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

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Date:	Wed, 15 May 2013 17:26:43 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Imre Deak <imre.deak@...el.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	John Stultz <john.stultz@...aro.org>,
	Ingo Molnar <mingo@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Eric Dumazet <edumazet@...gle.com>,
	Prarit Bhargava <prarit@...hat.com>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	Dong Zhu <bluezhudong@...il.com>
Subject: Re: [PATCH v2 1/8] time: add *_to_jiffies_timeout helpers to guarantee a minimum duration

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@...el.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
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