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

[Bjorn Helgaas]

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

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
>