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

From: Joe Perches <joe@perches.com>
To: Arnd Bergmann <arnd@arndb.de>,
	Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Alessandro Zummo <a.zummo@towertech.it>,
	Alexandre Belloni <alexandre.belloni@free-electrons.com>,
	linux-rtc@vger.kernel.org,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
	Dmitry Torokhov <dmitry.torokhov@gmail.com>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Guan Xuetao <gxt@mprc.pku.edu.cn>, Ingo Molnar <mingo@kernel.org>,
	Jason Wessel <jason.wessel@windriver.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Jonathan Hunter <jonathanh@nvidia.com>,
	Krzysztof Kozlowski <krzk@kernel.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Thierry Reding <thierry.reding@gmail.com>
Subject: Re: [PATCH v1 04/25] lib/vsprintf: Print time and date in human readable format via %pt
Date: Thu, 08 Jun 2017 08:48:06 -0700	[thread overview]
Message-ID: <1496936886.1929.20.camel@perches.com> (raw)
In-Reply-To: <CAK8P3a3P4rDqQaNmBR9C9nF3VCqcQOH06Nq8RyDHsNfc0Ms13g@mail.gmail.com>

On Thu, 2017-06-08 at 17:33 +0200, Arnd Bergmann wrote:
> On Thu, Jun 8, 2017 at 4:55 PM, Andy Shevchenko
> <andy.shevchenko@gmail.com> wrote:
> > On Thu, Jun 8, 2017 at 5:49 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> > > On Thu, Jun 8, 2017 at 3:47 PM, Andy Shevchenko
> > > <andriy.shevchenko@linux.intel.com> wrote:
> > > > There are users which print time and date represented by content of
> > > > struct rtc_time in human readable format.
> > > > 
> > > > Instead of open coding that each time introduce %pt[dt][rv] specifier.
> > > 
> > > 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.
> 
> > 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.

> > > I can see good reasons for pretty-printing any of them, but the namespace for
> > > format strings is rather limited.

The kernel already uses different types for the same leading
letter.  For
instance: %pI4 vs %pI6, %pap vs %pad

A single 't' letter could do reasonably well.