[PATCH v5 0/9] Adjust fbcon console device detection

[PATCH v5 0/9] Adjust fbcon console device detection

[Mario Limonciello]

From: Mario Limonciello <mario.limonciello@amd.com>

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 [1].  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:
https://gitlab.freedesktop.org/xorg/lib/libpciaccess/-/merge_requests/39
https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2038

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           | 13 ++++++++++-
 drivers/gpu/vga/vga_switcheroo.c        |  2 +-
 drivers/iommu/intel/iommu.c             |  2 +-
 drivers/pci/pci-sysfs.c                 | 14 ++++++++++++
 drivers/pci/vgaarb.c                    | 29 ++-----------------------
 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, 63 insertions(+), 36 deletions(-)

-- 
2.43.0

[PATCH v5 1/9] PCI: Add helper for checking if a PCI device is a display controller

[Mario Limonciello]

From: Mario Limonciello <mario.limonciello@amd.com>

Several places in the kernel do class shifting to match whether a
PCI device is display class.  Introduce a helper for those places to
use.

Reviewed-by: Daniel Dadap <ddadap@nvidia.com>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
---
 include/linux/pci.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/include/linux/pci.h b/include/linux/pci.h
index 05e68f35f3923..e77754e43c629 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -744,6 +744,21 @@ static inline bool pci_is_vga(struct pci_dev *pdev)
 	return false;
 }
 
+/**
+ * pci_is_display - Check if a PCI device is a display controller
+ * @pdev: Pointer to the PCI device structure
+ *
+ * This function determines whether the given PCI device corresponds
+ * to a display controller. Display controllers are typically used
+ * for graphical output and are identified based on their class code.
+ *
+ * Return: true if the PCI device is a display controller, false otherwise.
+ */
+static inline bool pci_is_display(struct pci_dev *pdev)
+{
+	return (pdev->class >> 16) == PCI_BASE_CLASS_DISPLAY;
+}
+
 #define for_each_pci_bridge(dev, bus)				\
 	list_for_each_entry(dev, &bus->devices, bus_list)	\
 		if (!pci_is_bridge(dev)) {} else
-- 
2.43.0

[PATCH v5 2/9] vfio/pci: Use pci_is_display()

[Mario Limonciello]

From: Mario Limonciello <mario.limonciello@amd.com>

The inline pci_is_display() helper does the same thing.  Use it.

Acked-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Daniel Dadap <ddadap@nvidia.com>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Suggested-by: Bjorn Helgaas <helgaas@kernel.org>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
---
 drivers/vfio/pci/vfio_pci_igd.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/vfio/pci/vfio_pci_igd.c b/drivers/vfio/pci/vfio_pci_igd.c
index ef490a4545f48..988b6919c2c31 100644
--- a/drivers/vfio/pci/vfio_pci_igd.c
+++ b/drivers/vfio/pci/vfio_pci_igd.c
@@ -437,8 +437,7 @@ static int vfio_pci_igd_cfg_init(struct vfio_pci_core_device *vdev)
 
 bool vfio_pci_is_intel_display(struct pci_dev *pdev)
 {
-	return (pdev->vendor == PCI_VENDOR_ID_INTEL) &&
-	       ((pdev->class >> 16) == PCI_BASE_CLASS_DISPLAY);
+	return (pdev->vendor == PCI_VENDOR_ID_INTEL) && pci_is_display(pdev);
 }
 
 int vfio_pci_igd_init(struct vfio_pci_core_device *vdev)
-- 
2.43.0

[PATCH v5 3/9] vga_switcheroo: Use pci_is_display()

[Mario Limonciello]

From: Mario Limonciello <mario.limonciello@amd.com>

The inline pci_is_display() helper does the same thing.  Use it.

Reviewed-by: Daniel Dadap <ddadap@nvidia.com>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Suggested-by: Bjorn Helgaas <helgaas@kernel.org>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
---
 drivers/gpu/vga/vga_switcheroo.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/vga/vga_switcheroo.c b/drivers/gpu/vga/vga_switcheroo.c
index 18f2c92beff8e..68e45a26e85f7 100644
--- a/drivers/gpu/vga/vga_switcheroo.c
+++ b/drivers/gpu/vga/vga_switcheroo.c
@@ -437,7 +437,7 @@ find_active_client(struct list_head *head)
  */
 bool vga_switcheroo_client_probe_defer(struct pci_dev *pdev)
 {
-	if ((pdev->class >> 16) == PCI_BASE_CLASS_DISPLAY) {
+	if (pci_is_display(pdev)) {
 		/*
 		 * apple-gmux is needed on pre-retina MacBook Pro
 		 * to probe the panel if pdev is the inactive GPU.
-- 
2.43.0

[PATCH v5 4/9] iommu/vt-d: Use pci_is_display()

[Mario Limonciello]

From: Mario Limonciello <mario.limonciello@amd.com>

The inline pci_is_display() helper does the same thing.  Use it.

Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Daniel Dadap <ddadap@nvidia.com>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Suggested-by: Bjorn Helgaas <helgaas@kernel.org>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
---
 drivers/iommu/intel/iommu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index 7aa3932251b2f..17267cd476ce7 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -34,7 +34,7 @@
 #define ROOT_SIZE		VTD_PAGE_SIZE
 #define CONTEXT_SIZE		VTD_PAGE_SIZE
 
-#define IS_GFX_DEVICE(pdev) ((pdev->class >> 16) == PCI_BASE_CLASS_DISPLAY)
+#define IS_GFX_DEVICE(pdev) pci_is_display(pdev)
 #define IS_USB_DEVICE(pdev) ((pdev->class >> 8) == PCI_CLASS_SERIAL_USB)
 #define IS_ISA_DEVICE(pdev) ((pdev->class >> 8) == PCI_CLASS_BRIDGE_ISA)
 #define IS_AZALIA(pdev) ((pdev)->vendor == 0x8086 && (pdev)->device == 0x3a3e)
-- 
2.43.0

[PATCH v5 5/9] ALSA: hda: Use pci_is_display()

[Mario Limonciello]

From: Mario Limonciello <mario.limonciello@amd.com>

The inline pci_is_display() helper does the same thing.  Use it.

