linux-kernel - Re: [PATCH 2/7] timekeeping: Provide new API to get the current time resolution

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Date:	Wed, 20 May 2015 19:53:05 +0200
From:	Harald Geyer <harald@...ib.org>
To:	John Stultz <john.stultz@...aro.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 2/7] timekeeping: Provide new API to get the current time resolution

Hi John,

John Stultz writes:
> From: Harald Geyer <harald@...ib.org>
> 
> This patch series introduces a new function
> u32 ktime_get_resolution_ns(void)
> which allows to clean up some driver code.

thanks for keeping track of this, but is this patch still useful?

I was thinking that the variable hrtimer_resolution, that Thomas
introduced in
https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/commit/include/linux/hrtimer.h?h=timers/wip&id=03eeacdb07e2fdfc4ef311c2593286c92eba609c
is meant to provide the same information. I haven't looked into this
in detail yet, so I might be wrong, but it is on my todo list for
after it appears in the trees I work with...

TIA,
Harald

> In particular the IIO subsystem has a function to provide timestamps for
> events but no means to get their resolution. So currently the dht11 driver
> tries to guess the resolution in a rather messy and convoluted way. We
> can do much better with the new code.
> 
> This API is not designed to be exposed to user space.
> 
> This has been tested on i386, sunxi and mxs.
> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Signed-off-by: Harald Geyer <harald@...ib.org>
> [jstultz: Tweaked to make it build after upstream changes]
> Signed-off-by: John Stultz <john.stultz@...aro.org>
> ---
>  include/linux/timekeeping.h |  1 +
>  kernel/time/timekeeping.c   | 17 +++++++++++++++++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
> index 99176af..9af5c12 100644
> --- a/include/linux/timekeeping.h
> +++ b/include/linux/timekeeping.h
> @@ -163,6 +163,7 @@ extern ktime_t ktime_get(void);
>  extern ktime_t ktime_get_with_offset(enum tk_offsets offs);
>  extern ktime_t ktime_mono_to_any(ktime_t tmono, enum tk_offsets offs);
>  extern ktime_t ktime_get_raw(void);
> +extern u32 ktime_get_resolution_ns(void);
>  
>  /**
>   * ktime_get_real - get the real (wall-) time in ktime_t format
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 3365e32..85d3763 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -702,6 +702,23 @@ ktime_t ktime_get(void)
>  }
>  EXPORT_SYMBOL_GPL(ktime_get);
>  
> +u32 ktime_get_resolution_ns(void)
> +{
> +	struct timekeeper *tk = &tk_core.timekeeper;
> +	unsigned int seq;
> +	u32 nsecs;
> +
> +	WARN_ON(timekeeping_suspended);
> +
> +	do {
> +		seq = read_seqcount_begin(&tk_core.seq);
> +		nsecs = tk->tkr_mono.mult >> tk->tkr_mono.shift;
> +	} while (read_seqcount_retry(&tk_core.seq, seq));
> +
> +	return nsecs;
> +}
> +EXPORT_SYMBOL_GPL(ktime_get_resolution_ns);
> +
>  static ktime_t *offsets[TK_OFFS_MAX] = {
>  	[TK_OFFS_REAL]	= &tk_core.timekeeper.offs_real,
>  	[TK_OFFS_BOOT]	= &tk_core.timekeeper.offs_boot,
> -- 
> 1.9.1
> 
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