Hi,

Am 19.01.22 um 11:29 schrieb Jocelyn Falempe:

On some servers with MGA G200_SE_A (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 the 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 (ifmemmapsl 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 code set it to 0x05 => memmapsl to b01 => 0xa0000 (graphic mode) But on x86, the VGA console is at 0xb8000 (text mode) In arch/x86/boot/compressed/misc.c debug strings are written to 0xb8000 As the driver doesn't use this mapping at 0xa0000, it is safe to set it to 0xb8000 instead, to avoid kernel hang on G200_SE_A rev42, with kexec/kdump.

Thus changing the value 0x05 to 0x0d

Signed-off-by: Jocelyn Falempe jfalempe@redhat.com Reviewed-by: Javier Martinez Canillas javierm@redhat.com Acked-by: Lyude Paul lyude@redhat.com Cc: stable@vger.kernel.org

---

v2: Add clear statement that it's not the right configuration, but it prevents an annoying bug with kexec/kdump.

drivers/gpu/drm/mgag200/mgag200_mode.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/mgag200/mgag200_mode.c b/drivers/gpu/drm/mgag200/mgag200_mode.c index b983541a4c53..cd9ba13ad5fc 100644 --- a/drivers/gpu/drm/mgag200/mgag200_mode.c +++ b/drivers/gpu/drm/mgag200/mgag200_mode.c @@ -529,7 +529,10 @@ 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);

• /* GCTL6 should be 0x05, but we configure memmapsl to 0xb8000 (text mode),

• * so that it doesn't hang when running kexec/kdump on G200_SE rev42.
  

• */
  

• WREG_GFX(6, 0x0d);

Appears to be working on my test machine.

But please rune scripts/checkpatch.pl on the patch before sending it. I get several errors

WARNING: Possible unwrapped commit description (prefer a maximum 75 chars per line)

#98:

0: Enables alpha mode, and the character generator addressing system is

ERROR: trailing whitespace

#149: FILE: drivers/gpu/drm/mgag200/mgag200_mode.c:532:

+^I/* GCTL6 should be 0x05, but we configure memmapsl to 0xb8000 (text mode),^M$

ERROR: trailing whitespace

#150: FILE: drivers/gpu/drm/mgag200/mgag200_mode.c:533:

+^I * so that it doesn't hang when running kexec/kdump on G200_SE rev42.^M$

Best regards Thomas

WREG_GFX(7, 0x0f); WREG_GFX(8, 0x0f);