Reviewed-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Daniel Dadap <ddadap@nvidia.com>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Suggested-by: Bjorn Helgaas <helgaas@kernel.org>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
---
 sound/hda/hdac_i915.c     | 2 +-
 sound/pci/hda/hda_intel.c | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/sound/hda/hdac_i915.c b/sound/hda/hdac_i915.c
index e9425213320ea..44438c799f957 100644
--- a/sound/hda/hdac_i915.c
+++ b/sound/hda/hdac_i915.c
@@ -155,7 +155,7 @@ static int i915_gfx_present(struct pci_dev *hdac_pci)
 
 	for_each_pci_dev(display_dev) {
 		if (display_dev->vendor != PCI_VENDOR_ID_INTEL ||
-		    (display_dev->class >> 16) != PCI_BASE_CLASS_DISPLAY)
+		    !pci_is_display(display_dev))
 			continue;
 
 		if (pci_match_id(denylist, display_dev))
diff --git a/sound/pci/hda/hda_intel.c b/sound/pci/hda/hda_intel.c
index 439cf1bda6e66..75badb5c69b8e 100644
--- a/sound/pci/hda/hda_intel.c
+++ b/sound/pci/hda/hda_intel.c
@@ -1465,7 +1465,7 @@ static struct pci_dev *get_bound_vga(struct pci_dev *pci)
 				 * the dGPU is the one who is involved in
 				 * vgaswitcheroo.
 				 */
-				if (((p->class >> 16) == PCI_BASE_CLASS_DISPLAY) &&
+				if (pci_is_display(p) &&
 				    (atpx_present() || apple_gmux_detect(NULL, NULL)))
 					return p;
 				pci_dev_put(p);
@@ -1477,7 +1477,7 @@ static struct pci_dev *get_bound_vga(struct pci_dev *pci)
 			p = pci_get_domain_bus_and_slot(pci_domain_nr(pci->bus),
 							pci->bus->number, 0);
 			if (p) {
-				if ((p->class >> 16) == PCI_BASE_CLASS_DISPLAY)
+				if (pci_is_display(p))
 					return p;
 				pci_dev_put(p);
 			}
-- 
2.43.0

[PATCH v5 6/9] Fix access to video_is_primary_device() when compiled without CONFIG_VIDEO

[Mario Limonciello]

From: Mario Limonciello <mario.limonciello@amd.com>

When compiled without CONFIG_VIDEO the architecture specific
implementations of video_is_primary_device() include prototypes and
assume that video-common.c will be linked. Guard against this so that the
fallback inline implementation that returns false will be used when
compiled without CONFIG_VIDEO.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202506221312.49Fy1aNA-lkp@intel.com/
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
---
v5:
 * add tag
v4:
 * new patch
---
 arch/parisc/include/asm/video.h | 2 +-
 arch/sparc/include/asm/video.h  | 2 ++
 arch/x86/include/asm/video.h    | 2 ++
 3 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/parisc/include/asm/video.h b/arch/parisc/include/asm/video.h
index c5dff3223194a..a9d50ebd6e769 100644
--- a/arch/parisc/include/asm/video.h
+++ b/arch/parisc/include/asm/video.h
@@ -6,7 +6,7 @@
 
 struct device;
 
-#if defined(CONFIG_STI_CORE)
+#if defined(CONFIG_STI_CORE) && defined(CONFIG_VIDEO)
 bool video_is_primary_device(struct device *dev);
 #define video_is_primary_device video_is_primary_device
 #endif
diff --git a/arch/sparc/include/asm/video.h b/arch/sparc/include/asm/video.h
index a6f48f52db584..773717b6d4914 100644
--- a/arch/sparc/include/asm/video.h
+++ b/arch/sparc/include/asm/video.h
@@ -19,8 +19,10 @@ static inline pgprot_t pgprot_framebuffer(pgprot_t prot,
 #define pgprot_framebuffer pgprot_framebuffer
 #endif
 
+#ifdef CONFIG_VIDEO
 bool video_is_primary_device(struct device *dev);
 #define video_is_primary_device video_is_primary_device
+#endif
 
 static inline void fb_memcpy_fromio(void *to, const volatile void __iomem *from, size_t n)
 {
diff --git a/arch/x86/include/asm/video.h b/arch/x86/include/asm/video.h
index 0950c9535fae9..08ec328203ef8 100644
--- a/arch/x86/include/asm/video.h
+++ b/arch/x86/include/asm/video.h
@@ -13,8 +13,10 @@ pgprot_t pgprot_framebuffer(pgprot_t prot,
 			    unsigned long offset);
 #define pgprot_framebuffer pgprot_framebuffer
 
+#ifdef CONFIG_VIDEO
 bool video_is_primary_device(struct device *dev);
 #define video_is_primary_device video_is_primary_device
+#endif
 
 #include <asm-generic/video.h>
 
-- 
2.43.0

[PATCH v5 7/9] PCI/VGA: Replace vga_is_firmware_default() with a screen info check

[Mario Limonciello]

From: Mario Limonciello <mario.limonciello@amd.com>

vga_is_firmware_default() checks firmware resources to find the owner
framebuffer resources to find the firmware PCI device.  This is an
open coded implementation of screen_info_pci_dev().  Switch to using
screen_info_pci_dev() instead.

