I've been playing for a six or so hours with one tiny bug. I was working on the high speed network adapter NetBSD driver and underlying PowerPC didn't want to send packet chain longer than one. Tons of messages in dmesg, many tries to debug DMA S/G transfers without anything reliable in return.
Finally I found out, how I'm stupid. I always set the end of the queue as transfer tail – send from end to end instead of send from this point to end. Aaargh.
I'm just curious, if there is any way to solve such issues not in so many hours.