DRM: Add a new 'boot_display' attribute

On systems with multiple GPUs there can be uncertainty which GPU is the
primary one used to drive the display at bootup. In some desktop
environments this can lead to increased power consumption because
secondary GPUs may be used for rendering and never go to a low power
state. In order to disambiguate this add a new sysfs attribute
'boot_display' that uses the output of video_is_primary_device() to
populate whether the PCI device was used for driving the display.

Suggested-by: Manivannan Sadhasivam <mani@kernel.org>
Acked-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://gitlab.freedesktop.org/xorg/lib/libpciaccess/-/issues/23
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20250811162606.587759-5-superm1@kernel.org
Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>

Author: superm1

Documentation/ABI/testing/sysfs-class-drm

@@ -0,0 +1,8 @@
+What:		/sys/class/drm/.../boot_display
+Date:		January 2026
+Contact:	Linux DRI developers <dri-devel@vger.kernel.org>
+Description:
+		This file indicates that displays connected to the device were
+		used to display the boot sequence.  If a display connected to
+		the device was used to display the boot sequence the file will
+		be present and contain "1".

drivers/gpu/drm/drm_sysfs.c

@@ -18,6 +18,7 @@
 #include <linux/gfp.h>
 #include <linux/i2c.h>
 #include <linux/kdev_t.h>
+#include <linux/pci.h>
 #include <linux/property.h>
 #include <linux/slab.h>
 
@@ -30,6 +31,8 @@
 #include <drm/drm_property.h>
 #include <drm/drm_sysfs.h>
 
+#include <asm/video.h>
+
 #include "drm_internal.h"
 #include "drm_crtc_internal.h"
 
@@ -508,6 +511,43 @@ void drm_sysfs_connector_property_event(struct drm_connector *connector,
 }
 EXPORT_SYMBOL(drm_sysfs_connector_property_event);
 
+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 struct attribute *display_attrs[] = {
+	&dev_attr_boot_display.attr,
+	NULL
+};
+
+static umode_t boot_display_visible(struct kobject *kobj,
+				    struct attribute *a, int n)
+{
+	struct device *dev = kobj_to_dev(kobj)->parent;
+
+	if (dev_is_pci(dev)) {
+		struct pci_dev *pdev = to_pci_dev(dev);
+
+		if (video_is_primary_device(&pdev->dev))
+			return a->mode;
+	}
+
+	return 0;
+}
+
+static const struct attribute_group display_attr_group = {
+	.attrs = display_attrs,
+	.is_visible = boot_display_visible,
+};
+
+static const struct attribute_group *card_dev_groups[] = {
+	&display_attr_group,
+	NULL
+};
+
 struct device *drm_sysfs_minor_alloc(struct drm_minor *minor)
 {
 	const char *minor_str;
@@ -531,6 +571,7 @@ struct device *drm_sysfs_minor_alloc(struct drm_minor *minor)
 
 		kdev->devt = MKDEV(DRM_MAJOR, minor->index);
 		kdev->class = drm_class;
+		kdev->groups = card_dev_groups;
 		kdev->type = &drm_sysfs_device_minor;
 	}