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

[Mario Limonciello <superm1@kernel.org>]

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
>>