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.limoncie...@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-...@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 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