[coreboot] HV121WX4-120 screen for X200

[Piotr Kubaj]

Hi,

I just wanted to share my experience of using alternative screens for ThinkPad X200 (HV121WX4-120) with coreboot.

After fitting the screen, it became apparent that something is wrong.
Having turned on the X200, it ran happily for about 1 minute, then the screen just powered off. It didn't matter whether I stayed in SeaBIOS, or just booted my OS. And when the screen powered off, I could awake it after several minutes, but only for another minute. Apart from that, the laptop worked.

nico_h from IRC helped me (thank you!) by providing alternative values for devicetree, which could make the screen work orderly. Well, it did work better, but still wasn't stable (it worked for 10-20 minutes, once I even got it to 2 hours).

Still, that was a huge difference. I tried to experiment a little with it and set some crazy values:
        register "gpu_panel_power_up_delay" = "600"             # T1+T2: 25ms
        register "gpu_panel_power_down_delay" = "3000"          # T3:    25ms
        register "gpu_panel_power_backlight_on_delay" = "500"   # T5:   250ms
        register "gpu_panel_power_backlight_off_delay" = "2500" # Tx:   250ms
        register "gpu_panel_power_cycle_delay" = "255"          # T4:   200ms

After that, things seem to work fine.

I hope someone finds it useful if they replace their X200's screen with alternative one.

Again thanks to Nico for help!

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