linux-kernel - Re: [RFC 2/2] x86, tsc: Enable clock for ealry printk timestamp

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Date:   Wed, 30 May 2018 22:17:29 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Andy Shevchenko <andy.shevchenko@...il.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 2/2] x86, tsc: Enable clock for ealry printk timestamp

Hi Andy,

Thanks for the review.

On Wed, May 30, 2018 at 04:25:07PM +0300, Andy Shevchenko wrote:
> On Wed, May 30, 2018 at 12:20 PM, Feng Tang <feng.tang@...el.com> wrote:
> > To show time info in kernel log earlier and help optimizing kernel
> > boot time, printk adds a debug hook "boot_printk_clock_fn()"  for
> > capable platform which has accurate clock in early boot phase.
> >
> > This patch will add early param setup option, so that user can
> > chose to provide a tsc based early printk clock simply by adding
> > in command line: "boot_tsc=xxxxM" (xxxxM is the stable TSC freq).
> 
> > +
> 
> This line is not needed.
Ok, will remove it.

> 
> > +       cur_tsc = rdtsc();
> > +       cur_tsc -= boot_tsc_offset;
> 
> In one expression?
Agree

> 
> > +       if (!p)
> > +               return -EINVAL;
> > +
> 
> I'm not sure it's needed at all.
> 
> > +       boot_tsc_mhz = div64_u64(tsc_hz, 1024 * 1024);
> 
> Hmm... 1024*1024 != 1000 * 1000. So, hz -> mhz here (as by suffixes)
> looks weird.

The memparse is mostly for memory stuff which takes 1M as 1024*1024,
but for TSC frequency 1M means 1000*1000,  and the code is trying to
do some awkward translation. 

But you are right, this code will break if user doesn't use expression
with 'm".

> 
> > +       if (boot_tsc_mhz == 0)
> > +               return -EINVAL;
> 
> > +       pr_info("TSC has run for %lld us\n",
> > +               div64_u64(boot_tsc_offset, boot_tsc_mhz));
> > +
> > +       /* Setup the early printk clock */
> > +       boot_printk_clock_fn = boot_tsc_clock;
> 
> > +       pr_info("TSC: Setup early printk timestamp with %lldM TSC.",
> > +               boot_tsc_mhz);
> 
> Perhaps remove period (above doesn't have it) and move this to one line?

You mean merge the two pr_info line? yes, we can do that. 


Thanks,
Feng