll try it out and send an updated RFC after the
> > weekend.
> >
> While I'm more inclined to Daniel's suggestion, I wonder why people
> moved away from Thierry's approach - creating a composite/wrapped dri
> module ? Is there anything wrong with it - be that from technical or
> conceptual POV ?
> 
> I believe it has a few advantages over the above two proposals - it
> allows greater flexibility as both drivers will be tightly coupled and
> can communicate directly, does not expand the internal/hidden ABI that
> we currently have between GBM and EGL, could (in theory) work with
> GLX.

I think composite driver makes sense if you e.g. have some 2d blitter on
the display block and a 3d gpu and want to make them one thing (and use
the blitter to shuffle bytes around for uploads). Wrt the internal API I'm
not concerned too much, since in the end all that coordination is the
exact same thing we need to add for compositor/client communication too.
They need to agree on what is most suitable as the frontbuffer format and
how/where to allocate it, otherwise you'll suffer tons of unnecessary
copies. In short we need much more powerful "what buffers can you
support/prefer" interfaces anyway, not just for kms/gpu dual drm device
support in gbm. And I actually think prototyping those in gbm is a great
idea, gets rid of the wayland/X proto complexities.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch