Re: [PATCH v2] Added "Preserve Boot Time Support"

[Oleksij Rempel]

Hi Bogdan,

are there any example what and how bootloader should do to provide 
correct values?

On 05/03/2017 12:59 PM, Bogdan Mirea 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.
>
> The sched_clock part is preserving boottime for kmsg which should be in
> sync with system uptime. The system uptime part is driver specific and I
> updated the arm_arch_timer with an arch_timer_setsystime() function
> which will call do_settimeofday64() with the values read from arch timer
> counter.
>
> This way both kmsg and uptime will be in sync, otherwise incosistencies
> will appear between the two.
>
> Signed-off-by: Bogdan Mirea <Bogdan-Stefan_mirea@xxxxxxxxxx>
> ---
>  drivers/clocksource/arm_arch_timer.c | 26 ++++++++++++++++++++++++++
>  kernel/time/Kconfig                  |  8 ++++++++
>  kernel/time/sched_clock.c            |  6 ++++++
>  3 files changed, 40 insertions(+)
>
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index 5152b38..7f9bf2a 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -475,6 +475,28 @@ struct timecounter *arch_timer_get_timecounter(void)
>  	return &timecounter;
>  }
>
> +#ifdef CONFIG_BOOT_TIME_PRESERVE
> +/*
> + * Set the real system time(including the time spent in bootloader)
> + * based on the timer counter.
> + */
> +void arch_timer_setsystime(void)
> +{
> +	static struct timespec64 boot_ts;
> +	static cycles_t cycles;
> +	unsigned long long nsecs;
> +
> +	cycles = arch_timer_read_counter() ? arch_timer_read_counter() : 0;
> +
> +	nsecs = clocksource_cyc2ns(cycles, clocksource_counter.mult,
> +				   clocksource_counter.shift);
> +	timespec64_add_ns(&boot_ts, nsecs);
> +
> +	if (do_settimeofday64(&boot_ts))
> +		pr_warn("arch_timer: unable to set systime\n");
> +}
> +#endif /* CONFIG_BOOT_TIME_PRESERVE */
> +
>  static void __init arch_counter_register(unsigned type)
>  {
>  	u64 start_count;
> @@ -504,6 +526,10 @@ static void __init arch_counter_register(unsigned type)
>
>  	/* 56 bits minimum, so we assume worst case rollover */
>  	sched_clock_register(arch_timer_read_counter, 56, arch_timer_rate);
> +#ifdef CONFIG_BOOT_TIME_PRESERVE
> +	/* Set systime */
> +	arch_timer_setsystime();
> +#endif /* CONFIG_BOOT_TIME_PRESERVE */
>  }
>
>  static void arch_timer_stop(struct clock_event_device *clk)
> diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
> index 4008d9f..7d70232 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 */
> +
>  	cd.actual_read_sched_clock = read;
>
>  	rd.read_sched_clock	= read;