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

[Andy Shevchenko]

On Wed, May 30, 2018 at 12:20 PM, Feng Tang <feng.tang@xxxxxxxxx> 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