\section{U-Boot Installation}

The U-Boot installation process is substantially more complicated than the

firmware update.  The investigator purchased the optional serial cable

along with the TPE-NWIFIROUTER, in order to complete the U-Boot installation

per the instructions in ``u-boot\verb0_0reflash''.

However, the investigator was only able to read data from the serial port; the

investigator was unable to send key events via the serial port so the U-Boot

console could not be accessed in that way.  The investigator did find another

way of accessing the U-Boot console, though, which was used to complete the

U-Boot installation and verification.  The likely issue with the serial port was

initial mis-wiring of the serial connector, causing the receive pin to be

permanently disabled.  Here are the steps the investigator tried, including the

alternate method of installation that did not require the serial console:

\begin{itemize}

\item The investigator found the serial cable included was a USB serial adapter

that had a male USB type A connector on one end and 4 female jumper wires at the

other end.  These female jumper wires were red, black, white, and green.

\item The instructions did not specify how to connect these wires, but the

investigator was able to determine this in part using the "v8.4" image (close to

the "v8.2" version string the investigator found on the bottom of the router) at

\url{http://wiki.openwrt.org/toh/tp-link/tl-wr841nd#serial.console} .  Aside

from power and ground (red and black), the investigator did have to guess which

of the wires was RX and TX.  By experimentation the investigator found that

green was RX and white was TX.  When the investigator tried the other way, no

data was received to the serial console at boot time.  While determining which

wires connected to which pins, the investigator may have connected the power pin

to the RX pin; this could explain why the receive (RX) pin later failed to work.

\item The investigator did have to use the included jumper pin gender changer

with the USB serial adapter, which the investigator put through the holes on the

router's mainboard and then connected to the USB serial adapter.  The fit was

fairly loose so it would be nice if future router versions included a tighter

gender changer or (ideally) had the jumper pins soldered onto the board to begin

with (so no gender changer would be required).  Since the serial cable is not

strictly required for U-Boot installation (see below), this may not be issue.

\item The investigator used 115200 8N1 as the serial console setting (with no

hardware or software flow control).  This was tested with both the

\verb0minicom0 and \verb0screen0 commands.  The investigator found that if all 4

wires were connected on the USB serial adapter that the router would start

without additional power and the console would receive the startup messages.

The investigator could replicate the same behavior by omitting the power cable

from the USB serial adapter (red wire) and connecting the main power adapter to

the router instead.

\item While the investigator did see the U-Boot and kernel boot logs in the

serial console, the investigator was unable to interrupt the boot process as

u-boot\verb0_0reflash indicated one should.  This is likely related to the

accidental connection of power to the RX pin mentioned earlier, which may have

disabled the pin, preventing the serial port on the router from receiving the

commands sent to it.

\item The investigator then contacted one of the libreCMC developers to

determine what the serial console issue might be and whether there was an

alternate way to install U-Boot that did not rely on the serial console working.

The developer agreed the the receive pin had likely been disabled so a different

installation method would be needed.  The developer indicated that the console

could be accessed via \verb0netcat0 when the router was booted into a special

mode by holding the reset button on the router for 7 seconds after turning on

the router.

\item The investigator turned on the router while pressing reset as mentioned

above and then ran \verb0nc -u -p 6666 192.168.1.1 66660 on the desktop that was

connected to the router (after changing its IP address to 192.168.1.2).  After

pressing Enter, a \verb0uboot>0 prompt appeared and the investigator was able to

confirm the running version by typing \verb0version0 to which the router

responded with "U-Boot 1.1.4  (Jul 28 2014)".

\item A TFTP server was then setup according to step 1 of the U-Boot

installation steps in u-boot\verb0_0reflash.  These instructions did not

explicitly state that the U-Boot image mentioned in step 4 of the build

instructions should be placed in /srv/tftp, but this was evident based on the

instructions that followed.  This should be corrected in a future version of

u-boot\verb0_0reflash but, because it was straight-forward based on the context,

did not amount to a compliance issue.

\item The u-boot\verb0_0reflash steps were then followed starting at step 4,

using the \verb0netcat0 console rather than the serial console (described in

steps 2 and 3).  The U-Boot image was downloaded onto the device and then copied

over top of the old U-Boot image.  The router was then restarted with the

\verb0reset0 command.

\item Since the serial cable was still connected, the investigator noticed at

startup that U-Boot now printed "U-Boot 1.1.4  (Oct 17 2014)" as its version

string.  This was also confirmed by using the \verb0netcat0 console and the

\verb0version0 command, as was previously done above.  The new version string

showed that the router was now running the version of U-Boot that the

investigator built, rather than the one it was shipped with, thus fulfilling the

GPL's requirements that one must be able to build and install the software and

any modified versions.

\end{itemize}

While the u-boot\verb0_0reflash instructions appear to be functional for those

able to use a serial console, we would prefer if these instructions were updated

to use the \verb0netcat0 console instead.  This provides a number of advantages,

such as no requirement for additional hardware to install a new version of

U-Boot, and less chance of mis-configuring one's serial connector (which would

reduce the risk of damage to the router).  The existing instructions appear to

be compliant without modification; this suggestion would merely make it easier

for users to take advantage of the freedoms provided to them by U-Boot and the

rest of the system.