Troubleshooting LVDS on GM45.

Rob Kramer rob at
Fri Apr 1 04:49:02 UTC 2016


My setup is an embedded GM45 based board with a screen connected via 24 
bit LVDS. When trying a to use a newer kernel (3.14.57) with KMS, the 
screen stays blank. I'm only testing KMS so far, no X. Older, pre-KMS 
versions of the kernel worked fine.

I'm hoping that a resident Intel DRM guru has an idea of what might be 
wrong here, or can tell me where to continue troubleshooting :)

I've uploaded a kernel log (drm.debug=0xe) here:

The video kernel parameter for this log is: video=LVDS-1:1920x1080. I've 
tried others, M and/or R suffixes, loading edid/1920x1080.bin etc, but 
nothing works. I'm not sure if all this EDID timing stuff is even used 
for LVDS..

One line in the log mentions a mode that is promising:

   [    5.534021] [drm:intel_dump_crtc_timings], crtc timings: 148571 
1920 2040 2080 2200 1080 1097 1108 1125, type: 0x0 flags: 0xa

In the BIOS, the panel type is configured as "1920x1080 at 148.5M", so that 
looks good. I have no idea where that modeline came from though.

 From what I can see, LVDS-1 and CRTC 3 are connected, but when it's 
time to set the mode, it seems to have been modified into a bogus 
adjusted mode:

   [    5.570523] [drm:intel_dump_pipe_config], requested mode:
   [    5.570525] [drm:drm_mode_debug_printmodeline], Modeline 
0:"1920x1080" 0 172780 1920 2040 2248 2576 1080 1081 1084 1118 0x0 0x6
   [    5.570526] [drm:intel_dump_pipe_config], adjusted mode:
   [    5.570529] [drm:drm_mode_debug_printmodeline], Modeline 
0:"640x480" 0 148571 640 656 752 800 480 490 492 525 0x8 0x0
   [    5.570531] [drm:intel_dump_crtc_timings], crtc timings: 148571 
640 656 752 800 480 490 492 525, type: 0x8 flags: 0x0

Also, the dot-clock mentioned in the HD mode is now 172780, for some 

All sorts of kernel warnings follow, after this (what is LVDS-6?):

   [    5.570569] [drm:intel_crtc_mode_set], [ENCODER:6:LVDS-6] set 

Please let me know if you'd like more logging or details.



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