Scott’s Weblog

The problem with hardware

July 21st, 2009

I’ve been using (and occasionally hacking on) MadWiFi for several years now as part of my Ph.D research. It amazes me that after all this time we can still manage to find pretty severe bugs in the code.

Today I posted a message to the madwifi-devel list detailing how Jamie and I realised that the timestamp on incoming packets is actually the timestamp for the previous packet, not the current. This may sound fairly mundane but there are several areas of the driver that this might affect. Ad-hoc mode has never really worked that well – the problem has been with the way ad-hoc cells merge (which is partly based on beacon timing). So, it will be interesting to see if this finding has any impact on that.

This is a bug that will have been around pretty much forever – and the really annoying part about it is that I don’t really think it’s a bug at all – instead I imagine it’s simply a hardware timing limitation which is probably well known for the fortunate few who have access to the Atheros documentation. Unfortunately for those of us working with open source drivers without access to hardware documentation we miss out on information such as this and end up making incorrect assumptions, which leads to driver bugs such as the ad-hoc merging mess.

I’ll be interested to see the list’s responses. I’ll be especially interested to see if the newer ath5k and ath9k drivers also suffer from this problem (especially seeing as ath9k was written and open sourced by Atheros).

Right, now I can get back to my actual research – though I’m 99% confident that another obscure bug will manage to stop me in my tracks at some point in the near future.

Entry Filed under: General