Re: [PATCH] vgacon: Fall back to the dummy console if mapping VGA memory fails

[Magnus Lindholm]

Hi Matt,

On Thu, Nov 12, 2026 at 1:30 AM Matt Turner <mattst88@xxxxxxxxx> wrote:
>
> On Alpha, setup_arch() registers vgacon whenever CONFIG_VGA_CONSOLE is
> enabled. On Marvel (EV7) systems whose SRM console is serial, no VGA
> hose is set, so marvel_ioremap() finds no hose for 0xb8000 and returns
> NULL. vgacon_startup() then probes video memory through that NULL
> pointer and oopses in console_init():
>
>   Unable to handle kernel paging request at virtual address 0000000000000000
>   pc is at vgacon_startup+0x174/0x910
>
> VGA_MAP_MEM() is ioremap() on Alpha and powerpc and can fail. Treat
> failure like a failed probe and fall back to the dummy console. If a
> VGA card is present, locate_and_init_vga() still switches to vgacon
> once PCI is up.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: Claude:claude-opus-5-5
> Signed-off-by: Matt Turner <mattst88@xxxxxxxxx>
> ---
>  drivers/video/console/vgacon.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c
> index e7e6750f1f87..09f7c376b468 100644
> --- a/drivers/video/console/vgacon.c
> +++ b/drivers/video/console/vgacon.c
> @@ -324,6 +324,8 @@ static const char *vgacon_startup(void)
>         }
>
>         vga_vram_base = VGA_MAP_MEM(vga_vram_base, vga_vram_size);
> +       if (!vga_vram_base)
> +               goto no_vga;
>         vga_vram_end = vga_vram_base + vga_vram_size;
>
>         /*
>
> ---
> base-commit: fa4ad798f6f681031775266044ba1b0cf3c670bd
> change-id: 20261111-vgacon-ioremap-null-ad4c7184b807
>
> Best regards,
> --
> Matt Turner <mattst88@xxxxxxxxx>
>
>

Nice fix! this must have been lingering here for a long time?
Cross-builds passed for generic Alpha, including Marvel. The shared
vgacon code also compiled successfully for x86-64.

Alpha QEMU fault injection confirmed the early dummy fallback and
successful VGA takeover when only the first mapping fails. Without
your check, the injected NULL mapping faults.

Forcing the later mapping to fail too exposes a separate takeover
issue: the console core installs vgacon despite the failed mapping,
leading to a NULL dereference. I don't see that as blocking this fix:
finding the VGA hose normally resolves the mapping failure on Marvel.

Reviewed-by: Magnus Lindholm <linmag7@xxxxxxxxx>
Tested-by: Magnus Lindholm <linmag7@xxxxxxxxx>

Thanks,
Magnus