[PATCH] mgag200 fix memmapsl configuration in GCTL6 register

[Javier Martinez Canillas]

Hello Jocelyn,

On 1/14/22 10:47, Jocelyn Falempe wrote:
> On some server with MGA G200e (rev 42), booting with Legacy BIOS,
> The hardware hangs when using kdump and kexec into the kdump kernel.
> This happens when the uncompress code tries to write "Decompressing Linux"
> to the VGA Console.
> 
> It can be reproduced by writing to the VGA console (0xB8000) after
> booting to graphic mode, it generates the following error:
> 
> kernel:NMI: PCI system error (SERR) for reason a0 on CPU 0.
> kernel:Dazed and confused, but trying to continue
> 
> The root cause is a bad configuration of the MGA GCTL6 register
> 
> According to the GCTL6 register documentation:
> 
> bit 0 is gcgrmode:
>     0: Enables alpha mode, and the character generator addressing system is activated.
>     1: Enables graphics mode, and the character addressing system is not used.
> 
> bit 1 is chainodd even:
>     0: The A0 signal of the memory address bus is used during system memory
>     addressing.
>     1: Allows A0 to be replaced by either the A16 signal of the system address (if
>     memmapsl is ‘00’), or by the hpgoddev (MISC<5>, odd/even page select) field,
>     described on page 3-294).
> 
> bit 3-2 are memmapsl:
>     Memory map select bits 1 and 0. VGA.
>     These bits select where the video memory is mapped, as shown below:
>         00 => A0000h - BFFFFh
>         01 => A0000h - AFFFFh
>         10 => B0000h - B7FFFh
>         11 => B8000h - BFFFFh
> 
> bit 7-4 are reserved.
> 
> Current driver code set it to 0x05 => memmapsl to b01 => 0xA0000
> but on x86, the VGA console is at 0xB8000

I think this need some rewording after imirkin's explanation that 0xA0000 is the
address of the VGA video memory and 0xB8000 the address of the VGA text buffer.

> arch/x86/boot/compressed/misc.c define vidmem to 0xb8000 in extract_kernel()
> so it's better to configure it to b11
> Thus changing the value 0x05 to 0x0d
> 
> Signed-off-by: Jocelyn Falempe <jfalempe at redhat.com>
> ---
>  drivers/gpu/drm/mgag200/mgag200_mode.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/mgag200/mgag200_mode.c b/drivers/gpu/drm/mgag200/mgag200_mode.c
> index b983541a4c53..c7f63610b278 100644
> --- a/drivers/gpu/drm/mgag200/mgag200_mode.c
> +++ b/drivers/gpu/drm/mgag200/mgag200_mode.c
> @@ -529,7 +529,7 @@ static void mgag200_set_format_regs(struct mga_device *mdev,
>  	WREG_GFX(3, 0x00);
>  	WREG_GFX(4, 0x00);
>  	WREG_GFX(5, 0x40);
> -	WREG_GFX(6, 0x05);
> +	WREG_GFX(6, 0x0d);

My worry is if this could cause other issues so I would only do this change
if (is_kdump_kernel()), to make it as non intrusive as possible. And also
add a verbose comment about why this is needed.

If you make those changes, feel free to add:

Reviewed-by: Javier Martinez Canillas <javierm at redhat.com>

Best regards,
-- 
Javier Martinez Canillas
Linux Engineering
Red Hat