Troubleshooting LVDS on GM45.

[Jani Nikula]

On Fri, 01 Apr 2016, Rob Kramer <rob at solution-space.com> wrote:
> 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'll put it bluntly, I don't think anyone's interested in
troubleshooting v3.14 anymore. Please try v4.5.

BR,
Jani.


>
> 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
>
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-- 
Jani Nikula, Intel Open Source Technology Center