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

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Jun 26, 2025 at 04:12:21PM -0500, Mario Limonciello wrote:
> 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().

I think for a singleton situation like this it makes more sense to
only expose the file for one device, not several files where only one
of them 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
> > > 
>