If using the SP Flash Tool on your favorite desktop GNU/Linux distro you're experiencing upload failures, like if the communication was broken a few seconds after it started, it's probably ModemManager's fault.
SP Flash Tool is communicating with the smartphones using a USB-modem-like technique (involving
cdc_acm), and doing so leads to ModemManager to think it has to do its thing, actually interfering with the communication, leading to errors like that:
cdc_acm 1-1.2:1.1: failed to set dtr/rts
Fixing this is trivial: just disable ModemManager while using SP Flash Tool, and you should be good to go.