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List:       linux-usb
Subject:    Re: USB 2.0 device has 8ms latency
From:       John Tapsell <johnflux () gmail ! com>
Date:       2015-10-22 17:33:00
Message-ID: CAHQ6N+ovU0WcCFMh1TJsgGE1+DM8x6bRRgE=DCuyFLKtVizvUA () mail ! gmail ! com
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On 22 October 2015 at 18:00, Greg KH <gregkh@linuxfoundation.org> wrote:
> On Thu, Oct 22, 2015 at 05:15:50PM +0100, John Tapsell wrote:
>> On 15 October 2015 at 15:37, Alan Stern <stern@rowland.harvard.edu> 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
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