Suggested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
---
v5:
 * split from next patch
---
 drivers/pci/vgaarb.c | 29 ++---------------------------
 1 file changed, 2 insertions(+), 27 deletions(-)

diff --git a/drivers/pci/vgaarb.c b/drivers/pci/vgaarb.c
index 78748e8d2dbae..c3457708c01e3 100644
--- a/drivers/pci/vgaarb.c
+++ b/drivers/pci/vgaarb.c
@@ -556,34 +556,9 @@ EXPORT_SYMBOL(vga_put);
 
 static bool vga_is_firmware_default(struct pci_dev *pdev)
 {
-#if defined(CONFIG_X86)
-	u64 base = screen_info.lfb_base;
-	u64 size = screen_info.lfb_size;
-	struct resource *r;
-	u64 limit;
+	struct screen_info *si = &screen_info;
 
-	/* Select the device owning the boot framebuffer if there is one */
-
-	if (screen_info.capabilities & VIDEO_CAPABILITY_64BIT_BASE)
-		base |= (u64)screen_info.ext_lfb_base << 32;
-
-	limit = base + size;
-
-	/* Does firmware framebuffer belong to us? */
-	pci_dev_for_each_resource(pdev, r) {
-		if (resource_type(r) != IORESOURCE_MEM)
-			continue;
-
-		if (!r->start || !r->end)
-			continue;
-
-		if (base < r->start || limit >= r->end)
-			continue;
-
-		return true;
-	}
-#endif
-	return false;
+	return pdev == screen_info_pci_dev(si);
 }
 
 static bool vga_arb_integrated_gpu(struct device *dev)
-- 
2.43.0

[PATCH v5 8/9] fbcon: Use screen info to find primary device

[Mario Limonciello]

From: Mario Limonciello <mario.limonciello@amd.com>

On systems with non VGA GPUs fbcon can't find the primary GPU because
video_is_primary_device() only checks the VGA arbiter.

Add a screen info check to video_is_primary_device() so that callers
can get accurate data on such systems.

Suggested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
---
v5:
 * Only change video-common.c
v4:
 * use helper
---
 arch/x86/video/video-common.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/video/video-common.c b/arch/x86/video/video-common.c
index 81fc97a2a837a..917568e4d7fb1 100644
--- a/arch/x86/video/video-common.c
+++ b/arch/x86/video/video-common.c
@@ -9,6 +9,7 @@
 
 #include <linux/module.h>
 #include <linux/pci.h>
+#include <linux/screen_info.h>
 #include <linux/vgaarb.h>
 
 #include <asm/video.h>
@@ -27,6 +28,7 @@ EXPORT_SYMBOL(pgprot_framebuffer);
 
 bool video_is_primary_device(struct device *dev)
 {
+	struct screen_info *si = &screen_info;
 	struct pci_dev *pdev;
 
 	if (!dev_is_pci(dev))
@@ -34,7 +36,16 @@ bool video_is_primary_device(struct device *dev)
 
 	pdev = to_pci_dev(dev);
 
-	return (pdev == vga_default_device());
+	if (!pci_is_display(pdev))
+		return false;
+
+	if (pdev == vga_default_device())
+		return true;
+
+	if (pdev == screen_info_pci_dev(si))
+		return true;
+
+	return false;
 }
 EXPORT_SYMBOL(video_is_primary_device);
 
-- 
2.43.0

[PATCH v5 9/9] PCI: Add a new 'boot_display' attribute

[Mario Limonciello]

From: Mario Limonciello <mario.limonciello@amd.com>

On systems with multiple GPUs there can be uncertainty which GPU is the
primary one used to drive the display at bootup. In order to disambiguate
this add a new sysfs attribute 'boot_display' that uses the output of
video_is_primary_device() to populate whether a PCI device was used for
driving the display.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
---
v4:
 * new patch
---
 Documentation/ABI/testing/sysfs-bus-pci |  9 +++++++++
 drivers/pci/pci-sysfs.c                 | 14 ++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
index 69f952fffec72..897cfc1b0de0f 100644
--- a/Documentation/ABI/testing/sysfs-bus-pci
+++ b/Documentation/ABI/testing/sysfs-bus-pci
@@ -612,3 +612,12 @@ Description:
 
 		  # ls doe_features
 		  0001:01        0001:02        doe_discovery
+
+What:		/sys/bus/pci/devices/.../boot_display
+Date:		October 2025
+Contact:	Linux PCI developers <linux-pci@vger.kernel.org>
+Description:
+		This file indicates whether the device was used as a boot
+		display. If the device was used as the boot display, the file
+		will contain "1". If the device is a display device but wasn't
+		used as a boot display, the file will contain "0".
diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
index 268c69daa4d57..5bbf79b1b953d 100644
--- a/drivers/pci/pci-sysfs.c
+++ b/drivers/pci/pci-sysfs.c
@@ -30,6 +30,7 @@
 #include <linux/msi.h>
 #include <linux/of.h>
 #include <linux/aperture.h>
+#include <asm/video.h>
 #include "pci.h"
 
 #ifndef ARCH_PCI_DEV_GROUPS
@@ -679,6 +680,13 @@ const struct attribute_group *pcibus_groups[] = {
 	NULL,
 };
 
+static ssize_t boot_display_show(struct device *dev, struct device_attribute *attr,
+				 char *buf)
+{
+	return sysfs_emit(buf, "%u\n", video_is_primary_device(dev));
+}
+static DEVICE_ATTR_RO(boot_display);
+
 static ssize_t boot_vga_show(struct device *dev, struct device_attribute *attr,
 			     char *buf)
 {
@@ -1698,6 +1706,7 @@ late_initcall(pci_sysfs_init);
 
 static struct attribute *pci_dev_dev_attrs[] = {
 	&dev_attr_boot_vga.attr,
+	&dev_attr_boot_display.attr,
 	NULL,
 };
 
@@ -1710,6 +1719,11 @@ static umode_t pci_dev_attrs_are_visible(struct kobject *kobj,
 	if (a == &dev_attr_boot_vga.attr && pci_is_vga(pdev))
 		return a->mode;
 
+#ifdef CONFIG_VIDEO
+	if (a == &dev_attr_boot_display.attr && pci_is_display(pdev))
+		return a->mode;
+#endif
+
 	return 0;
 }
 
-- 
2.43.0

Re: [PATCH v5 7/9] PCI/VGA: Replace vga_is_firmware_default() with a screen info check

[kernel test robot]

Hi Mario,

kernel test robot noticed the following build errors:

