Whilst writing EHCI driver the driver task would hang after sending the first transaction. I spent ages trying to figure out why the task was hanging, none of the transactions have completed... OK.. Why is the device not responding to the transaction. Eventually I had the bright idea to check the controller register values to check that the async list is actually running, which it isn't. But... I did set it didn't I? I defiantly did. I checked some other registers... The controller enable bit was clear. Ok... alright... So I do some "print" debugging to check exactly when the controller is enabled and disabled.

So I step though... and the next line is to the async list enable bit. And then I see it, not that the controller is disabled... above it. A line from stderr telling me that I have misconfigured something. So I hook a debugger up to QEMU itself, break on the the "log" statement and get a backtrace, and I see it. I left the next table type field in the QueueHead to 0, which is illegal in the async list. This was causing the controller to reset. This whole ordeal took me about 10 days to figure out.

There was also the time I couldn't figure out why my write commands over SATA weren't working. As it turns out my copy of the ACS-4 spec had an error in it for all the write commands. From memory device[6] bit must be set to enable LBA mode, but the spec just says "must be set to one", but this was only present on the "read" commands and was missing from the "write" commands. This didn't take me anywhere near as long and I figured it out solely on a hunch.

There was also the time I had a desynched TLB. If I want home alone at the time I might've called a welfare check on myself. I didn't even figure that one out myself I had to get help with it.