dload mode can be enabled by adb shell command.
echo 1 > /sys/module/restart/parameters/download_mode
once dload is enabled, whenever Android faces panic(), it will reset the system and go to download mode
and if you want to make device go to download mode once abnormal reset is detected, you should enable dload for abnormal reset. can be done by below adb shell command.
echo 1 > /sys/module/restart/parameters/reset_detection
Note 1: You need to enable MSM_DLOAD_MODE by kernel\arch\arm\mach-msm\Kconfig to use this feature.
Note 2: reset_detection is merged into download_mode from 8960 and APQ8064. so you just need to enable download_mode, then the device will go to download mode when sudden reset is detected.