[auto build test ERROR on pci/next]
[also build test ERROR on pci/for-linus tiwai-sound/for-next tiwai-sound/for-linus tip/x86/core linus/master v6.16-rc3 next-20250625]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Mario-Limonciello/PCI-Add-helper-for-checking-if-a-PCI-device-is-a-display-controller/20250625-043200
base:   https://git.kernel.org/pub/scm/linux/kernel/git/pci/pci.git next
patch link:    https://lore.kernel.org/r/20250624203042.1102346-8-superm1%40kernel.org
patch subject: [PATCH v5 7/9] PCI/VGA: Replace vga_is_firmware_default() with a screen info check
config: x86_64-defconfig (https://download.01.org/0day-ci/archive/20250625/202506251749.fPKnHMH5-lkp@intel.com/config)
compiler: gcc-11 (Debian 11.3.0-12) 11.3.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250625/202506251749.fPKnHMH5-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202506251749.fPKnHMH5-lkp@intel.com/

All errors (new ones prefixed by >>):

   ld: vmlinux.o: in function `vga_arbiter_add_pci_device':
>> vgaarb.c:(.text+0x5f8f90): undefined reference to `screen_info_pci_dev'
>> ld: vgaarb.c:(.text+0x5f91f8): undefined reference to `screen_info_pci_dev'

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH v5 1/9] PCI: Add helper for checking if a PCI device is a display controller

[Bjorn Helgaas]

On Tue, Jun 24, 2025 at 03:30:34PM -0500, Mario Limonciello wrote:
> From: Mario Limonciello <mario.limonciello@amd.com>
> 
> Several places in the kernel do class shifting to match whether a
> PCI device is display class.  Introduce a helper for those places to
> use.
> 
> Reviewed-by: Daniel Dadap <ddadap@nvidia.com>
> Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>

Acked-by: Bjorn Helgaas <bhelgaas@google.com>

Not sure how this should be merged, let me know if you want me to do
something with it.

> ---
>  include/linux/pci.h | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 05e68f35f3923..e77754e43c629 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -744,6 +744,21 @@ static inline bool pci_is_vga(struct pci_dev *pdev)
>  	return false;
>  }
>  
> +/**
> + * pci_is_display - Check if a PCI device is a display controller
> + * @pdev: Pointer to the PCI device structure
> + *
> + * This function determines whether the given PCI device corresponds
> + * to a display controller. Display controllers are typically used
> + * for graphical output and are identified based on their class code.
> + *
> + * Return: true if the PCI device is a display controller, false otherwise.
> + */
> +static inline bool pci_is_display(struct pci_dev *pdev)
> +{
> +	return (pdev->class >> 16) == PCI_BASE_CLASS_DISPLAY;
> +}
> +
>  #define for_each_pci_bridge(dev, bus)				\
>  	list_for_each_entry(dev, &bus->devices, bus_list)	\
>  		if (!pci_is_bridge(dev)) {} else
> -- 
> 2.43.0
> 

Re: [PATCH v5 9/9] PCI: Add a new 'boot_display' attribute

[Bjorn Helgaas]

On Tue, Jun 24, 2025 at 03:30:42PM -0500, Mario Limonciello wrote:
> From: Mario Limonciello <mario.limonciello@amd.com>
> 
> On systems with multiple GPUs there can be uncertainty which GPU is the
> primary one used to drive the display at bootup. In order to disambiguate
> this add a new sysfs attribute 'boot_display' that uses the output of
> video_is_primary_device() to populate whether a PCI device was used for
> driving the display.
> 
> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>

Acked-by: Bjorn Helgaas <bhelgaas@google.com>

Question below.

> ---
> v4:
>  * new patch
> ---
>  Documentation/ABI/testing/sysfs-bus-pci |  9 +++++++++
>  drivers/pci/pci-sysfs.c                 | 14 ++++++++++++++
>  2 files changed, 23 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
> index 69f952fffec72..897cfc1b0de0f 100644
> --- a/Documentation/ABI/testing/sysfs-bus-pci
> +++ b/Documentation/ABI/testing/sysfs-bus-pci
> @@ -612,3 +612,12 @@ Description:
>  
>  		  # ls doe_features
>  		  0001:01        0001:02        doe_discovery
> +
> +What:		/sys/bus/pci/devices/.../boot_display
> +Date:		October 2025
> +Contact:	Linux PCI developers <linux-pci@vger.kernel.org>
> +Description:
> +		This file indicates whether the device was used as a boot
> +		display. If the device was used as the boot display, the file
> +		will contain "1". If the device is a display device but wasn't
> +		used as a boot display, the file will contain "0".

Is there a reason to expose this file if it wasn't a boot display
device?  Maybe it doesn't need to exist at all unless it contains "1"?

> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> index 268c69daa4d57..5bbf79b1b953d 100644
> --- a/drivers/pci/pci-sysfs.c
> +++ b/drivers/pci/pci-sysfs.c
> @@ -30,6 +30,7 @@
>  #include <linux/msi.h>
>  #include <linux/of.h>
>  #include <linux/aperture.h>
> +#include <asm/video.h>
>  #include "pci.h"
>  
>  #ifndef ARCH_PCI_DEV_GROUPS
> @@ -679,6 +680,13 @@ const struct attribute_group *pcibus_groups[] = {
>  	NULL,
>  };
>  
> +static ssize_t boot_display_show(struct device *dev, struct device_attribute *attr,
> +				 char *buf)
> +{
> +	return sysfs_emit(buf, "%u\n", video_is_primary_device(dev));
> +}
> +static DEVICE_ATTR_RO(boot_display);
> +
>  static ssize_t boot_vga_show(struct device *dev, struct device_attribute *attr,
>  			     char *buf)
>  {
> @@ -1698,6 +1706,7 @@ late_initcall(pci_sysfs_init);
>  
>  static struct attribute *pci_dev_dev_attrs[] = {
>  	&dev_attr_boot_vga.attr,
> +	&dev_attr_boot_display.attr,
>  	NULL,
>  };
>  
> @@ -1710,6 +1719,11 @@ static umode_t pci_dev_attrs_are_visible(struct kobject *kobj,
>  	if (a == &dev_attr_boot_vga.attr && pci_is_vga(pdev))
>  		return a->mode;
>  
> +#ifdef CONFIG_VIDEO
> +	if (a == &dev_attr_boot_display.attr && pci_is_display(pdev))
> +		return a->mode;
> +#endif
> +
>  	return 0;
>  }
>  
> -- 
> 2.43.0
> 

Re: [PATCH v5 7/9] PCI/VGA: Replace vga_is_firmware_default() with a screen info check

