Re: [PATCH v9 0/9] Adjust fbcon console device detection

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Jul 17, 2025 at 03:56:58PM -0500, Bjorn Helgaas wrote:
> On Thu, Jul 17, 2025 at 12:38:03PM -0500, Mario Limonciello wrote:
> > From: Mario Limonciello <mario.limonciello@amd.com>
> > 
> > Systems with more than one GPU userspace doesn't know which one to be
> > used to treat as primary.  The concept of primary is important to be
> > able to decide which GPU is used for display and  which is used for
> > rendering.  If it's guessed wrong then both GPUs will be kept awake
> > burning a lot of power.
> > 
> > Historically it would use the "boot_vga" attribute but this isn't
> > present on modern GPUs.
> > 
> > This series started out as changes to VGA arbiter to try to handle a case
> > of a system with 2 GPUs that are not VGA devices and avoid changes to
> > userspace.  This was discussed but decided not to overload the VGA arbiter
> > for non VGA devices.
> > 
> > Instead move the x86 specific detection of framebuffer resources into x86
> > specific code that the fbcon can use to properly identify the primary
> > device. This code is still called from the VGA arbiter, and the logic does
> > not change there. To avoid regression default to VGA arbiter and only fall
> > back to looking up with x86 specific detection method.
> > 
> > In order for userspace to also be able to discover which device was the
> > primary video display device create a new sysfs file 'boot_display'.
> > 
> > A matching userspace implementation for this file is available here:
> > Link: https://gitlab.freedesktop.org/xorg/lib/libpciaccess/-/merge_requests/39
> > Link: https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2038
> > 
> > Dave Airlie has been pinged for a comment on this approach.
> > Dave had suggested in the past [1]:
> > 
> > "
> >  But yes if that doesn't work, then maybe we need to make the boot_vga
> >  flag mean boot_display_gpu, and fix it in the kernel
> > "
> > 
> > This was one of the approached tried in earlier revisions and it was
> > rejected in favor of creating a new sysfs file (which is what this
> > version does).
> > 
> > It is suggested that this series merge entirely through the PCI tree.
> > 
> > Link: https://gitlab.freedesktop.org/xorg/lib/libpciaccess/-/merge_requests/37#note_2938602 [1]
> > 
> > v9:
> >  * Add more to cover letter
> >  * Add bug link to last patch
> >  * Update commit message for last patch
> >  * Update boot_display documentation description
> > 
> > Mario Limonciello (9):
> >   PCI: Add helper for checking if a PCI device is a display controller
> >   vfio/pci: Use pci_is_display()
> >   vga_switcheroo: Use pci_is_display()
> >   iommu/vt-d: Use pci_is_display()
> >   ALSA: hda: Use pci_is_display()
> >   Fix access to video_is_primary_device() when compiled without
> >     CONFIG_VIDEO
> >   PCI/VGA: Replace vga_is_firmware_default() with a screen info check
> >   fbcon: Use screen info to find primary device
> >   PCI: Add a new 'boot_display' attribute
> > 
> >  Documentation/ABI/testing/sysfs-bus-pci |  9 +++++
> >  arch/parisc/include/asm/video.h         |  2 +-
> >  arch/sparc/include/asm/video.h          |  2 ++
> >  arch/x86/include/asm/video.h            |  2 ++
> >  arch/x86/video/video-common.c           | 17 ++++++++-
> >  drivers/gpu/vga/vga_switcheroo.c        |  2 +-
> >  drivers/iommu/intel/iommu.c             |  2 +-
> >  drivers/pci/pci-sysfs.c                 | 46 +++++++++++++++++++++++++
> >  drivers/pci/vgaarb.c                    | 31 +++--------------
> >  drivers/vfio/pci/vfio_pci_igd.c         |  3 +-
> >  include/linux/pci.h                     | 15 ++++++++
> >  sound/hda/hdac_i915.c                   |  2 +-
> >  sound/pci/hda/hda_intel.c               |  4 +--
> >  13 files changed, 102 insertions(+), 35 deletions(-)
> 
> Applied to pci/boot-display for v6.17, thanks!

I kept the pci_is_display() changes but deferred the following for now:

  Fix access to video_is_primary_device() when compiled without CONFIG_VIDEO
  PCI/VGA: Replace vga_is_firmware_default() with a screen info check
  fbcon: Use screen info to find primary device
  PCI: Add a new 'boot_display' attribute

I think the boot_display attribute isn't quite baked yet and I don't
want to add something when it looks like we're immediately going to
change the implementation and maybe the sysfs location.

Bjorn