[PATCH v7 9/9] PCI: Add a new 'boot_display' attribute

[Mario Limonciello <superm1@kernel.org>]

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.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
---
v7:
 * fix lkp failure
 * Add tag
v6:
 * Only show for the device that is boot display
 * Only create after PCI device sysfs files are initialized to ensure
   that resources are ready.
v4:
 * new patch
---
 Documentation/ABI/testing/sysfs-bus-pci |  8 +++++
 drivers/pci/pci-sysfs.c                 | 46 +++++++++++++++++++++++++
 2 files changed, 54 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
index 69f952fffec72..8b455b1a58852 100644
--- a/Documentation/ABI/testing/sysfs-bus-pci
+++ b/Documentation/ABI/testing/sysfs-bus-pci
@@ -612,3 +612,11 @@ 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 the device was used as a boot
+		display. If the device was used as the boot display, the file
+		will be present and contain "1".
diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
index 268c69daa4d57..6b1a0ae254d3a 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, "1\n");
+}
+static DEVICE_ATTR_RO(boot_display);
+
 static ssize_t boot_vga_show(struct device *dev, struct device_attribute *attr,
 			     char *buf)
 {
@@ -1051,6 +1059,37 @@ void pci_remove_legacy_files(struct pci_bus *b)
 }
 #endif /* HAVE_PCI_LEGACY */
 
+/**
+ * pci_create_boot_display_file - create a file in sysfs for @dev
+ * @pdev: dev in question
+ *
+ * Creates a file `boot_display` in sysfs for the PCI device @pdev
+ * if it is the boot display device.
+ */
+static int pci_create_boot_display_file(struct pci_dev *pdev)
+{
+#ifdef CONFIG_VIDEO
+	if (video_is_primary_device(&pdev->dev))
+		return sysfs_create_file(&pdev->dev.kobj, &dev_attr_boot_display.attr);
+#endif
+	return 0;
+}
+
+/**
+ * pci_remove_boot_display_file - remove the boot display file for @dev
+ * @pdev: dev in question
+ *
+ * Removes the file `boot_display` in sysfs for the PCI device @pdev
+ * if it is the boot display device.
+ */
+static void pci_remove_boot_display_file(struct pci_dev *pdev)
+{
+#ifdef CONFIG_VIDEO
+	if (video_is_primary_device(&pdev->dev))
+		sysfs_remove_file(&pdev->dev.kobj, &dev_attr_boot_display.attr);
+#endif
+}
+
 #if defined(HAVE_PCI_MMAP) || defined(ARCH_GENERIC_PCI_MMAP_RESOURCE)
 /**
  * pci_mmap_resource - map a PCI resource into user memory space
@@ -1654,9 +1693,15 @@ static const struct attribute_group pci_dev_resource_resize_group = {
 
 int __must_check pci_create_sysfs_dev_files(struct pci_dev *pdev)
 {
+	int retval;
+
 	if (!sysfs_initialized)
 		return -EACCES;
 
+	retval = pci_create_boot_display_file(pdev);
+	if (retval)
+		return retval;
+
 	return pci_create_resource_files(pdev);
 }
 
@@ -1671,6 +1716,7 @@ void pci_remove_sysfs_dev_files(struct pci_dev *pdev)
 	if (!sysfs_initialized)
 		return;
 
+	pci_remove_boot_display_file(pdev);
 	pci_remove_resource_files(pdev);
 }
 
-- 
2.43.0