When the handset is powered up with a low battery, if PC USB is detected , It will
attempt to charge up the battery by dead/fast USB charging upon the battery voltage
before boot-up continues , if wall-charger/usb charger (with D+ and D- shorted)
is detected the phone boots directly without charging in boot stage.
In different platform, accord to the architecture of bootloader, the function which handle
above situation is oemsbl_low_batt_chg or osbl_pm_auto_low_batt_chg .
In oemsbl_low_batt_chg / osbl_pm_auto_low_batt_chg function, charger type will be distinguished by
pm_sbl_usb_sess_is_valid/pm_sbl_ext_chg_is_invalid and D+/D- shorted checking method .
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