[PATCH] xf86drm: add drmOpenByFB

Alex Deucher alexdeucher at gmail.com
Fri May 29 14:20:23 UTC 2020

On Fri, May 29, 2020 at 3:49 AM Pekka Paalanen <ppaalanen at gmail.com> wrote:
> On Thu, 28 May 2020 17:46:08 +0800
> Chih-Wei Huang <cwhuang at linux.org.tw> wrote:
> > The main problem we're trying to solve is to
> > find the DRM device of the primary framebuffer (fb0).
> Hi,
> I would say that is a completely wrong starting point. Please, let
> fbdev die in peace/pieces. Do not make any new code that relies on
> fbdev existing.
> Why do you think you want to follow the setup fbdev has? How do you
> know fb0 is the device you want and not fb1? How do you guarantee that
> fb0 is the device you want?
> "It was right on where I tested it" is no guarantee if you do not
> understand how it actually is chosen under the hood. If you know how it
> is chosen under the hood, you can do the same yourself without relying
> on deprecated stuff (fbdev).
> It is fbdev that should follow the DRM setup, not pick a DRM device
> based on fbdev.
> People already mentioned looking at device properties via libudev API.
> If you cannot have libudev (I believe it does not need udev daemon,
> btw.), then you can look at the same information directly in sysfs. Use
> the information about the DRM devices themselves from sysfs to decide
> which one to pick, and never look at fbdev anything. Or polish the
> patch Emil proposed if boot_vga indeed matches what you actually want
> to find.
> I think Daniel Vetter explained nicely what boot_vga means. Is your
> problem that the device may not be a PCI device, but a platform device
> for instance?
> Display servers use heuristics, for example if no device has boot_vga,
> then pick the platform device (since there usually is only one with KMS
> capabilities). Here are couple of examples.
> Weston wants a device with KMS capabilities, because it currently
> doesn't support using more than one KMS device and it also doesn't
> explicitly support a separate render device:
> https://gitlab.freedesktop.org/wayland/weston/-/blob/8.0.0/libweston/backend-drm/drm.c#L2546-2631
> Mutter is primarily looking for a hardware rendering capable device,
> because it can use any number of KMS devices in addition to a rendering
> device, separate or same device:
> https://gitlab.gnome.org/GNOME/mutter/-/blob/3.37.1/src/backends/native/meta-renderer-native.c#L3904-3953
> Neither is probably perfect, but they are totally better than picking
> based on fb0.

As an example, on my system with multiple GPUs, the one that the bios
lights up is the last one Linux enumerates when loading the GPU


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