Re: [PATCH v6 9/9] PCI: Add a new 'boot_display' attribute

[Mario Limonciello <superm1@kernel.org>]

On 6/27/2025 2:07 AM, Thomas Zimmermann wrote:
> Hi
> 
> Am 27.06.25 um 06:31 schrieb 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>
>> ---
>> 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 +++++++++++++++++++++++++
> 
> The code looks good. Just one more question: could this be added 
> independently from the PCI bus (at a reasonable cost)? There are other 
> busses that can host the boot display. Alternatively, we'd add this 
> attribute per bus as needed.

It depends upon the underlying hardware implementation.  On x86 it's 
always PCI and so I realized there is a requirement that PCI resources 
are setup before screen_info event works.

That is the v5 version of this patch would have had a potential race 
condition with userspace where boot_display didn't always show '1' if 
userspace read it too quickly.

Other architecture's hardware implementation might have similar problem.

So in summary I think it would be better to do it per-bus.  If we 
realize there is indeed code duplication we can always move this to a 
common helper at that point.

> 
> Best regards
> Thomas
> 
>>   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..cc766461de1da 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)
>>   {
>> @@ -1246,6 +1254,37 @@ static int pci_create_attr(struct pci_dev 
>> *pdev, int num, int write_combine)
>>       return 0;
>>   }
>> +/**
>> + * 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
>> +}
>> +
>>   /**
>>    * pci_create_resource_files - create resource files in sysfs for @dev
>>    * @pdev: dev in question
>> @@ -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);
>>   }
>