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    Subject[patch] timer: Add a mod_timer_msec() API function
    Subject: timer: add a mod_timer_msec() API function
    From: Arjan van de Ven <arjan@linux.intel.com>

    There is a common pattern in the kernel, where the caller wants
    to set a timer a specified number of milliseconds into the future.

    The typical solution then ends up

    mod_timer(&timer, jiffies + msecs_to_jiffies(msec_amount));

    or

    mod_timer(&timer, jiffies + HZ/some_value);

    This patch aims to simplify this pattern for the caller, and at
    the same time, make for a more power-friendly API.

    The new pattern for identical behavior will be

    mod_timer_msec(&timer, msec_amount, 0);

    while

    mod_timer_msec(&time, msec_amount, 100);

    would give the kernel 100 milliseconds of slack to find a power
    friendly time to group timers into one CPU wakeup.

    A patch to convert a number of users, to show how the API is used,
    will be sent as reply to this email.


    Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>

    diff --git a/include/linux/timer.h b/include/linux/timer.h
    index a2d1eb6..006d4da 100644
    --- a/include/linux/timer.h
    +++ b/include/linux/timer.h
    @@ -162,6 +162,7 @@ static inline int timer_pending(const struct timer_list * timer)
    extern void add_timer_on(struct timer_list *timer, int cpu);
    extern int del_timer(struct timer_list * timer);
    extern int mod_timer(struct timer_list *timer, unsigned long expires);
    +extern int mod_timer_msec(struct timer_list *timer, unsigned long delay_ms, unsigned long slack_ms);
    extern int mod_timer_pending(struct timer_list *timer, unsigned long expires);
    extern int mod_timer_pinned(struct timer_list *timer, unsigned long expires);

    diff --git a/kernel/timer.c b/kernel/timer.c
    index 5db5a8d..c9d5cbf 100644
    --- a/kernel/timer.c
    +++ b/kernel/timer.c
    @@ -751,6 +751,47 @@ int mod_timer(struct timer_list *timer, unsigned long expires)
    EXPORT_SYMBOL(mod_timer);

    /**
    + * mod_timer_msec - modify a timer's timeout, using relative milliseconds
    + * @timer: the timer to be modified
    + * @delay_ms: the desired minimum delay in milliseconds
    + * @slack_ms: the amount of slack is ok in firing the timer
    + *
    + * Changes the timeout of a timer similar to mod_timer().
    + *
    + * mod_timer_msec() takes relative milliseconds rather than absolute
    + * jiffies as argument and also takes a "slack" amount, which is
    + * an explicit permission to treat the time of the timer as less
    + * precise in order to allow grouping of timers.
    + *
    + * The function returns whether it has modified a pending timer or not.
    + * (ie. mod_timer() of an inactive timer returns 0, mod_timer() of an
    + * active timer returns 1.)
    + */
    +int mod_timer_msec(struct timer_list *timer, unsigned long delay_ms, unsigned long slack_ms)
    +{
    + unsigned long next_jiffies, max_jiffies;
    + unsigned long rounded_jiffies;
    +
    + next_jiffies = jiffies + msecs_to_jiffies(delay_ms);
    + max_jiffies = jiffies + msecs_to_jiffies(delay_ms + slack_ms);
    + rounded_jiffies = round_jiffies(next_jiffies);
    +
    + /* check if rounding up next_jiffies is within the max */
    + if (rounded_jiffies <= max_jiffies && rounded_jiffies >= next_jiffies) {
    + next_jiffies = rounded_jiffies;
    + } else {
    + /* regular rounding failed; try a rounding to multiple-of-16 */
    + rounded_jiffies = (next_jiffies + 15) & ~0xF;
    + if (rounded_jiffies <= max_jiffies)
    + next_jiffies = rounded_jiffies;
    + }
    +
    + return mod_timer(timer, next_jiffies);
    +}
    +EXPORT_SYMBOL_GPL(mod_timer_msec);
    +
    +
    +/**
    * mod_timer_pinned - modify a timer's timeout
    * @timer: the timer to be modified
    * @expires: new timeout in jiffies

    --
    Arjan van de Ven Intel Open Source Technology Centre
    For development, discussion and tips for power savings,
    visit http://www.lesswatts.org


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