linux-kernel - Re: [RFC 1/2] printk: Enable platform to provide a early boot clock

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Date:   Thu, 31 May 2018 10:18:46 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Petr Mladek <pmladek@...e.com>, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        alek.du@...el.com
Subject: Re: [RFC 1/2] printk: Enable platform to provide a early boot clock

On Wed, May 30, 2018 at 12:20 PM, Feng Tang <feng.tang@...el.com> wrote:
> Currently printk timestamp mostly come from the sched_clock which
> depends on the clock setup, so there are many kernel logs started
> with "[    0.000000]   " before the clock is calibrated.
>
> This patch will provide an debug option for specific platform to
> provide a early boot time clock, so that we can have time info in
> kernel log much earlier, which can show the time info for the early
> kernel boot, and make boottime tuning/optimization easier (boot time
> is critical for phone/tablet and embedded devices).
>
> Capable platform only need to setup the "boot_printk_clock_fn"
> which could return time in nano seconds.
>
> Together with a TSC patch on x86 system, we have easily captured
> some early boottime killer like unwind_init() which takes about
> 300ms in boot phase.

> +static u64 printk_clock(void)
> +{
> +       /* If platform provides early boot printk clock, then use it */
> +       if (unlikely(system_state == SYSTEM_BOOTING && boot_printk_clock_fn))
> +               return boot_printk_clock_fn();
> +       else
> +               return local_clock();

'else' is redundant.

> +}


-- 
With Best Regards,
Andy Shevchenko