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I have came across Boot flow and when there is based on boot state of a device, it shows different colot screen.

YELLOW: Warning screen for LOCKED devices with custom root of trust set
ORANGE: Warning screen for UNLOCKED devices
RED (eio): Warning screen for dm-verity corruption
RED (no os found): No valid OS found

Let me know where or from which path those screens are displaying?

Eg:Yellow Screen

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This is all happen in OEM bootloader (proprietary) before android is started. This screen is displayed before splash screen only in yellow/orange/red state

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    @Abracadabra Bootloaders aren't part of AOSP. They are OEM-specific thing. AOSP contains just a basic reference implementation of final bootloader (which loads kernel); check AVB documentation and code. Final bootloader in the device may or may not be based on it. It's entirely up to the OEM. Sep 8, 2021 at 7:08
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    don't know what you mean. bootloader is proprietary. do you have source code of aboot or lk for your device?
    – alecxs
    Sep 8, 2021 at 12:20
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    @Abracadabra please spend time to learn the basics. It'll help you self answer many of your questions. You can start with forum.xda-developers.com/t/… and forum.xda-developers.com/t/… and android.stackexchange.com/a/220678/218526. Do check the AOSP links in the last link. Sep 9, 2021 at 5:31
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    IrfanLatif has linked source code of aboot but I am not even sure this is the stage where OEMs implement boot state warning screen. After reading source code I believe it is happen earlier in OEM bootloader which is what I should have called SBL (which is proprietary closed source). AVB/dm-verity on the other hand is implemented in aboot. google documentation is bit confusing because bootloader check (yellow screen) is actually predecessor to verifiedboot newandroidbook.com/Articles/aboot.html
    – alecxs
    Sep 9, 2021 at 11:01
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    for mediatek devices it is in lk (little kernel, aka fastboot) which is bit different to qualcomm devices. XopmoH97 has written hex patch for disable yellow screen on mediatek devices orange_state_disabler_v0.3.zip
    – alecxs
    Sep 9, 2021 at 11:14

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