Message-ID: <20250722015934.GA2763711@bhelgaas>
Date: Mon, 21 Jul 2025 20:59:34 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Mario Limonciello <superm1@...nel.org>
Cc: David Airlie <airlied@...il.com>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Alex Deucher <alexander.deucher@....com>,
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	Thomas Zimmermann <tzimmermann@...e.de>,
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	Lu Baolu <baolu.lu@...ux.intel.com>, Joerg Roedel <joro@...tes.org>,
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	Daniel Dadap <ddadap@...dia.com>,
	Mario Limonciello <mario.limonciello@....com>
Subject: Re: [PATCH v9 9/9] PCI: Add a new 'boot_display' attribute

On Mon, Jul 21, 2025 at 07:28:07PM -0500, Mario Limonciello wrote:
> On 7/21/25 6:00 PM, Bjorn Helgaas wrote:
> > 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@....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@...r.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 assume the loop is the "while (pdev =
pci_get_base_class(PCI_BASE_CLASS_DISPLAY))" in
__screen_info_pci_dev(), which indeed walks through all known PCI
devices]

> > 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().
> 
> Yes it was screen_info_pci_dev() and __screen_info_pci_dev().  The
> resources weren't ready on the first call into
> __screen_info_pci_dev().
>
> That's why the attribute needed to be created later.

I don't understand this.  IIUC, screen_info contains addresses
programmed by BIOS.  If we want to use that to match with a PCI
device, we have to compare with the BAR contents *before* Linux does
any assignments of its own.

So the only thing this should depend on is the BAR value at BIOS ->
Linux handoff, which we know at the time of device_add(), and we
should be able to do something like this:

  bool pci_video_is_primary_device(struct pci_dev *pdev)
  {
    struct screen_info *si = &screen_info;
    struct resource res[SCREEN_INFO_MAX_RESOURCES];
    ssize_t i, numres;

    numres = screen_info_resources(si, res, ARRAY_SIZE(res));
    ...

    for (i = 0; i < numres; ++i) {
      if (pci_find_resource(pdev, &res[i]))
        return true;
    }

    return false;
  }

  static umode_t pci_dev_boot_display_is_visible(...)
  {
    struct pci_dev *pdev = to_pci_dev(kobj_to_dev(kobj));

    if (pci_video_is_primary_device(pdev))
      return a->mode;

    return 0;
  }

We should be able to check each BAR of each device in this path, with
no loop through the devices at all:

  pci_device_add
    device_add
      device_add_attrs
        device_add_groups
          ...
            create_files
              grp->is_visible()
                pci_dev_boot_display_is_visible

Bjorn

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