[Bjorn Helgaas]

On Tue, Jun 24, 2025 at 03:30:40PM -0500, Mario Limonciello wrote:
> From: Mario Limonciello <mario.limonciello@amd.com>
> 
> vga_is_firmware_default() checks firmware resources to find the owner
> framebuffer resources to find the firmware PCI device.  This is an
> open coded implementation of screen_info_pci_dev().  Switch to using
> screen_info_pci_dev() instead.
> 
> Suggested-by: Thomas Zimmermann <tzimmermann@suse.de>
> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>

Acked-by: Bjorn Helgaas <bhelgaas@google.com>

(after the kernel robot issue is fixed, of course)

> ---
> v5:
>  * split from next patch
> ---
>  drivers/pci/vgaarb.c | 29 ++---------------------------
>  1 file changed, 2 insertions(+), 27 deletions(-)
> 
> diff --git a/drivers/pci/vgaarb.c b/drivers/pci/vgaarb.c
> index 78748e8d2dbae..c3457708c01e3 100644
> --- a/drivers/pci/vgaarb.c
> +++ b/drivers/pci/vgaarb.c
> @@ -556,34 +556,9 @@ EXPORT_SYMBOL(vga_put);
>  
>  static bool vga_is_firmware_default(struct pci_dev *pdev)
>  {
> -#if defined(CONFIG_X86)
> -	u64 base = screen_info.lfb_base;
> -	u64 size = screen_info.lfb_size;
> -	struct resource *r;
> -	u64 limit;
> +	struct screen_info *si = &screen_info;
>  
> -	/* Select the device owning the boot framebuffer if there is one */
> -
> -	if (screen_info.capabilities & VIDEO_CAPABILITY_64BIT_BASE)
> -		base |= (u64)screen_info.ext_lfb_base << 32;
> -
> -	limit = base + size;
> -
> -	/* Does firmware framebuffer belong to us? */
> -	pci_dev_for_each_resource(pdev, r) {
> -		if (resource_type(r) != IORESOURCE_MEM)
> -			continue;
> -
> -		if (!r->start || !r->end)
> -			continue;
> -
> -		if (base < r->start || limit >= r->end)
> -			continue;
> -
> -		return true;
> -	}
> -#endif
> -	return false;
> +	return pdev == screen_info_pci_dev(si);
>  }
>  
>  static bool vga_arb_integrated_gpu(struct device *dev)
> -- 
> 2.43.0
> 

Re: [PATCH v5 9/9] PCI: Add a new 'boot_display' attribute

[Mario Limonciello]

On 6/26/2025 3:45 PM, Bjorn Helgaas wrote:
> On Tue, Jun 24, 2025 at 03:30:42PM -0500, Mario Limonciello wrote:
>> From: Mario Limonciello <mario.limonciello@amd.com>
>>
>> On systems with multiple GPUs there can be uncertainty which GPU is the
>> primary one used to drive the display at bootup. In order to disambiguate
>> this add a new sysfs attribute 'boot_display' that uses the output of
>> video_is_primary_device() to populate whether a PCI device was used for
>> driving the display.
>>
>> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
> 
> Acked-by: Bjorn Helgaas <bhelgaas@google.com>
> 
> Question below.
> 
>> ---
>> v4:
>>   * new patch
>> ---
>>   Documentation/ABI/testing/sysfs-bus-pci |  9 +++++++++
>>   drivers/pci/pci-sysfs.c                 | 14 ++++++++++++++
>>   2 files changed, 23 insertions(+)
>>
>> diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
>> index 69f952fffec72..897cfc1b0de0f 100644
>> --- a/Documentation/ABI/testing/sysfs-bus-pci
>> +++ b/Documentation/ABI/testing/sysfs-bus-pci
>> @@ -612,3 +612,12 @@ Description:
>>   
>>   		  # ls doe_features
>>   		  0001:01        0001:02        doe_discovery
>> +
>> +What:		/sys/bus/pci/devices/.../boot_display
>> +Date:		October 2025
>> +Contact:	Linux PCI developers <linux-pci@vger.kernel.org>
>> +Description:
>> +		This file indicates whether the device was used as a boot
>> +		display. If the device was used as the boot display, the file
>> +		will contain "1". If the device is a display device but wasn't
>> +		used as a boot display, the file will contain "0".
> 
> Is there a reason to expose this file if it wasn't a boot display
> device?  Maybe it doesn't need to exist at all unless it contains "1"?

I was mostly thinking that it's a handy way for userspace to know 
whether the kernel even supports this feature.  If userspace sees that 
file on any GPU as it walks a list then it knows it can use that for a hint.

But if you would rather it only shows up for the boot display yes it's 
possible to do I think.  It's just more complexity to the visibility 
lookup to also call video_is_primary_device().

LMK which way you want to go and I'll respin the series with your tags 
and the robot fix.

> 
>> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
>> index 268c69daa4d57..5bbf79b1b953d 100644
>> --- a/drivers/pci/pci-sysfs.c
>> +++ b/drivers/pci/pci-sysfs.c
>> @@ -30,6 +30,7 @@
>>   #include <linux/msi.h>
>>   #include <linux/of.h>
>>   #include <linux/aperture.h>
>> +#include <asm/video.h>
>>   #include "pci.h"
>>   
>>   #ifndef ARCH_PCI_DEV_GROUPS
>> @@ -679,6 +680,13 @@ const struct attribute_group *pcibus_groups[] = {
>>   	NULL,
>>   };
>>   
>> +static ssize_t boot_display_show(struct device *dev, struct device_attribute *attr,
>> +				 char *buf)
>> +{
>> +	return sysfs_emit(buf, "%u\n", video_is_primary_device(dev));
>> +}
>> +static DEVICE_ATTR_RO(boot_display);
>> +
>>   static ssize_t boot_vga_show(struct device *dev, struct device_attribute *attr,
>>   			     char *buf)
>>   {
>> @@ -1698,6 +1706,7 @@ late_initcall(pci_sysfs_init);
>>   
>>   static struct attribute *pci_dev_dev_attrs[] = {
>>   	&dev_attr_boot_vga.attr,
>> +	&dev_attr_boot_display.attr,
>>   	NULL,
>>   };
>>   
>> @@ -1710,6 +1719,11 @@ static umode_t pci_dev_attrs_are_visible(struct kobject *kobj,
>>   	if (a == &dev_attr_boot_vga.attr && pci_is_vga(pdev))
>>   		return a->mode;
>>   
>> +#ifdef CONFIG_VIDEO
>> +	if (a == &dev_attr_boot_display.attr && pci_is_display(pdev))
>> +		return a->mode;
>> +#endif
>> +
>>   	return 0;
>>   }
>>   
>> -- 
>> 2.43.0
>>

