[mirsadm]

I don't think 'safe mode' is an Android thing. Are you referring to a Sony device? They seem to have some sort of safe mode.

[somesay]

It's definitely not vendor specific since I've seen manuals for various devices.

When successfully started Android, you should be able to access it by long-pressing the power button to get into a popup menu, then long-tapping on the power off text button there. Also different hardware key combinations might work while booting.

Also, just speculative, Android itself might detect a boot loop or crashes while booting and then automatically restart into safe mode.