I have a charging related issue with my LG UltraFine 4K display. It’s charging at an excruciatingly slow speed and the estimate at 50% to full is 11 hours! I’m on Archlinux.
The display and the hub on the display works fine. The display is Thunderbolt 3 and the spec claims “Power Delivery: Up to 85W over Thunderbolt 3 or USB-C for host power and charging”. The display does indeed charge my Macbook super fast.
Anyone ideas?
With the help of @DHowett (thanks), I’ve managed to figure out where the problem may lie. The laptop and the screen are negotiating 9V at 1.6A based on the PD logs:
[183555.279942 CYPD_RESPONSE_PORT_CONNECT 3]
[183555.389105 CYPD_RESPONSE_PD_CONTRACT_NEGOTIATION_COMPLETE 3]
[183555.556424 PD Source supply changed! old=0x0, new=0x08]
[183555.573982 update charger!!]
[183555.576449 AC on]
[183555.576916 event set 0x0000000000000008]
[183555.581786 event clear 0x0000000000000008]
[183555.582411 ACPI query = 4]
PORT80: 0004
[183555.614846 CYPD_RESPONSE_PD_CONTRACT_NEGOTIATION_COMPLETE 3]
[183555.625038 Updating board_set_active_charge_port port 3]
[183555.628428 CL: p3 s0 i1660 v9000]
[183555.629187 TODO Implement pd_set_new_power_request port 3]
[183555.631259 INTR_REG CTRL:0 TODO Device 0x2]
[183555.632409 cypd_update_power_status power_stat 0xe]
[183555.640997 INTR_REG CTRL:1 TODO Device 0x2]
[183555.657804 CYPD_RESPONSE_PD_CONTRACT_NEGOTIATION_COMPLETE 3]
[183555.685395 CYP_RESPONSE_RX_EXT_MSG]
[183555.716239 CYP_RESPONSE_RX_EXT_MSG]
Port:3 Unknown data type: 0x06 Hdr:0x8f86 ExtHdr:0x0002 Data:0x0000
[183555.762599 CYP_RESPONSE_RX_EXT_MSG]
The CL: p3 s0 i1660 v9000 is the interesting bit. p3 is the port number, s0 is the supplier number, i1660 (in milliamps) is the charge current and v9000 (in millivolts) is the charge voltage. This amounts to a mere 15 watts of power. The laptop often pulls more than this based on the workload hence the slow charge rate.
In contrast, when connecting my 60W power bank or wall adapter, I see CL: p2 s0 i3000 v20000.
The screen definitely claims to output 80+ watts (and confirmed independently) but having used a PD trigger to see all the offered PD profiles, it only lists:
5V @ 3A
9V @ 1.66A
15V @ 1A
20V @ 0.75A
All of which point to a max. of 15 watts! So I did a bit more research and it turns out that LG intentionally limits the output if a non-Apple device is connected. WTF!
EDIT: Another post highlights the PD shenanigans over Thunderbolt involving Apple devices.
EDIT: The larger LG UltraFine 5K doesn’t have this issue. It reports 20V at 4.7A or 94 Watts. But it has other issues on linux! 
That was literally my response reading this, strange why would they do this? That Reddit post is a year old and not an official response, I would want to hear from LG on the matter. Thanks for sharing the information, aparently “Life’s Good” only if you use an Apple.