Re: [PATCH v5 1/9] PCI: Add helper for checking if a PCI device is a display controller

[Mario Limonciello]

On 6/26/2025 3:43 PM, Bjorn Helgaas wrote:
> On Tue, Jun 24, 2025 at 03:30:34PM -0500, Mario Limonciello wrote:
>> From: Mario Limonciello <mario.limonciello@amd.com>
>>
>> Several places in the kernel do class shifting to match whether a
>> PCI device is display class.  Introduce a helper for those places to
>> use.
>>
>> Reviewed-by: Daniel Dadap <ddadap@nvidia.com>
>> Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
>> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
> 
> Acked-by: Bjorn Helgaas <bhelgaas@google.com>
> 
> Not sure how this should be merged, let me know if you want me to do
> something with it.

Unless there is opposition I think it's best to take it through PCI.

It's a trivial change to most the other drivers.  If there is opposition 
I think there will need to be an immutable tag for the others to merge.

> 
>> ---
>>   include/linux/pci.h | 15 +++++++++++++++
>>   1 file changed, 15 insertions(+)
>>
>> diff --git a/include/linux/pci.h b/include/linux/pci.h
>> index 05e68f35f3923..e77754e43c629 100644
>> --- a/include/linux/pci.h
>> +++ b/include/linux/pci.h
>> @@ -744,6 +744,21 @@ static inline bool pci_is_vga(struct pci_dev *pdev)
>>   	return false;
>>   }
>>   
>> +/**
>> + * pci_is_display - Check if a PCI device is a display controller
>> + * @pdev: Pointer to the PCI device structure
>> + *
>> + * This function determines whether the given PCI device corresponds
>> + * to a display controller. Display controllers are typically used
>> + * for graphical output and are identified based on their class code.
>> + *
>> + * Return: true if the PCI device is a display controller, false otherwise.
>> + */
>> +static inline bool pci_is_display(struct pci_dev *pdev)
>> +{
>> +	return (pdev->class >> 16) == PCI_BASE_CLASS_DISPLAY;
>> +}
>> +
>>   #define for_each_pci_bridge(dev, bus)				\
>>   	list_for_each_entry(dev, &bus->devices, bus_list)	\
>>   		if (!pci_is_bridge(dev)) {} else
>> -- 
>> 2.43.0
>>

Re: [PATCH v5 9/9] PCI: Add a new 'boot_display' attribute

[Bjorn Helgaas]

On Thu, Jun 26, 2025 at 04:12:21PM -0500, Mario Limonciello wrote:
> On 6/26/2025 3:45 PM, Bjorn Helgaas wrote:
> > On Tue, Jun 24, 2025 at 03:30:42PM -0500, Mario Limonciello wrote:
> > > From: Mario Limonciello <mario.limonciello@amd.com>
> > > 
> > > On systems with multiple GPUs there can be uncertainty which GPU is the
> > > primary one used to drive the display at bootup. In order to disambiguate
> > > this add a new sysfs attribute 'boot_display' that uses the output of
> > > video_is_primary_device() to populate whether a PCI device was used for
> > > driving the display.
> > > 
> > > Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
> > 
> > Acked-by: Bjorn Helgaas <bhelgaas@google.com>
> > 
> > Question below.
> > 
> > > ---
> > > v4:
> > >   * new patch
> > > ---
> > >   Documentation/ABI/testing/sysfs-bus-pci |  9 +++++++++
> > >   drivers/pci/pci-sysfs.c                 | 14 ++++++++++++++
> > >   2 files changed, 23 insertions(+)
> > > 
> > > diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
> > > index 69f952fffec72..897cfc1b0de0f 100644
> > > --- a/Documentation/ABI/testing/sysfs-bus-pci
> > > +++ b/Documentation/ABI/testing/sysfs-bus-pci
> > > @@ -612,3 +612,12 @@ Description:
> > >   		  # ls doe_features
> > >   		  0001:01        0001:02        doe_discovery
> > > +
> > > +What:		/sys/bus/pci/devices/.../boot_display
> > > +Date:		October 2025
> > > +Contact:	Linux PCI developers <linux-pci@vger.kernel.org>
> > > +Description:
> > > +		This file indicates whether the device was used as a boot
> > > +		display. If the device was used as the boot display, the file
> > > +		will contain "1". If the device is a display device but wasn't
> > > +		used as a boot display, the file will contain "0".
> > 
> > Is there a reason to expose this file if it wasn't a boot display
> > device?  Maybe it doesn't need to exist at all unless it contains "1"?
> 
> I was mostly thinking that it's a handy way for userspace to know whether
> the kernel even supports this feature.  If userspace sees that file on any
> GPU as it walks a list then it knows it can use that for a hint.
> 
> But if you would rather it only shows up for the boot display yes it's
> possible to do I think.  It's just more complexity to the visibility lookup
> to also call video_is_primary_device().

I think for a singleton situation like this it makes more sense to
only expose the file for one device, not several files where only one
of them contains "1".

> > > diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> > > index 268c69daa4d57..5bbf79b1b953d 100644
> > > --- a/drivers/pci/pci-sysfs.c
> > > +++ b/drivers/pci/pci-sysfs.c
> > > @@ -30,6 +30,7 @@
> > >   #include <linux/msi.h>
> > >   #include <linux/of.h>
> > >   #include <linux/aperture.h>
> > > +#include <asm/video.h>
> > >   #include "pci.h"
> > >   #ifndef ARCH_PCI_DEV_GROUPS
> > > @@ -679,6 +680,13 @@ const struct attribute_group *pcibus_groups[] = {
> > >   	NULL,
> > >   };
> > > +static ssize_t boot_display_show(struct device *dev, struct device_attribute *attr,
> > > +				 char *buf)
> > > +{
> > > +	return sysfs_emit(buf, "%u\n", video_is_primary_device(dev));
> > > +}
> > > +static DEVICE_ATTR_RO(boot_display);
> > > +
> > >   static ssize_t boot_vga_show(struct device *dev, struct device_attribute *attr,
> > >   			     char *buf)
> > >   {
> > > @@ -1698,6 +1706,7 @@ late_initcall(pci_sysfs_init);
> > >   static struct attribute *pci_dev_dev_attrs[] = {
> > >   	&dev_attr_boot_vga.attr,
> > > +	&dev_attr_boot_display.attr,
> > >   	NULL,
> > >   };
> > > @@ -1710,6 +1719,11 @@ static umode_t pci_dev_attrs_are_visible(struct kobject *kobj,
> > >   	if (a == &dev_attr_boot_vga.attr && pci_is_vga(pdev))
> > >   		return a->mode;
> > > +#ifdef CONFIG_VIDEO
> > > +	if (a == &dev_attr_boot_display.attr && pci_is_display(pdev))
> > > +		return a->mode;
> > > +#endif
> > > +
> > >   	return 0;
> > >   }
> > > -- 
> > > 2.43.0
> > > 
> 

Re: [PATCH v5 9/9] PCI: Add a new 'boot_display' attribute

[Mario Limonciello]

On 6/26/25 4:47 PM, Bjorn Helgaas wrote:
> On Thu, Jun 26, 2025 at 04:12:21PM -0500, Mario Limonciello wrote:
>> On 6/26/2025 3:45 PM, Bjorn Helgaas wrote:
>>> On Tue, Jun 24, 2025 at 03:30:42PM -0500, Mario Limonciello wrote:
>>>> From: Mario Limonciello <mario.limonciello@amd.com>
>>>>
>>>> On systems with multiple GPUs there can be uncertainty which GPU is the
>>>> primary one used to drive the display at bootup. In order to disambiguate
>>>> this add a new sysfs attribute 'boot_display' that uses the output of
>>>> video_is_primary_device() to populate whether a PCI device was used for
>>>> driving the display.
>>>>
>>>> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
>>>
>>> Acked-by: Bjorn Helgaas <bhelgaas@google.com>
>>>
>>> Question below.
>>>
>>>> ---
>>>> v4:
>>>>    * new patch
>>>> ---
>>>>    Documentation/ABI/testing/sysfs-bus-pci |  9 +++++++++
>>>>    drivers/pci/pci-sysfs.c                 | 14 ++++++++++++++
>>>>    2 files changed, 23 insertions(+)
>>>>
>>>> diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
>>>> index 69f952fffec72..897cfc1b0de0f 100644
>>>> --- a/Documentation/ABI/testing/sysfs-bus-pci
>>>> +++ b/Documentation/ABI/testing/sysfs-bus-pci
>>>> @@ -612,3 +612,12 @@ Description:
>>>>    		  # ls doe_features
>>>>    		  0001:01        0001:02        doe_discovery
>>>> +
>>>> +What:		/sys/bus/pci/devices/.../boot_display
>>>> +Date:		October 2025
>>>> +Contact:	Linux PCI developers <linux-pci@vger.kernel.org>
>>>> +Description:
>>>> +		This file indicates whether the device was used as a boot
>>>> +		display. If the device was used as the boot display, the file
>>>> +		will contain "1". If the device is a display device but wasn't
>>>> +		used as a boot display, the file will contain "0".
>>>
>>> Is there a reason to expose this file if it wasn't a boot display
>>> device?  Maybe it doesn't need to exist at all unless it contains "1"?
>>
>> I was mostly thinking that it's a handy way for userspace to know whether
>> the kernel even supports this feature.  If userspace sees that file on any
>> GPU as it walks a list then it knows it can use that for a hint.
>>
>> But if you would rather it only shows up for the boot display yes it's
>> possible to do I think.  It's just more complexity to the visibility lookup
>> to also call video_is_primary_device().
> 
> I think for a singleton situation like this it makes more sense to
> only expose the file for one device, not several files where only one
> of them contains "1".

I did an experiment with this but the PCI resources aren't ready at the 
time visibility is determined.

So either:
* the sysfs file creation needs to be deferred similar to 
pci_create_resource_files() does

or

* call to sysfs_update_group() is needed to recalculate visibility.

> 
>>>> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
>>>> index 268c69daa4d57..5bbf79b1b953d 100644
>>>> --- a/drivers/pci/pci-sysfs.c
>>>> +++ b/drivers/pci/pci-sysfs.c
>>>> @@ -30,6 +30,7 @@
>>>>    #include <linux/msi.h>
>>>>    #include <linux/of.h>
>>>>    #include <linux/aperture.h>
>>>> +#include <asm/video.h>
>>>>    #include "pci.h"
>>>>    #ifndef ARCH_PCI_DEV_GROUPS
>>>> @@ -679,6 +680,13 @@ const struct attribute_group *pcibus_groups[] = {
>>>>    	NULL,
>>>>    };
>>>> +static ssize_t boot_display_show(struct device *dev, struct device_attribute *attr,
>>>> +				 char *buf)
>>>> +{
>>>> +	return sysfs_emit(buf, "%u\n", video_is_primary_device(dev));
>>>> +}
>>>> +static DEVICE_ATTR_RO(boot_display);
>>>> +
>>>>    static ssize_t boot_vga_show(struct device *dev, struct device_attribute *attr,
>>>>    			     char *buf)
>>>>    {
>>>> @@ -1698,6 +1706,7 @@ late_initcall(pci_sysfs_init);
>>>>    static struct attribute *pci_dev_dev_attrs[] = {
>>>>    	&dev_attr_boot_vga.attr,
>>>> +	&dev_attr_boot_display.attr,
>>>>    	NULL,
>>>>    };
>>>> @@ -1710,6 +1719,11 @@ static umode_t pci_dev_attrs_are_visible(struct kobject *kobj,
>>>>    	if (a == &dev_attr_boot_vga.attr && pci_is_vga(pdev))
>>>>    		return a->mode;
>>>> +#ifdef CONFIG_VIDEO
>>>> +	if (a == &dev_attr_boot_display.attr && pci_is_display(pdev))
>>>> +		return a->mode;
>>>> +#endif
>>>> +
>>>>    	return 0;
>>>>    }
>>>> -- 
>>>> 2.43.0
>>>>
>>

