linux-kernel - Re: [PATCH v3] Added "Preserve Boot Time Support"

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Message-ID: <alpine.DEB.2.20.1705212251570.3023@nanos>
Date:   Sun, 21 May 2017 23:36:21 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Bogdan Mirea <Bogdan-Stefan_mirea@...tor.com>
cc:     linux-kernel@...r.kernel.org, john.stultz@...aro.org,
        ore@...gutronix.de, kernel@...gutronix.de
Subject: Re: [PATCH v3] Added "Preserve Boot Time Support"

On Fri, 19 May 2017, Bogdan Mirea wrote:
> +#ifdef CONFIG_BOOT_TIME_PRESERVE
> +/*
> + * Set the real system time(including the time spent in bootloader)
> + * based on the timer counter.
> + */
> +
> +#ifndef BOOT_TIME_PRESERVE_CMDLINE
> +	#define BOOT_TIME_PRESERVE_CMDLINE "preserve_boot_time"
> +#endif
> +void arch_timer_setsystime(void)
> +{
> +	static struct timespec64 boot_ts;
> +	static cycles_t cycles;
> +	unsigned long long nsecs;
> +
> +	if (!strstr(boot_command_line, BOOT_TIME_PRESERVE_CMDLINE))
> +		return;

This adds a arch_timer specific command line option. Why is this arch_timer
specific? So if any other platform wants to gain this feature then we end
up copying that mess to every single timer implementation? Certainly NOT!

> +
> +	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");

What the heck is this? What has boot_ts to do with do_settimeofday()?

Exactly nothing. settimeofday() modifies CLOCK_REALTIME and if the platform
has an early accessible RTC, you hereby wreckaged wall_time. If the RTC
readout comes later then CLOCK_REALTIME is overwritten. So what is this
supposed to do?

It has absolutely nothing to do with CLOCK_BOOTTIME. /proc/uptime is based
on CLOCK_BOOTTIME, which is the CLOCK_MONOTONIC time since system boot. The
difference between CLOCK_MONOTONIC and CLOCK_BOOTTIME is that
CLOCK_MONOTONIC does not advance during suspend, but CLOCK_BOOTTIME takes
the suspended time into account.

I have not the faintest idea how you can claim that this patch actually
does what it is supposed to do. It's simply crap and CANNOT work at all.

Thanks,

	tglx