K3 [KT3 / SY69JL] Recovered a Kindle stuck on the boot logo

[b4ldm4ch1n3]

I recovered a KT3 (model SY69JL, Kindle Basic 8th gen) that was frozen on the boot screen (logo on top, tree-with-a-kid picture below) and that Windows didn't detect at all. I'm posting what worked, including the dead ends, in case it helps someone with the same symptom. It was a single unit, so please treat it as one data point, not a guaranteed recipe.

Short version

• Try another USB cable. The first one was charge-only. With a data cable the device showed up as VID_15A2 PID_0063, which uuu -lsusb reports as MX6SL in SDP mode. So the CPU was alive and it was recoverable over USB. (It was not the battery, and there is no "BIOS" to flash.)
  
• Loaded a fastboot-capable u-boot over SDP with uuu (MollySophia's imx6_android_u-boot_kindle). The fastboot trigger there is two power-button long presses within LP_FASTBOOT_THRESHOLD seconds (15 by default; "LP" = long press). I raised it to 600 so it can be met while loading over USB. It needed a 40 s forced restart between the two loads.
  
• With fastboot, flashed the bootloader and stock partitions from the CracKdroid kindle8.230118.zip package (restore.img, then userdata, diags, diags_kernel, kernel, system). Each command needed its own fastboot window (~8 s, VID_18D1 PID_4E40), so I automated "wait for the device, run one command".
  
• The system booted but the home screen stayed blank (lock screen, power menu, Wi-Fi and keyboard worked). Dropping an empty DO_FACTORY_RESTORE file in the root and rebooting brought up the setup wizard and everything worked. (That trick comes from the KT3 demo-mode thread here.)
  
• Jailbroke with KindleBreak (the unit ended up on 5.12.2), then Hotfix, then KUAL. KUAL only opened after I typed ;log mrpi in the search bar (I can't say with certainty that this was the fix; it was the last thing changed before it worked). KOReader (kindlepw2 build) then ran fine.

Things that cost me the most time

• The cable (twice: detection, and later fastboot writes failing with AdbWriteEndpointSync ... semaphore timeout).
  
• Google's fastboot.exe didn't see the device after installing WinUSB with Zadig, because Zadig uses its own interface GUID. Setting DeviceInterfaceGUIDs to {F72FE0D4-CBCB-407D-8814-9ED673D0DD6B} fixed it.
  
• The package's fastdownload.exe hangs (100 % CPU) if the fastboot window closes mid-transfer. Doing the same writes one command per window with fastboot was more reliable.
  
• Timing is everything. The window is short (~8 s), so a polling script plus a device that keeps rebooting is what makes it work. Once the system boots, the window no longer opens by itself: it needs the double long press (first press ~10 s until it resets, then start the second press while the logo/tree screen is showing, before the progress bar). If the second press comes after the system has loaded, nothing happens.
  
• Windows Defender flagged the CracKdroid zip as Trojan:Script/Phonzy.A!ml. I read the batch script before using it (it only calls fastboot/adb); I suspect a false positive but could not audit fastdownload.exe, which is closed and unsigned.

What I don't know

• Why the first boot with the new bootloader showed the "repair needed" screen and later ones didn't.
  
• Why the home screen was blank before DO_FACTORY_RESTORE.
  
• Whether ;log mrpi is what really fixed KUAL.

Build notes: the u-boot tree from 2013 doesn't compile with a modern GCC (I used 10.3). I needed a few small fixes (extern inline in headers, inline on __foo functions that have a weak alias, and a missing compiler-gcc10.h). I've put them in two separate patches: one for the build fixes and one for the threshold. [PATCH_LINK]

Full write-up (step by step, commands, and a "what I don't know" section): [GUIDE_LINK]

Thanks to the people in the KT3 debricking thread and to MollySophia for the u-boot; none of this would have been possible without that work. Happy to answer questions or test things if anyone has the same fault.