On Wed, Mar 11, 2009 at 10:30 AM, Johannes Berg <johannes@xxxxxxxxxxxxxxxx> wrote: > On Wed, 2009-03-11 at 10:20 -0700, Luis R. Rodriguez wrote: > >> > Hmm. I really don't understand this at all. Most operations won't be >> > single writes and reads, and if you need multiple like a write+read for >> > indirect register accesses then I'm sure you need to serialise them >> > against each other in some other way too, no? >> >> The issue is not serializing against separate routines calling some >> reads or writes, this serialization prevents the PCI hardware FIFO >> queue from getting more than two requests as otherwise the hardware >> poops out. But as you can see this is only an issue with PCI devices, >> not PCI express devices. > > Well yes, but if you ever do multi-read/write operations then you need > to synchronise at a higher level anyway -- hence I don't quite > understand why this is necessary at all. Agreed as the PCI FIFO issue should be visible on even UP boxes too. I was trying to find out out exactly why in an SMP system this would occur and it seems the only thing that would cause this is two separate threads reading/writing. I did review this but I was unable to find a specific synchronization issue. Technically the most obvious case is where we handle the beacon tasklet on one CPU and then the RX/TX tasklet (this is done in just one tasklet) in another but we don't handle the beaconing tasklet in STA mode and this issue is seen immediately upon association as a STA. I've been unable to locate that source of possible synchronization issue. If you get an idea let me know. Your point about the simplicity of the code by always doing the locking makes sense and its difficult to decide whether or not we want that without doing elaborate tests and analysis. Only reason to consider this carefully is that we will in fact be changing the behavior of each read/write. Luis -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
