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

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Date:   Thu, 14 Jun 2018 16:38:51 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     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@...r.kernel.org
Cc:     alek.du@...el.com, feng.tang@...el.com
Subject: Re: [RFC 1/2] printk: Enable platform to provide a early boot clock

On Wed, May 30, 2018 at 05:20:58PM +0800, Feng Tang 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.

Hi Petr and all,

As the 2/2 tsc related patch is still under review/discussion, can
we consider taking this first? As this may benefit other archs.
For example, Intel Curie platform has an always-on 32KHz osc clock,
which is accurate but low frequency, and it could be used as
early printk timestamp until the high-resolution timer is initialized
and used as sched_clock. Don't know if ARM or other platforms
have similar use case.

Thanks,
Feng