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

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Date:   Thu, 14 Jun 2018 11:08:04 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Feng Tang <feng.tang@...el.com>
cc:     Petr Mladek <pmladek@...e.com>, Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, alek.du@...el.com
Subject: Re: [RFC 1/2] printk: Enable platform to provide a early boot
 clock

On Thu, 14 Jun 2018, Feng Tang wrote:
> 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.

Can we please _NOT_ add half sorted core stuff in a hurry?

Thanks,

	tglx