On Fri, 24 May 2013, Philippe De Muyter wrote: > On Thu, May 23, 2013 at 08:31:18AM -0700, Greg Kroah-Hartman wrote: > > On Thu, May 23, 2013 at 03:07:09PM +0200, Philippe De Muyter wrote: > > > Hi all, > > > > > > I have a lot of linux computers equipped with a GlobalSat Br-353 GPS receiver, > > > which is connected via USB (an integrated PL2303). The GPS receiver emits > > > one multi-line message every second, giving position and time. I listen > > > to this messages in a user program running in the highest priority, > > > and I have noticed that under load, the messages are not delivered to my > > > process every second, but often delayed, which is not great for a time source. > > > > > > I looked at the sources of pl2303 and added a diagnostic message if the > > > beginning of a message came more than one second later than the beginning > > > of the previous one, and I noticed that the delay was already present > > > in the kernel: often even more than one second delay after the expected > > > beginning time. > > > > Then that implies that the device itself is holding on to the message, > > right? > > I hoped it was the configuration on the host-side. I have read internet > pages that implied that for USB mice it was possible to increase the polling > rate. I hoped that it would be the same for serial lines. Have you tried usbmon? The detailed information about the timing of the URBs might give you some clues. > > USB is not something that you can rely on for very high-frequency, low > > latency, timing things. Although to be fair, second delays are quite > > rare, which implies that your hardware is to blame here. > > Yes, I think that there must be something wrong but more on a software > side, because I do not get such big delays on an unloaded computer. It's possible that the delays are at the user level and not in the kernel. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
