[coreboot] Getting an WXGA+ LED MVA panel to work on T400: "G141C1-L01"

Nico Huber nico.h at gmx.de
Sun Sep 25 23:36:05 CEST 2016

Hi Merlin,

On 24.09.2016 21:25, Merlin Büge wrote:
> Hello everyone!
> This is my first post to this mailing list :)
welcome to coreboot ;)

I didn't have the time for a closer look at your logs. But there is one
thing coreboot does definitely wrong: The clock configuration. You can
see it from the Linux log as it dumps the settings coreboot made first:

> [    6.523864] [drm:drm_mode_debug_printmodeline] Modeline 0:"1440x900" 50 87075 1440 1520 1672 1820 900 903 909 961 0x40 0xa
> [    6.523872] [drm:intel_dump_pipe_config] port clock: 87075

vs what Linux sets up later:

> [    8.787651] [drm:drm_mode_debug_printmodeline] Modeline 0:"1440x900" 60 104500 1440 1520 1672 1820 900 903 909 961 0x48 0xa
> [    8.787668] [drm:intel_dump_pipe_config] port clock: 104500

Not sure why Linux doesn't get the display running though. But as Linux
sometimes reuses register settings set by the firmware there is a slight
chance that fixing coreboot would make it work in Linux too.

There is a related patch up for review for i945 [1] (older chipset) that
fixes a similar issue there. Maybe it applies to gm45 too. The code
looks very similar.


[1] https://review.coreboot.org/#/c/16504/

> I'm trying to get a specific panel working on my Thinkpad T400 which runs
> libreboot. It's an WXGA+ MVA panel (CMO G141C1-L01) and I have soldered an
> adapter cable for it. From what I have found, nobody succeeded on this mod yet -
> there are some attempts on 51nb.com, but only for the T410. I basically took
> some already existing adapter cable, and modified a few pins, so that it would
> match up exactly with the datasheets. I used the LG LP141WP2-TLB1 for reference.
> Some details about my adapter cable can be found here:
> http://forum.thinkpads.com/viewtopic.php?f=45&t=121151
> In the first post there are also links to the datasheets of the two panels in
> question.
> Of course I thoroughly checked all my self-made connections and modifications,
> but only with some general purpose DMM for continuity. They all seem fine.
> So far backlight + PWM control works, but I have no image displayed, both in
> libreboot (with GRUB2 as payload) and Linux (up-to-date Arch Linux).
> I tried the panel both with the latest stock ROM (3.23/EC 1.06) and with
> the latest libreboot release, which is currently 2016090
> (t400_8mb_usqwerty_vesafb.rom in my case).
> Since I have not much experience with such low-level hardware stuff, could
> someone maybe help me with this or direct me to some useful direction, how I
> can debug this?
> I already tried booting with i915.lvds_channel_mode=2 (since I was not able
> to figure out what mode was actually used, because this module parameter is
> normally set to "0", which means the channel mode is taken from BIOS).
> I attached the EDIDs of both panels, the coreboot log (cbmem -c) of both
> panels, and a kernel log of both panels. I also attached a kernel log of the
> MVA panel with drm debbuging enabled (drm.debug=0x1e log_buf_len=1M). Note that
> all of the logs were taken from the T400 with the latest libreboot release.
> It would be great if somebody could have a look at it. (I don't understand much
> of it.)
> Here is a picture of the adapter cable, so that you have an idea what it looks
> like: https://lut.im/funPftuHF0/3sndAspbdiwOpTN4.jpg
> I already put *a lot* of effort in this project, and I would be very happy if it
> succeeded at the end of the day. I would design some PCBs then, so that this
> would basically become a drop-in replacement. Background is: The T400 is a very
> desirable target for libreboot, since it's build on the latest Intel hardware
> currently supported in libreboot for laptops. And (not only in the libreboot
> community) it is sadly well known for its rather bad TN panels. Also, it would
> be something that I could give back to the libreboot community.
> Since I have not much experience with mailing lists, please correct my of any
> 'bad' behavior.
> Thank you for reading this,
> and for any advice in advance :)
> Regards,
> Merlin

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