[PATCH] PCI/VGA: Fix boot device detection after framebuffer relocation

[PATCH] PCI/VGA: Fix boot device detection after framebuffer relocation

[wuqianhai]

From: wuqianhai <wuqianhai@loongson.cn>

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@loongson.cn>
---
 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();
+
 	rc = misc_register(&vga_arb_device);
 	if (rc < 0)
 		pr_err("error %d registering device\n", rc);
-- 
2.34.1

Re: [PATCH] PCI/VGA: Fix boot device detection after framebuffer relocation

[Bjorn Helgaas]

On Wed, Oct 28, 2026 at 05:46:34PM +0800, wuqianhai@loongson.cn wrote:
> From: wuqianhai <wuqianhai@loongson.cn>
> 
> 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@loongson.cn>
> ---
>  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();

I'm really confused about this.  vga_is_firmware_default() relies on
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.

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.

>  	rc = misc_register(&vga_arb_device);
>  	if (rc < 0)
>  		pr_err("error %d registering device\n", rc);
> -- 
> 2.34.1
> 

[PATCH v2] PCI/VGA: Enable firmware default device detection on LoongArch

[wuqianhai]

From: wuqianhai <wuqianhai@loongson.cn>

On LoongArch, PCI resource assignment completes during subsys_initcall
(acpi_init), before vga_arb_device_init() runs.  By that time the
firmware framebuffer BAR may already have been relocated, leaving
screen_info's lfb_base stale.  Enable vga_is_firmware_default() for
LoongArch and refresh screen_info in vga_arb_device_init() before the
boot device is matched.

Unlike x86, sysfb_primary_display is only defined when CONFIG_SYSFB is
enabled on LoongArch, so have VGA_ARB select SYSFB there.

Signed-off-by: wuqianhai <wuqianhai@loongson.cn>
---
Changes in v2:
- Add VGA_ARB's select of SYSFB for LoongArch to avoid an undefined
  reference when CONFIG_SCREEN_INFO is not set.
- Restrict the screen_info_apply_fixups() call to CONFIG_LOONGARCH so
  x86 and other architectures are unaffected.
- Enable vga_is_firmware_default() for LoongArch.
---
 drivers/pci/Kconfig  |  1 +
 drivers/pci/vgaarb.c | 12 +++++++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
index 0c7408509ba2..55cc60f5d312 100644
--- a/drivers/pci/Kconfig
+++ b/drivers/pci/Kconfig
@@ -256,6 +256,7 @@ config VGA_ARB
 	default y
 	depends on !S390
 	select SCREEN_INFO if X86
+	select SYSFB if LOONGARCH
 	help
 	  Some "legacy" VGA devices implemented on PCI typically have the same
 	  hard-decoded addresses as they did on ISA. When multiple PCI devices
diff --git a/drivers/pci/vgaarb.c b/drivers/pci/vgaarb.c
index 3e58b2c05c85..1b7e915629f9 100644
--- a/drivers/pci/vgaarb.c
+++ b/drivers/pci/vgaarb.c
@@ -566,7 +566,7 @@ EXPORT_SYMBOL(vga_put);
 
 static bool vga_is_firmware_default(struct pci_dev *pdev)
 {
-#if defined CONFIG_X86
+#if defined(CONFIG_X86) || defined(CONFIG_LOONGARCH)
 	return pdev == screen_info_pci_dev(&sysfb_primary_display.screen);
 #else
 	return false;
@@ -1527,6 +1527,16 @@ static int __init vga_arb_device_init(void)
 	int rc;
 	struct pci_dev *pdev;
 
+#if defined(CONFIG_LOONGARCH)
+	/*
+	 * On LoongArch, PCI resource assignment completes during
+	 * subsys_initcall (acpi_init), before the VGA arbiter runs.  Refresh
+	 * screen_info so vga_is_firmware_default() can match the relocated
+	 * BAR.
+	 */
+	screen_info_apply_fixups();
+#endif
+
 	rc = misc_register(&vga_arb_device);
 	if (rc < 0)
 		pr_err("error %d registering device\n", rc);
-- 
2.34.1

Re: [PATCH] PCI/VGA: Fix boot device detection after framebuffer relocation

[wuqianhai]

Hi Bjorn,

Thanks for the detailed analysis.

You're right that screen_info_fixup_lfb()/screen_info_apply_fixups()
stores a PCI bus address in screen_info_lfb_res_start and compares it
with the CPU address in pdev->resource[].start, which is broken on
systems where bus and CPU addresses differ.

On LoongArch, though, bus and CPU addresses are identical: with modern
firmware, acpi_prepare_root_resources() takes the PCIH path and leaves
the window offset at 0 (see commit 49f33840dcc9).  So the existing
screen_info_apply_fixups() works correctly for LoongArch, and the
address-space confusion you describe does not trigger there.

I've sent a v2 that:
- enables vga_is_firmware_default() for LoongArch,
- restricts the screen_info_apply_fixups() call to LoongArch so x86 and
  other architectures are unaffected,
- makes VGA_ARB select SYSFB on LoongArch to avoid the undefined
  reference when CONFIG_SCREEN_INFO is not set.

The general bus != CPU problem in screen_info_pci.c is still worth
fixing for ARM and friends, but I'd rather keep that separate from this
LoongArch enablement.  Happy to help with that follow-up if useful.

Regards,
wuqianhai