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@xxxxxxxxxx> > --- > 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@xxxxxxxxxx> Best regards, -- Javier Martinez Canillas Linux Engineering Red Hat
