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

Harald Geyer
2015-05-20T17:53:15Z

Hi John,

John Stultz writes:
> From: Harald Geyer <harald@ccbib.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@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Signed-off-by: Harald Geyer <harald@ccbib.org>
> [jstultz: Tweaked to make it build after upstream changes]
> Signed-off-by: John Stultz <john.stultz@linaro.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
>