[PATCH 4.14 43/77] fbdev: sm712fb: fix white screen of death on reboot, dont set CR3B-CR3F

[Greg Kroah-Hartman]

From: Yifeng Li <tomli@xxxxxxxx>

commit 8069053880e0ee3a75fd6d7e0a30293265fe3de4 upstream.

On a Thinkpad s30 (Pentium III / i440MX, Lynx3DM), rebooting with
sm712fb framebuffer driver would cause a white screen of death on
the next POST, presumably the proper timings for the LCD panel was
not reprogrammed properly by the BIOS.

Experiments showed a few CRTC Scratch Registers, including CRT3D,
CRT3E and CRT3F may be used internally by BIOS as some flags. CRT3B is
a hardware testing register, we shouldn't mess with it. CRT3C has
blanking signal and line compare control, which is not needed for this
driver.

Stop writing to CR3B-CR3F (a.k.a CRT3B-CRT3F) registers. Even if these
registers don't have side-effect on other systems, writing to them is
also highly questionable.

Signed-off-by: Yifeng Li <tomli@xxxxxxxx>
Tested-by: Sudip Mukherjee <sudipm.mukherjee@xxxxxxxxx>
Cc: Teddy Wang <teddy.wang@xxxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>  # v4.4+
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/video/fbdev/sm712fb.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- a/drivers/video/fbdev/sm712fb.c
+++ b/drivers/video/fbdev/sm712fb.c
@@ -1173,8 +1173,12 @@ static void sm7xx_set_timing(struct smtc
 			smtc_crtcw(i, vgamode[j].init_cr00_cr18[i]);
 
 		/* init CRTC register CR30 - CR4D */
-		for (i = 0; i < SIZE_CR30_CR4D; i++)
+		for (i = 0; i < SIZE_CR30_CR4D; i++) {
+			if ((i + 0x30) >= 0x3B && (i + 0x30) <= 0x3F)
+				/* side-effect, don't write to CR3B-CR3F */
+				continue;
 			smtc_crtcw(i + 0x30, vgamode[j].init_cr30_cr4d[i]);
+		}
 
 		/* init CRTC register CR90 - CRA7 */
 		for (i = 0; i < SIZE_CR90_CRA7; i++)