fbdev: sm712fb: fix VRAM detection, don't set SR70/71/74/75

commit dcf9070595e100942c539e229dde4770aaeaa4e9 upstream.

On a Thinkpad s30 (Pentium III / i440MX, Lynx3DM), the amount of Video
RAM is not detected correctly by the xf86-video-siliconmotion driver.
This is because sm712fb overwrites the GPR71 Scratch Pad Register, which
is set by BIOS on x86 and used to indicate amount of VRAM.

Other Scratch Pad Registers, including GPR70/74/75, don't have the same
side-effect, but overwriting to them is still questionable, as they are
not related to modesetting.

Stop writing to SR70/71/74/75 (a.k.a GPR70/71/74/75).

Signed-off-by: Yifeng Li <tomli@tomli.me>
Tested-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Cc: Teddy Wang <teddy.wang@siliconmotion.com>
Cc: <stable@vger.kernel.org>  # v4.4+
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/video/fbdev/sm712fb.c b/drivers/video/fbdev/sm712fb.c
index e4836bbeb9d15d097975932b4b99001a803246f5..7dcf94fbc3b0ec93494f1f9d66ab2fa2f26ddd61 100644 (file)
--- a/drivers/video/fbdev/sm712fb.c
+++ b/drivers/video/fbdev/sm712fb.c
@@ -1145,7 +1145,9 @@ static void sm7xx_set_timing(struct smtcfb_info *sfb)
                /* init SEQ register SR30 - SR75 */
                for (i = 0; i < SIZE_SR30_SR75; i++)
                        if ((i + 0x30) != 0x30 && (i + 0x30) != 0x62 &&
-                           (i + 0x30) != 0x6a && (i + 0x30) != 0x6b)
+                           (i + 0x30) != 0x6a && (i + 0x30) != 0x6b &&
+                           (i + 0x30) != 0x70 && (i + 0x30) != 0x71 &&
+                           (i + 0x30) != 0x74 && (i + 0x30) != 0x75)
                                smtc_seqw(i + 0x30,
                                          vgamode[j].init_sr30_sr75[i]);