[Galaxy S6 General] low_voltage:
Good news - I have successfully flashed LineageOS 17.1 on one G925X and two G920X LDU mainboards, which I bought from ebay, so I could repurpose these devices in some electronics projects.

Here's how I did it:

First of all, the boards both came with Android 5.0.2 (G920XXXU1AOBF_LLK for G920X, G925XXXU1AOD7_LLK for G925X). However, I did not manage to boot the kernel from LineageOS 17.1 with the bootloader of this firmware (seems to load/start the kernel to a different address, but I didn't figure it out).

Nevertheless, a newer version is available (as of the time of writing) as an OTA update. Make sure you don't tamper with the device prior to getting that update, because otherwise it won't download the OTA update; I was still able to fix one of by G920X by extracting the bootloader from the other, already upgraded one and flashing it using Odin).

This OTA update gave me Android version 5.1.1 (G925XXXU1CPC1_LLK on the G925X).

The next steps were as follows:

1. Enable Developer Options / ADB

(To take screenshots and to use adb to look around; don't know if it's strictly necessary.)

Then I also went to the retail mode panel, and unset the "Disable the factory data reset" switch. Not sure if that step is strictly necessary.

2. Issue "adb reboot download"

from the PC connected via USB data cable (Alternative: Enter download mode via key combination VolDown+Home+Power)

3. Flash Lineage 17.1 recovery

(here I used a self-compiled version from 2020-10, built according to scripts from los-legacy.de)

I was using heimdall for that:

heimdall flash --no-reboot --verbose --RECOVERY recovery.img

4. Reboot directly to recovery: Switch off via VolDown+Power, then directly hold Home+VolUp+Power

5. In LOS Recovery (17.1 20201009):

Advanced Options -> Enable ADB

6. (Optional): Make a complete backup of partitions using e.g.

adb pull /dev/block/sda1 sda1_BOTA0
adb pull /dev/block/sda2 sda2_BOTA1
adb pull /dev/block/sda3 sda3_EFS
adb pull /dev/block/sda4 sda4_PARAM
adb pull /dev/block/sda5 sda5_BOOT
# adb pull /dev/block/sda6 sda6_RECOVERY # is gone already, this is the LOS version
adb pull /dev/block/sda7 sda7_OTA
adb pull /dev/block/sda8 sda8_RADIO
adb pull /dev/block/sda9 sda9_TOMBSTONES
adb pull /dev/block/sda10 sda10_DNT
adb pull /dev/block/sda11 sda11_PERSISTENT
adb pull /dev/block/sda12 sda12_STEADY
adb pull /dev/block/sda13 sda13_PERSDATA
adb pull /dev/block/sda14 sda14_SBFS
adb pull /dev/block/sda15 sda15_SYSTEM
adb pull /dev/block/sda16 sda16_CACHE
adb pull /dev/block/sda17 sda17_HIDDEN
adb pull /dev/block/sda18 sda18_USERDATA

7. Choose "Apply Update" / "Apply from ADB"

,

and on the PC issue

adb sideload lineage-1.17-20201009-UNOFFICIAL-zeroltexx.zip

(Alternative:

7a. adb push lineage-1.17-20201009-UNOFFICIAL-zeroltexx.zip /data/media/0/

7b. Choose "Apply Update" / "Choose from internal storage" / navigate to .zip

)

NB: I hat to do some extra steps to fix permissions in the LineageOS image (privapp-permissions-lineage-xml was missing).

8. Choose "Factory reset"

then "Format cache partition"

then "Format data/factory reset"

You may choose Reboot System now, but it will bootloop/hang while loading.

Here's the critical step:

9. Patch Kernel CMDLINE in Bootloader

This is the critical step to get the bootloader to start the LineageOS 17.1 ROM correctly!

For this step, I build an USB serial jig cable with 619 kilo-Ohm resistor (cf. https://xdaforums.com/t/sgs5-uart-jig.3535833/ or https://xdaforums.com/t/guide-samsung-galaxy-s7-uart.3743895/; Actually, I used 3 serial 220k resistors, which luckily was close enough).

(Note 1: In case you're interested: the kernel can access this UART as ttySAC1 on S6 devices; use console=ttySAC1,115200 for serial console output from the kernel. This is how I found out what was wrong.)

(Note 2: There may be ways to modify the kernel commandline directly from within the recovery system by modifying the PARAM partition, but I did not bother to find out how.)

Using such a cable, I did the following:

a) Power off phone

b) Connect jig cable to PC (in my case, a FTDI serial-to-USB converter); Run serial terminal program (e.g. minicom) on this line, e.g.

minicom -D /dev/ttyUSB0

c) Power on while holding VolDown on the phone

d) Repeatedly hit enter on the serial terminal, until you reach the

"S-BOOT # " prompt of the bootloader.

e) Type on that prompt:

setenv CMDLINE androidboot.hardware=samsungexynos7420 console=ram loglevel=4
saveenv
reset

The added "androidboot.hardware=samsungexynos7420" seems to be necessary for correct initialization of the ROM (in its initrd).

10. Done.

Lineage 17.1 starts its configuration wizard.

Here's how things look afterwards (German language settings):