Qualcomm EDL mode

The Qualcomm Emergency Download mode, commonly known as Qualcomm EDL mode and officially known as Qualcomm HS-USB QD-Loader 9008 is a feature implemented in the boot ROM of a system on a chip by Qualcomm which can be used to recover bricked smartphones. On Google's Pixel 3, the feature was accidentally shown to users after the phone was bricked.

Device support
For a device to support EDL it must be using Qualcomm hardware. The Snapdragon family is very widely used.

ADB
The Android Debug Bridge can be utilized to get access to EDL mode, with the command.

Windows
The Qualcomm Product Support Tool (QPST) is normally used internally by service center executives for low-level firmware flashing to revive Android devices from a hard-brick or to fix persistent software issues. To flash the firmware, the tool communicates with supported devices via EDL. The QPST has not been officially released by Qualcomm.

Linux
Qualcomm Download (QDL) is a tool to communicate with Qualcomm System On a Chip bootroms to install or execute code. The source code is maintained by Bjorn Andersson aka andersson.

Test points
Qualcomm implemented motherboards always include a test point. These can vary between phone models. Generally, test points are a pair of contacts, which can be some way apart. EDL can be accessed by opening the back of the phone, finding the location of the test point, which depends on the model, and by using a pair of metal tweezers to short the connectors and boot the phone into EDL. Further software tools are needed for actions in EDL mode.

EDL Deep Flash Cable
Qualcomm implemented motherboards, with the presence of EDL, can be booted to EDL via the use of a EDL Deep Flash Cable. This specific cable has a general appearance of a button present in the cable. The button can be represented as a switch, to be able to make the phone boot into EDL mode. With the use of the cable, in most devices and cases, it will not be necessary to use the test points. The cable also works on hard-bricked devices to boot them into EDL mode. This cable works by having a button present between D+ and GND.