On Thu, 12 Apr 2007, Danny Budik wrote:
> Would it be possible to add my own constant to /usr/include/linux/
> usbdevice_fs.h and write an underlying function that would call
> usb_get_current_frame_number()? It
> looks like I can add a constant say USBDEVFS_FRAMENUM in
> usbdevice_fs.h, add new case to .../drivers/usb/core/devio.c, add a
> function called proc_framenum
> that would call usb_get_current_frame_number() and return the frame
> number. Is this a reasonable and/or feasible solution?
Yes, that would work. Subject to the restriction that the frame number
is constantly changing so that any value you receive can quickly become
out-of-date.
Also bear in mind that high-speed buses use microframes rather than
frames; they increment eight times more quickly.
Alan Stern
Hi Alan, that is not right. See USB 2.0 spec 8.4.3.1 USB Frames and
Microframes
Where it says:
<quote>
High-speed devices see an SOF packet with the same frame number eight times
(every 125 μs) during each
1 ms period. If desired, a high-speed device can locally determine a
particular microframe “number” by
detecting the SOF that had a different frame number than the previous SOF
and treating that as the zeroth
microframe. The next seven SOFs with the same frame number can be treated as
microframes 1 through 7.
</quote>
I guess the USB 2.0 spec designers did not see any use for a number that
changed every 125 usec, and where the field would roll over every 1/4
second instead of every 2 seconds.
It is not clear what use (in a non-real time OS), the frame count is in
userspace?
Regards, Steve
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