linux-kernel - Re: [PATCH v1 04/25] lib/vsprintf: Print time and date in human readable format via %pt

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Date:   Thu, 08 Jun 2017 16:09:23 -0700
From:   Joe Perches <joe@...ches.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Arnd Bergmann <arnd@...db.de>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        linux-rtc@...r.kernel.org,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Guan Xuetao <gxt@...c.pku.edu.cn>,
        Ingo Molnar <mingo@...nel.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Jonathan Corbet <corbet@....net>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Thierry Reding <thierry.reding@...il.com>
Subject: Re: [PATCH v1 04/25] lib/vsprintf: Print time and date in human
 readable format via %pt

On Thu, 2017-06-08 at 21:02 +0300, Andy Shevchenko wrote:
> On Thu, Jun 8, 2017 at 6:33 PM, Arnd Bergmann <arnd@...db.de> wrote:
> > On Thu, Jun 8, 2017 at 4:55 PM, Andy Shevchenko
> > <andy.shevchenko@...il.com> wrote:
> > > On Thu, Jun 8, 2017 at 5:49 PM, Arnd Bergmann <arnd@...db.de> wrote:
> > > > On Thu, Jun 8, 2017 at 3:47 PM, Andy Shevchenko
> > > > <andriy.shevchenko@...ux.intel.com> wrote:
> > > > I really like the idea, and the implementation seems fine for this use case, but
> > > > before we reserve %pt for rtc_time, could we discuss whether we want
> > > > that for printing struct tm, struct timespec64, time64_t or ktime_t instead?
> > > 
> > > How many users?
> > 
> > It's hard to predict, I would assume we get more users once there is an
> > easy way to print the time.
> 
> So, at least for now we can guess using existing users, right?
> 
> I don't check yet how to calculate those cases of time64_t,
> timespec64, ktime_t and alike if they are about pretty ptintong time
> and date.
> I'm speculating that there are (almost) none.
> 
> > > For struct tm it's somelike 4 (which want to print its content).
> > 
> > Good point. I notice that they all convert from time64_t or time_t into
> > struct tm immediately before printing it, so we can scratch that one
> > as long as there is a way to pretty-print a time64_t. We also don't
> > need to print a time_t as we want to kill that one off anyway.
> > 
> > If we only care about printing time64_t and rtc_time, we can easily
> > use %pT for one and %pt for the other, but there may still be good
> > reasons to print a timespec64 or ktime_t.
> 
> No need, we may still use 3rd/4th letter in the format for that.
> 
> %pt(t/d) time/date + whatever modifications, like raw, validate, timespec, etc.
> 
> 's' for timespec64, for example.

My preference would be for %pt[type]<output style>
where <type> is mandatory and could be:

	r for struct rtc_time
	6 for time64_t
	k for ktime_t
	T for struct timespec64
	etc

and <output style> has an unspecified default of
YYYY-MM-DD:hh:mm:ss

Perhaps use the "date" formats without the leading
% uses for <output style> for additional styles.