DFU Mode

DFU or Device Firmware Upgrade mode allows the S5L8900 to be restored from any state. It resides in the VROM and is vulnerable to the pwnage 2.0 exploit.

Entering DFU
Software cannot be used to reliably enter DFU. Software methods rely on sending a WTF file which either calls the "real" DFU mode in bootrom or emulates it. If you are attempting to exploit the DFU, it is advisable to always use the hardware method. If your NOR firmware is corrupted, of course you have no recourse but to use the hardware method.

Method 1 (True Hardware DFU)

 * 1) Turn off the device.
 * 2) Hold the home and power buttons simultaneously to turn on.
 * 3) Hold them both around 10 seconds, then let the power button go. Keep the home button pressed until the screen flashes and stays "black" but lit.

If the Restore Logo is present on the screen, you are in Restore Mode, NOT DFU.

Method 2

 * 1) When your phone is on and connected to iTunes hold the home and power buttons simultaneously until it turns off (ignoring the red 'power off' slider)
 * 2) Wait approximately 4 seconds.
 * 3) Release the power/mute button and keep home pressed until iTunes shows a message that it found a phone in Restore Mode.

Your phone should have a blank screen, which is DFU mode. If you see the Restore Logo, you are in Restore Mode and will have to try again.

Exiting DFU Without Restoring
While in DFU, hold the power button for 30-60 seconds. When I have tested it, it has varied, so I don't know an exact length of time to hold it. Note that sometimes if you do this, when the device reboots from DFU, it will go into recovery mode for reasons unknown.