Re: [PATCH v9 9/9] PCI: Add a new 'boot_display' attribute

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Jul 18, 2025 at 12:44:11PM -0500, Mario Limonciello wrote:
> On 7/18/2025 12:36 PM, Bjorn Helgaas wrote:
> > On Fri, Jul 18, 2025 at 12:29:05PM -0500, Mario Limonciello wrote:
> > > On 7/18/2025 12:25 PM, Bjorn Helgaas wrote:
> > > > On Thu, Jul 17, 2025 at 12:38:12PM -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 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 a PCI device was used for driving the display.
> > > > 
> > > > > +What:		/sys/bus/pci/devices/.../boot_display
> > > > > +Date:		October 2025
> > > > > +Contact:	Linux PCI developers <linux-pci@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".
> > > > 
> > > > >    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);
> > > > 
> > > > In addition to Mani's question about whether /sys/bus/pci/ is
> > > > the right place for this (which is a very good question), it's
> > > > also been pointed out to me that we've been trying to get rid
> > > > of pci_create_sysfs_dev_files() for years.
> > > > 
> > > > If it's possible to make this a static attribute that would be
> > > > much, much cleaner.
> > > 
> > > Right - I tried to do this, but the problem is at the time the
> > > PCI device is created the information needed to make the
> > > judgement isn't ready.  The options end up being:
> > > * a sysfs file for every display device with 0/1
> > > * a sysfs file that is not accurate until later in the boot
> > 
> > What's missing?  The specifics might be helpful if someone has
> > another crack at getting rid of pci_create_sysfs_dev_files() in
> > the future.
> 
> The underlying SCREEN_INFO code tries to walk through all the PCI
> devices in a loop, but at the time all the devices are walked the
> memory regions associated with the device weren't populated.

Which loop are you referring to that walks through all the PCI
devices?  I see this:

  efifb_set_system
    for_each_pci_dev(dev)

but that only looks at VGA devices and IIUC you also want to look at
non-VGA GPUs.

I don't see a loop in *this* series, where the screen_info path looks
like this:

  pci_create_boot_display_file
    video_is_primary_device
      screen_info_pci_dev      # added by "fbcon: Use screen info to find primary device"
        screen_info_resources
        __screen_info_pci_dev

and we're basically matching the screen_info base/address with BAR
values.

The usual problem is that BARs may not have been assigned by the time
pci_device_add() -> device_add() creates the static attributes.

So we call pci_assign_unassigned_root_bus_resources() to assign all
the BARs.  Then we call pci_create_sysfs_dev_files(), where
pci_create_resource_files() creates a "resource%d" file for each BAR.

But since we're trying to find the GPU that was used by BIOS, I assume
its BARs were programmed by BIOS and we shouldn't have to wait until
after pci_assign_unassigned_root_bus_resources().

Bjorn