Re: [PATCH] xf86drm: add drmOpenByFB

[Emil Velikov <emil.l.velikov@gmail.com>]

On Thu, 28 May 2020 at 10:46, /wrote:
>
> Simon Ser <contact@emersion.fr> 於 2020年5月25日 週一 上午3:25寫道：
> > On Sunday, May 24, 2020 8:53 PM, Daniel Vetter <daniel@ffwll.ch> wrote:
> > > On Sat, May 23, 2020 at 5:44 PM Mauro Rossi issor.oruam@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.
>
> Thank you for the reply.
> I'm not the expert so
> I can't fully understand what you mean.
> What does 'boot_vga' mean and how could it help our problem?
>
The boot_vga is a flag which indicating which VGA (graphics) device
the system booted from.

> The main problem we're trying to solve is to
> find the DRM device of the primary framebuffer (fb0).
> I believe /sys/class/graphics/fb0/device/drm
> is the correct one we could use.
> At least it works on all devices I've tested
> including nvidia optimus notebooks.
> If there is a simpler way to get the DRM device of fb0
> I'm happy to learn.
>
> > Yes. See [1] for an example of how to do this.
> >
> > [1]: https://github.com/swaywm/wlroots/blob/5c942bd5972afee9a68cb15c14aa83b4b0aaf82d/backend/session/session.c#L331
>
> Thank you for the example.
> However, our target platform is Android and
> Android doesn't have udev. So I'm afraid we can't use it.

Indeed - udev isn't as wide-spread as it could have been. A quick WIP
should be in your inbox.
It implements boot_vga support for drmDevice.

Do give it some test/love and resubmit once it works on your end.

-Emil
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