During this process, EDL implements the Firehose/Sahara protocol and acts as a Secondary Bootloader to accept commands for flashing. The figure on the right shows the boot process when EDL mode is executed.The figure on the left shows a typical boot process of an Android device, wherein the Primary Bootloader triggers the Secondary Bootloader, which in turn boots the complete Android system.
To have a better understanding, please take a look at the figures below. Unlike Fastboot, Download, and Recovery modes on Android, which reside in the Secondary Bootloader (SBL), PBL resides within the ROM and so it could not be corrupted due to software errors (again, like a wrong flash). EDL itself is a part of the Primary Bootloader (PBL) on Qualcomm Devices. That’s exactly when you’d need to use EDL mode. We often like to refer to this device state as a “Hard-brick”. And thus, there would be no chance of flashing the firmware to revive/unbrick the device. Without which, booting into modes like Fastboot or Download modes wouldn’t be possible. Some times, flashing the wrong file can also potentially corrupt the Android bootloader itself.
Generally if the device’s software is corrupted due to a wrong flash or any other software issue, it could be revived by flashing the firmware through Fastboot and Download modes. The OEM flash tools can only communicate with a device and flash it through the said modes. If you’re familiar with flashing firmware or custom binaries (like TWRP, root, etc), you’d know that it is required to boot the Android device into specific boot modes like Fastboot or Download Modes. Why and when would you need to use EDL Mode? When in this mode, the device identifies itself as “ Qualcomm HS-USB QDLoader 9008” over a USB connection.
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