Re: [PATCH] PCI/VGA: Fix boot device detection after framebuffer relocation
From: Thomas Zimmermann
Date: Thu Oct 29 2026 - 03:56:58 EST
Hi
Am 29.10.26 um 00:15 schrieb Bjorn Helgaas:
On Wed, Oct 28, 2026 at 05:46:34PM +0800, wuqianhai@xxxxxxxxxxx wrote:
From: wuqianhai <wuqianhai@xxxxxxxxxxx>I'm really confused about this. vga_is_firmware_default() relies on
PCI resource assignment can relocate the firmware framebuffer BAR, which
leaves the base address in screen_info stale. The VGA arbiter then calls
screen_info_pci_dev() from vga_is_firmware_default(), which fails to match
the framebuffer against any PCI device's BARs. As a result, the boot VGA
device is not recognized and the arbiter may select the wrong default
device.
Fix this by calling screen_info_apply_fixups() in vga_arb_device_init()
before adding the VGA devices. This relocates lfb_base to the assigned
BAR address so that vga_is_firmware_default() matches the boot device
again.
Tested on a LoongArch platform, with vga_is_firmware_default()
additionally enabled for CONFIG_LOONGARCH via local changes.
Fixes: 337bf13aa9ddab8ae47f2677ab75cf69ae33437b ("PCI/VGA: Replace vga_is_firmware_default() with a screen info check")
Signed-off-by: wuqianhai <wuqianhai@xxxxxxxxxxx>
---
drivers/pci/vgaarb.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/drivers/pci/vgaarb.c b/drivers/pci/vgaarb.c
index 3e58b2c05c85..aca594651f58 100644
--- a/drivers/pci/vgaarb.c
+++ b/drivers/pci/vgaarb.c
@@ -1527,6 +1527,14 @@ static int __init vga_arb_device_init(void)
int rc;
struct pci_dev *pdev;
+ /*
+ * PCI resource assignment may have relocated the firmware
+ * framebuffer BAR. Update screen_info's lfb_base so that
+ * vga_is_firmware_default() can match it against the boot
+ * device's relocated resource.
+ */
+ screen_info_apply_fixups();
screen_info_pci_dev(&sysfb_primary_display.screen):
vga_is_firmware_default
screen_info_pci_dev(&sysfb_primary_display.screen)
numres = screen_info_resources(&sysfb_primary_display.screen, res, ...)
for (i = 0; i < numres; ++i)
pdev = __screen_info_pci_dev(&res[i])
screen_info_resources() returns resource structs with PCI bus
addresses in them. These are not valid CPU addresses, so that seems
broken on any system where bus addresses are not the same as CPU
addresses.
What we get from the firmware for the framebuffer is a memory range. If the underlying PCI memory gets relocated during the boot, we need to update the framebuffer data accordingly. This has worked well for the last 20 yrs.
screen_info_fixup_lfb() and screen_info_apply_fixups() may fix the
issue you see, but I think the way this happens is questionable.
screen_info_fixup_lfb() captures the framebuffer PCI bus address in
screen_info_lfb_res_start. Then screen_info_apply_fixups() compares
that PCI bus address with the CPU address in pdev->resource[]. That's
broken on any system where bus addresses are not the same as CPU
addresses. I think it's also broken because screen_info_lfb_res_start
may not be at the beginning of the BAR.
I think screen_info_fixup_lfb() should only capture the pci_dev, the
BAR number, and the framebuffer offset into the BAR. When we need the
CPU or bus address, we can always compute it from those values.
This is not sufficient, I think. We did this until a while ago and got problems with relocated bars. IIRC aarch64 had a number of affected systems. See commit 2f29b5c23101 ("video: screen_info: Relocate framebuffers behind PCI bridges") for details.
I'm happy to improve that code though, if we can do something better.
Best regards
Thomas
rc = misc_register(&vga_arb_device);
if (rc < 0)
pr_err("error %d registering device\n", rc);
--
2.34.1
--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Stefan Gaiser, Jochen Jaser, Abhinav Puri, (HRB 36809, AG Nürnberg)