I have been playing with OpenWrt on a TL-MR3020 and as happens got it to the point that I could no longer access the unit so reset was in order but how !!1

Failsafe Mode
– Set your computer’s IP to 192.168.1.2, subnet 255.255.255.0
– Connect the TL-MR3020 to your computer via ethernet
– Power on the TL-MR3020
– When the WPS Button starts to blink , push it until it blinks faster
– The device is now in Failsafe-Mode
– You may access it by using telnet 192.168.1.1

Once you are connected via telnet issue the following commands

mount_root
passwd
mtd -r erase rootfs_data

And that should be it simple.

Explanation of the commands below

In failsafe mode
The root file system in failsafe mode is the only the SquashFS partition. The JFFS2 is not present. To mount JFFS2 in read-write mode run this command:
– mount_root

and then repair your system:
– In case you forgot your password, you need to set a new one. Type:
passwd

– In case you filled up the entire JFFS2 by installing too big/too many packages, clean the entire JFFS2 partition. All settings will be reset and all installed packages are removed. (OpenWrt equivalent of a factory reset)
firstboot

or (this will reboot the device as part of the process)
mtd -r erase rootfs_data

or
rm -r /overlay/*

If you are done with failsafe mode use
– reboot -f