Re: [PATCH] xf86drm: add drmOpenByFB - Chih-Wei Huang

From: Chih-Wei Huang <cwhuang@linux.org.tw>
To: Simon Ser <contact@emersion.fr>
Cc: Robert Foss <robert.foss@collabora.com>,
	Mauro Rossi <issor.oruam@gmail.com>,
	Emil Velikov <emil.l.velikov@gmail.com>,
	dri-devel <dri-devel@lists.freedesktop.org>
Subject: Re: [PATCH] xf86drm: add drmOpenByFB
Date: Thu, 28 May 2020 17:46:08 +0800	[thread overview]
Message-ID: <CAHUn7wPfaEuWAc7iDGtfCdT71dwYkt7fNgMdPtfZL=COmKBaiQ@mail.gmail.com> (raw)
In-Reply-To: <2xZF5q7722pK-_L_0jQjYv33oMS6WS4Jm06tbVwUo9SQO9QBv48_HaVhzVHTb2OLnz3JUj3O1uRYn_y-JWGrWpD3c8J0y8Mr_qgwbNzsxDE=@emersion.fr>

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 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.
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