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    SubjectRe: printing current system time from kernel space

    john stultz wrote:
    > On Mon, Sep 22, 2008 at 1:28 PM, john stultz <> wrote:
    >> On Sun, Sep 21, 2008 at 10:18 PM, George Nychis <> wrote:
    >>> I am looking to measure the latency of USB data between kernel space and
    >>> user space. The user space driver uses a URB to get data from the device to
    >>> the kernel and finally to user space.
    >>> To measure this latency, I was thinking of printing the current system time
    >>> when a read occurs/succeeds in drivers/usb/core/devio.c at the function
    >>> usbdev_read(), and then again in user space when the URB succeeds in
    >>> reading. Then, I could subtract the two times to get the latency.
    >>> I spent some time googling, but could not find out how or if it is possible
    >>> to read the current system time in kernel space. I could insert a printk()
    >>> somewhere in usbdev_read() then.
    >>> If it is not possible to read the current system time, is there some other
    >>> shared clock between kernel and user space that I could use for this?
    >> Kernel: getnstimeofday()
    >> Userland: clock_gettime(CLOCK_MONOTONIC, ...)
    >> Those two should give you the same data. So printing timespecs from
    >> kernel space that come from getnstimeofday() and comparing it to
    >> userland clock_gettime(CLOCK_MONOTONIC,...) hopefully will give you
    >> what you want.
    > Gah! Typed too fast. The above is wrong. You want to use
    > clock_gettime(CLOCK_REALTIME,...) not CLOCK_MONOTONIC with
    > getnstimeofday().
    > Sorry for the confusion.
    > -john

    Thanks John!

    This is exactly what I am looking for.

    - George

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