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

[Bjorn Helgaas <helgaas@kernel.org>]

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
>