On Fri, May 24, 2013 at 09:27:17AM +0200, 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. You can increase the interrupt request rate some, but if the device doesn't have any data to send, it doesn't matter how often you ask it for data. > Is it possible to make bulk_read_requests at the "interrupt" priority ? There is no difference in the "priority" of the two requests at all, they are the same. > > 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. That's odd, let us know if you find anything out with your tests. greg k-h -- 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
