Re: [PATCH v2 1/8] time: add *_to_jiffies_timeout helpers to guarantee a minimum duration

Arnd Bergmann
2013-05-15T15:27:06Z

On Tuesday 14 May 2013, Imre Deak wrote:
> The *_to_jiffies(x) macros return a jiffy value, which if used as a
> delta to wait for a specific amount of time, may result in a wait time
> that is less than x. Many callers already compensate for this by adding
> one to the returned value. Instead of having this adjustment open-coded
> at every call site add helpers that return the adjusted value. This will
> make the intention for the adjustment more explicit and also provide
> documentation for why it is needed.
>
> Later patches will convert the currently open-coded call sites to use
> the new helpers.
>
> Also this can serve as a basis for auditing those users of *_to_jiffies
> that most likely do the wrong thing - for example set a timeout value of
> msecs_to_jiffies(1) - and converting them to use the new helpers.
>
> Kudos to Daniel Vetter for the idea of the new helpers.
>
> Signed-off-by: Imre Deak <imre.deak@intel.com>

This certainly looks like a reasonable change, but I wonder if we could take
it one step further and add milisecond based interfaces for some of those
functions that currently take a jiffies value, something like

int timer_set_msecs(struct timer_list *timer, unsigned long msecs)
{
unsigned long j = msec_to_jiffies(msecs);
return mod_timer(timer, min_t(msecs, MAX_JIFFY_OFFSET));
}

> +#define __define_time_to_jiffies_timeout(tname, ttype) \
> +unsigned long tname ## _to_jiffies_timeout(const ttype v) \
> +{ \
> + unsigned long j = tname ## _to_jiffies(v); \
> + return min_t(unsigned long, MAX_JIFFY_OFFSET, j + 1); \
> +} \
> +EXPORT_SYMBOL(tname ## _to_jiffies_timeout);

The macro has a few disadvantages:

* It's impossible to grep for the function or use tags if you generate
the identifier using the macro.

* msecs_to_jiffies is what puts MAX_JIFFY_OFFSET there in the first
place, which means you add an extra comparison here that should
not really be needed.

Arnd