linux-kernel - Re: [PATCH] Added "Preserve Boot Time Support"

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Date:	Wed, 17 Aug 2016 12:24:18 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Bogdan Mirea <Bogdan-Stefan_mirea@...tor.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] Added "Preserve Boot Time Support"

On Fri, Jul 29, 2016 at 2:50 AM, Bogdan Mirea
<Bogdan-Stefan_mirea@...tor.com> wrote:
> This option enables Boot Time Preservation between Bootloader and
> Linux Kernel. It is based on the idea that the Bootloader (or any
> other early firmware) will start the HW Timer and Linux Kernel will
> count the time starting with the cycles elapsed since timer start.
>
> Signed-off-by: Bogdan Mirea <Bogdan-Stefan_mirea@...tor.com>
> ---
>  kernel/time/Kconfig       | 8 ++++++++
>  kernel/time/sched_clock.c | 6 ++++++
>  2 files changed, 14 insertions(+)
>
> diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
> index 4008d9f..d001839 100644
> --- a/kernel/time/Kconfig
> +++ b/kernel/time/Kconfig
> @@ -193,5 +193,13 @@ config HIGH_RES_TIMERS
>           hardware is not capable then this option only increases
>           the size of the kernel image.
>
> +config BOOT_TIME_PRESERVE
> +       bool "Preserve Boot Time Support"
> +       help
> +         This option enables Boot Time Preservation between Bootloader and
> +         Linux Kernel. It is based on the idea that the Bootloader (or any
> +         other early firmware) will start the HW Timer and Linux Kernel will
> +         count the time starting with the cycles elapsed since timer start.
> +
>  endmenu
>  endif
> diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
> index a26036d..1d6e35a 100644
> --- a/kernel/time/sched_clock.c
> +++ b/kernel/time/sched_clock.c
> @@ -193,7 +193,13 @@ sched_clock_register(u64 (*read)(void), int bits, unsigned long rate)
>         /* Update epoch for new counter and update 'epoch_ns' from old counter*/
>         new_epoch = read();
>         cyc = cd.actual_read_sched_clock();
> +
> +#ifdef CONFIG_BOOT_TIME_PRESERVE
> +       ns = rd.epoch_ns + cyc_to_ns((new_epoch - rd.epoch_cyc) & new_mask, new_mult, new_shift);
> +#else
>         ns = rd.epoch_ns + cyc_to_ns((cyc - rd.epoch_cyc) & rd.sched_clock_mask, rd.mult, rd.shift);
> +#endif /* CONFIG_BOOT_TIME_PRESERVE */
> +

So... we already have the read_boot_clock64() interface which
platforms can define to provide the power-on-to-now delta which the
timekeeping logic will make use of at init to initialize the monotonic
clock.

Is there some reason that interface can't be used here?

thanks
-john