[PATCH] xf86drm: add drmOpenByFB

[Simon Ser]

On Sunday, May 24, 2020 8:53 PM, Daniel Vetter <daniel at ffwll.ch> wrote:

> On Sat, May 23, 2020 at 5:44 PM Mauro Rossi issor.oruam at gmail.com wrote:
>
> > OpenByFB is introduced to overcome GPU driver loading order issue
> > on a device with multiple GPUs, e.g. Intel iGPU and Nvidia dGPU
> > where the first drmfb kernel module loaded will become device file
> > /dev/dri/card0 and the second will become /dev/dri/card1
> > The use case is to prefer Intel iGPU over dGPU, or viceversa,
> > in a deterministic and reliable manner.
> > OpenByFB function opens the DRM device with specified fb and node type,
> > thus enabling gralloc to open the correct device node of the primary fb,
> > regardless of the GPU driver loading order.
>
> The fbdev load ordering is as inconsistent/random as the dri node
> loading. Well more so, because you might have random firmware fbdev
> drivers hanging out there. Hence I'm not following how this solves
> anything for your problem.
>
> I think usually what userspace does it look at the boot_vga property
> of the underlying device in sysfs, and prefer that one.

Yes. See [1] for an example of how to do this.

[1]: https://github.com/swaywm/wlroots/blob/5c942bd5972afee9a68cb15c14aa83b4b0aaf82d/backend/session/session.c#L331