linux-kernel - Re: [PATCH v2 1/4] time: rtc-lib: Add rtc_show_time(const char *prefix

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Date:   Tue, 18 Jul 2017 23:52:16 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mark Salyzyn <salyzyn@...roid.com>
cc:     linux-kernel@...r.kernel.org, rjw@...ysocki.net,
        len.brown@...el.com, pavel@....cz, linux-pm@...r.kernel.org,
        a.zummo@...ertech.it, alexandre.belloni@...e-electrons.com,
        linux-rtc@...r.kernel.org, andy.shevchenko@...il.com,
        Mark Salyzyn <salyzyn@...gle.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Thierry Strudel <tstrudel@...gle.com>,
        John Stultz <john.stultz@...aro.org>,
        Richard Cochran <richardcochran@...il.com>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2 1/4] time: rtc-lib: Add rtc_show_time(const char
 *prefix_msg)

On Tue, 18 Jul 2017, Mark Salyzyn wrote:

> Go directly to the rtc for persistent wall clock time and print.
> Useful if REALTIME is required to be logged in a low level power
> management function or when clock activities are suspended.  An
> aid to permit user space alignment of kernel activities.

That's a horrible idea, really. And there is no point at all.

> +void rtc_show_time(const char *prefix_msg)
> +{
> +	struct timespec ts;
> +	struct rtc_time tm;
> +
> +	getnstimeofday(&ts);

It calls getnstimeofday(), which is wrong to begin with as we switch
everything in kernel to the 64bit variants.

> +	rtc_time64_to_tm(ts.tv_sec, &tm);

This is even more wrong as rtc_time64_to_tm is for 64 bit wide second
values....

> +	pr_info("%s %d-%02d-%02d %02d:%02d:%02d.%09lu UTC\n",
> +		prefix_msg ? prefix_msg : "Time:",
> +		tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday,
> +		tm.tm_hour, tm.tm_min, tm.tm_sec, ts.tv_nsec);

Why on earth do you need to print that information in RTC format? What's
wrong with just doing:

      pr_info("My so important log message %lld\n", ktime_get_real_seconds());

and then decoding the seconds in post processing? That's completely
sufficient as you intend that for logging. Correlation with user space
CLOCK_REALTIME is even simpler as this is the same as reading it from user
space.

If your main intention is logging/debugging, then you can just use
tracepoints. The tracer allows correlation to user space tracing already.

So unless you can come up with a reasonable explanation why all this voodoo
is required, this is going nowhere.

Thanks,

	tglx