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

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Message-ID: <20250717205658.GA2654514@bhelgaas>
Date: Thu, 17 Jul 2025 15:56:58 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Mario Limonciello <superm1@...nel.org>
Cc: David Airlie <airlied@...il.com>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Alex Deucher <alexander.deucher@....com>,
	Christian König <christian.koenig@....com>,
	Simona Vetter <simona@...ll.ch>, Lukas Wunner <lukas@...ner.de>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Woodhouse <dwmw2@...radead.org>,
	Lu Baolu <baolu.lu@...ux.intel.com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>,
	"open list:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:INTEL IOMMU (VT-d)" <iommu@...ts.linux.dev>,
	"open list:PCI SUBSYSTEM" <linux-pci@...r.kernel.org>,
	"open list:VFIO DRIVER" <kvm@...r.kernel.org>,
	"open list:SOUND" <linux-sound@...r.kernel.org>,
	Daniel Dadap <ddadap@...dia.com>,
	Mario Limonciello <mario.limonciello@....com>
Subject: Re: [PATCH v9 0/9] Adjust fbcon console device detection

On Thu, Jul 17, 2025 at 12:38:03PM -0500, Mario Limonciello wrote:
> From: Mario Limonciello <mario.limonciello@....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!