Troubleshooting LVDS on GM45 (kernel 4.5.0)

[Rob Kramer]

Here's the same issue on a more reasonable kernel. The log looks quite 
different, but the end result is still a blank screen. Uploaded here: 
http://pastebin.com/WLpT5mPc

This section looks promising:

[    4.763951] [drm:intel_dump_pipe_config] requested mode:
[    4.763953] [drm:drm_mode_debug_printmodeline] Modeline 0:"1920x1080" 
60 148571 1920 2040 2080 2200 1080 1097 1108 1125 0x40 0xa
[    4.763955] [drm:intel_dump_pipe_config] adjusted mode:
[    4.763958] [drm:drm_mode_debug_printmodeline] Modeline 0:"1920x1080" 
60 148571 1920 2040 2080 2200 1080 1097 1108 1125 0x40 0xa
[    4.763961] [drm:intel_dump_crtc_timings] crtc timings: 148571 1920 
2040 2080 2200 1080 1097 1108 1125, type: 0x40 flags: 0xa

.. but I don't know how relevant it is.  I also have no idea where those 
timings came from.

Could anyone tell me what I could try next?

Cheers!

      Rob


On 04/01/2016 12:49 PM, Rob Kramer wrote:
> Hi,
>
> 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: 
> http://pastebin.com/nYNRGW5B
>
> 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 
> reason..
>
> All sorts of kernel warnings follow, after this (what is LVDS-6?):
>
>   [    5.570569] [drm:intel_crtc_mode_set], [ENCODER:6:LVDS-6] set 
> [MODE:0:1920x1080]
>
> Please let me know if you'd like more logging or details.
>
> Cheers,
>
>      Rob
>