[PATCH] mgag200 fix memmapsl configuration in GCTL6 register

[Lyude Paul]

We should probably  Cc: stable at vger.kernel.org this as well, see: 

https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html for
more info. As well, some useful tools for adding the appropriate Fixes: tags:

https://drm.pages.freedesktop.org/maintainer-tools/dim.html

At least on my end this is:

Acked-by: Lyude Paul <lyude at redhat.com>

I'd very much like Thomas Zimmerman to verify that this patch is OK though
with an R-b before we push anything upstream.

On Fri, 2022-01-14 at 10:47 +0100, 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
> 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);
>         WREG_GFX(7, 0x0f);
>         WREG_GFX(8, 0x0f);
>  

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat