Alexander Couzens has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/28950 )
Change subject: lenovo/x230: introduce FHD variant ......................................................................
Patch Set 9:
U1 acts as a smart translator for EN/PWM signals - whenever EN is high, PWM from the system is translated to the FHD display. Otherwise, EN and PWM are translated high if there's a DC bias on DisplayPort TX lane, which means something is being transmitted to the display. Initially I had an idea that some displays may have PWM brightness regulation, so I also set output PWM frequency to around 1kHz to make it less disgusting. With proper support in the system firmware the adapter board could be completely passive. U1 expects the default 230Hz frequency (or is it 220Hz?), but I believe it should work with 1kHz just as well.
With the default assembly, panel power is sourced directly from VCC3M power rail, so you can't control it. By removing F1 and bridging J1 panel power is switched to VCC3P, which is sourced from a load switch on the motherboard, so you can control it. Note that actually moving F1 to J1 isn't necessary because there's already a fuse on the motherboard.
Thanks for the explanation. Now I understand also the bug which I have with the display. My lid does not trigger suspend (I configured it this way). So when my lid is closed, the EN line must be low, but the display is on full brightness. I will try to desolder the sense pin.
@nitrocaster I could imagine the firmware of the STM8 is not much. Do you thought about open sourcing it?
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Alexander Couzens (Code Review)