Friday, 5 October 2012

TP-WN822N AP/Master Mode

I have been running Debian on a hacked Buffalo LS500GL NAS for at least the past four years now and this little beast is still going strong.  I've since upgraded the HDD to a 1TB disk and use it for various things, such as Firewall, NIS server, NFS server, DLNA server, Web server and just about any other kind of server you can cram into the 128 MB of RAM.  It only has a 200 MHz ARM CPU, so it's no good for use as a transcoder, but for general services, it serves me well.

However, having just upgrades to a fibre connection, I thought I'd take advantage of the PPPoE availability and run a wireless access point, instead of having the switch plugged in.  The NAS uses some 11 watts, so is really economic.  Having the switch plugged in as well, draws an extra 8 or 9 watts; which, when you consider it is not really necessary, I may as well ditch it in favour of a USB access point.


There is of course a dilemma.  Most wireless drivers on Linux don't support access point mode.  The simple work around is to use NDIS wrapper to wrap native Windows kernel drivers, providing the Windows kernel API, whilst interfacing with the Linux kernel.  However, this is not possible when you are not running a supported Windows architecture, or at least one that you can finder drivers for.  In this case, there are no Windows drivers for ARM.


The solution is simple, so it would seem.  Find a device that has a Linux driver that supports AP mode.  These are few and far between and figuring out which of the devices will be suitable before you buy is a tough one.  Luckily I was able to find a few resources that indicated that the TP-WN822N USB adapter supports AP mode using the Atheros drivers (ath9k variant).  This driver is available in the 3.X.X kernels and onwards, so first port of call is to upgrade my 2.6.X kernel.

Cross compiling means running up a computer for hours on end while it builds, so I opt for building them on the NAS.  It takes a lot longer, given the memory restrictions and slow CPU, but you can log in, run up '/usr/bin/screen' and leave it running, while you go off and do something else as equally constructive for 7 hours, like sleep!

So here is my procedure:

ssh root@nasdrive
exec screen
cd /usr/src/linux-3.X.X
make menuconfig

Installation is slightly different given the U-Boot loader that runs out of the NAS flash.  For the NAS, you need to build the zImage and modify it slightly:

make zImage
mkimage -O linux -T kernel -C none -a 0x00008000 -e 0x00008000 -n 'linux' -d <( devio 'wl 0xe3a01c06,4' 'wl 0xe3811031,4'; cat arch/arm/boot/zImage )
mv uImage-3.X.X
mv .config config-3.X.X

Once all this has been built, it can be copied to the /boot partition and the default links updated appropriately.  It's no big deal if the NAS doesn't come up on reboot.  You can dismantle it, remove the HDD, relink the old kernel and start over.  This doesn't happen often, but when it does, is usually the result of not having built the uImage with the correct devio header.

So, let's leave it there for now.  I am going to set my kernel building and will come back and report the next stages of getting the ath9k driver loaded and getting the wlan interface up for the TP-LINK adapter.  Stay tuned...

No comments:

Post a Comment