[Samsung Galaxy A21s ROMs, Kernels, Recoveries, & O] grccorps:
After doing some research on this device, I was able to port LineageOS. The original sources are based on Galaxy Tab S6 Lite Wi-Fi version (gta4xlwifi), with some heavy modifications made for the A21s. In the process, I was able to fix some of the problems that were found on GSI (MTP, Fingerprint removal issues, NFC, Wi-Fi hotspot and direct), and added in some enhancements to the kernel.

Unfortunately, installation of this ROM requires Lineage Recovery, and since FBE (File-based-encryption) is enabled by default, it is incompatible with the TWRP.

Screenshot:

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NOT WORKING:

- Cell Broadcast (Emergency Messages) is broken in South Korea. The phone shows notifications, but the messages appear broken. Not sure about other countries. FIXED in South Korea as of 2021/9/2. However, its functionality in other countries hasn't been tested.

- Samsung's Proprietary Apps

- In order to use Wi-Fi Direct, you need to install a Wi-Fi direct sharing app from Google Play, as LineageOS doesn't have a sharing app installed by default

- Auto Brightness (The phone doesn't have the sensors for it anyway)

- On very rare occasions(probably once in more than 20 boots), the device's RIL (Phone calls, Data) might start up late when booting. If you see "No service" after booting, please wait for 30-60 seconds.

- VoLTE & VoWifi(not tested) - Unfortunately, this is located deep within Samsung's framework. AOSP-based roms for other Samsung devices don't support it either. Someday, this might be fixed.....

WORKING:

- Everything Else

GITHUB:

Device Source Tree: android_device_samsung_a21s, android_device_samsung_universal3830-common

Kernel: android_kernel_samsung_universal3830-common

Instructions for Installation:

1. BEFORE INSTALLING, make sure that you have UNLOCKED the bootloader of your device. This is a great guide that show how to do it. Also, the phone must be on the Stock ANDROID 11. If you are on binary U6, please use the recovery tar for U6.

2. Download the Lineage Recovery, ROM, Google Apps, ADB, and Odin. Unzip ADB and Odin to somewhere appropriate.

- ROM

- Recovery / Recovery(for U6)

- Google Apps

- ADB

- Odin

3. Enable USB Debugging on the device. Go to settings->About phone->Software information, and tap the build number several times until you see the notification that the devloper settings have been enabled. After enabling developer settings, go to settings->developer settings and check USB debugging.

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4. Open Command Prompt, and type "cd <the path where you saved ADB to>".

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4-1. As a test, type "adb devices". After typing it, the phone should show a screen that asks you to grant permissions for USB debugging. Select Yes.

5. On the phone, start holding down the Volume Up and Down keys. KEEP THE KEYS PRESSED UNTIL THE PHONE REBOOTS AFTER YOU TYPE THE COMMAND IN THE NEXT STEP.

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6. While holding the keys down. type in the command "adb reboot recovery" on the command line.

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11. After the phone reboots into download mode, press volume up to continue. Make sure that you see the "OEM LOCK: OFF" before continuing.

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12. Open Odin. After opening Odin, click on AP, and select the Lineage Recovery Tar file that you downloaded in the step 2. After that, click on Start.

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13. After a few seconds, if everything goes well, Odin should show "PASS" and the device should reboot into Lineage Recovery. After the device enters recovery, tap on Apply update->Apply from ADB. In some cases, touchscreen may be unresponsive or slow. If that happens, use Volume Keys to navigate and Power key to select. KEEP THE DEVICE CONNECTED TO THE COMPUTER.

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14. After choosing Apply from ADB, type "adb sideload <the path where you saved the ROM file>" on the command prompt and press enter.

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