On 22 October 2015 at 18:00, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > On Thu, Oct 22, 2015 at 05:15:50PM +0100, John Tapsell wrote: >> On 15 October 2015 at 15:37, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: >> > On Thu, 15 Oct 2015, John Tapsell wrote: >> > >> >> I did have one wacky idea. I'm sure it's stupid, but here it is: Is >> >> it at all possible that there's a bug in the linux usb code where a >> >> bInterval value of 1ms is being converted into microframes (8 >> >> microframes) but then because it's a full speed device it's then >> >> incorrectly read as an 8ms delay? I did have a look into the code, >> >> but got thoroughly lost. Any pointers on how I could check my wacky >> >> theory? >> > >> > There is no such bug. Such a thing would have been spotted long, long >> > ago. >> > >> >> I'm just wondering where this 8ms delay comes from. >> > >> > Multiple places: time to submit the request, time to reserve >> > bandwidth for the previously unused interrupt endpoint, time to >> > complete the transfer, all multiplied by 2. >> > >> > You can get more information from usbmon (see >> > Documentation/usb/usbmon.txt in the kernel source). But Greg is right; >> > the protocol you described is terrible. There's no need for a multiple >> > ping-pong interchange like that; all you should need to do is wait for >> > the device to send the next bit (or whatever) of data as soon as it >> > becomes available. >> > >> > Alan Stern >> >> >> I had a look at the windows driver, and found that it is implemented >> in pretty much exactly the same way as the linux driver, but it >> operates at twice the speed. And that's for a user-space USB driver. >> >> Any ideas why? > > "pretty much"? What exactly is the difference? Well, just that one uses the windows API and one uses the linux API :-) > And you should be able > to try out a userspace driver in Linux as well, libusb works on Windows > and Linux so you can do a good comparison. Interesting - I'll give it a try, thanks. John -- 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