Re: [PATCH v5 9/9] PCI: Add a new 'boot_display' attribute

[Bjorn Helgaas]

On Thu, Jun 26, 2025 at 06:33:15PM -0500, Mario Limonciello wrote:
> On 6/26/25 4:47 PM, Bjorn Helgaas wrote:
> > On Thu, Jun 26, 2025 at 04:12:21PM -0500, Mario Limonciello wrote:
> > > On 6/26/2025 3:45 PM, Bjorn Helgaas wrote:
> > > > On Tue, Jun 24, 2025 at 03:30:42PM -0500, Mario Limonciello wrote:
> > > > > From: Mario Limonciello <mario.limonciello@amd.com>
> > > > > 
> > > > > On systems with multiple GPUs there can be uncertainty which GPU is the
> > > > > primary one used to drive the display at bootup. In order to disambiguate
> > > > > this add a new sysfs attribute 'boot_display' that uses the output of
> > > > > video_is_primary_device() to populate whether a PCI device was used for
> > > > > driving the display.
> > > > > 
> > > > > Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
> > > > 
> > > > Acked-by: Bjorn Helgaas <bhelgaas@google.com>
> > > > 
> > > > Question below.
> > > > 
> > > > > ---
> > > > > v4:
> > > > >    * new patch
> > > > > ---
> > > > >    Documentation/ABI/testing/sysfs-bus-pci |  9 +++++++++
> > > > >    drivers/pci/pci-sysfs.c                 | 14 ++++++++++++++
> > > > >    2 files changed, 23 insertions(+)
> > > > > 
> > > > > diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
> > > > > index 69f952fffec72..897cfc1b0de0f 100644
> > > > > --- a/Documentation/ABI/testing/sysfs-bus-pci
> > > > > +++ b/Documentation/ABI/testing/sysfs-bus-pci
> > > > > @@ -612,3 +612,12 @@ Description:
> > > > >    		  # ls doe_features
> > > > >    		  0001:01        0001:02        doe_discovery
> > > > > +
> > > > > +What:		/sys/bus/pci/devices/.../boot_display
> > > > > +Date:		October 2025
> > > > > +Contact:	Linux PCI developers <linux-pci@vger.kernel.org>
> > > > > +Description:
> > > > > +		This file indicates whether the device was used as a boot
> > > > > +		display. If the device was used as the boot display, the file
> > > > > +		will contain "1". If the device is a display device but wasn't
> > > > > +		used as a boot display, the file will contain "0".
> > > > 
> > > > Is there a reason to expose this file if it wasn't a boot display
> > > > device?  Maybe it doesn't need to exist at all unless it contains "1"?
> > > 
> > > I was mostly thinking that it's a handy way for userspace to know whether
> > > the kernel even supports this feature.  If userspace sees that file on any
> > > GPU as it walks a list then it knows it can use that for a hint.
> > > 
> > > But if you would rather it only shows up for the boot display yes it's
> > > possible to do I think.  It's just more complexity to the visibility lookup
> > > to also call video_is_primary_device().
> > 
> > I think for a singleton situation like this it makes more sense to
> > only expose the file for one device, not several files where only one
> > of them contains "1".
> 
> I did an experiment with this but the PCI resources aren't ready at the time
> visibility is determined.
> 
> So either:
> * the sysfs file creation needs to be deferred similar to
> pci_create_resource_files() does
> 
> or
> 
> * call to sysfs_update_group() is needed to recalculate visibility.

Sigh, yeah, that's an old annoying problem.  I think deferring as you
did is fine.

> > > > > diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> > > > > index 268c69daa4d57..5bbf79b1b953d 100644
> > > > > --- a/drivers/pci/pci-sysfs.c
> > > > > +++ b/drivers/pci/pci-sysfs.c
> > > > > @@ -30,6 +30,7 @@
> > > > >    #include <linux/msi.h>
> > > > >    #include <linux/of.h>
> > > > >    #include <linux/aperture.h>
> > > > > +#include <asm/video.h>
> > > > >    #include "pci.h"
> > > > >    #ifndef ARCH_PCI_DEV_GROUPS
> > > > > @@ -679,6 +680,13 @@ const struct attribute_group *pcibus_groups[] = {
> > > > >    	NULL,
> > > > >    };
> > > > > +static ssize_t boot_display_show(struct device *dev, struct device_attribute *attr,
> > > > > +				 char *buf)
> > > > > +{
> > > > > +	return sysfs_emit(buf, "%u\n", video_is_primary_device(dev));
> > > > > +}
> > > > > +static DEVICE_ATTR_RO(boot_display);
> > > > > +
> > > > >    static ssize_t boot_vga_show(struct device *dev, struct device_attribute *attr,
> > > > >    			     char *buf)
> > > > >    {
> > > > > @@ -1698,6 +1706,7 @@ late_initcall(pci_sysfs_init);
> > > > >    static struct attribute *pci_dev_dev_attrs[] = {
> > > > >    	&dev_attr_boot_vga.attr,
> > > > > +	&dev_attr_boot_display.attr,
> > > > >    	NULL,
> > > > >    };
> > > > > @@ -1710,6 +1719,11 @@ static umode_t pci_dev_attrs_are_visible(struct kobject *kobj,
> > > > >    	if (a == &dev_attr_boot_vga.attr && pci_is_vga(pdev))
> > > > >    		return a->mode;
> > > > > +#ifdef CONFIG_VIDEO
> > > > > +	if (a == &dev_attr_boot_display.attr && pci_is_display(pdev))
> > > > > +		return a->mode;
> > > > > +#endif
> > > > > +
> > > > >    	return 0;
> > > > >    }
> > > > > -- 
> > > > > 2.43.0
> > > > > 
> > > 
>