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

From: Arnd Bergmann <arnd@arndb.de>
To: 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, 8 Jun 2017 17:33:42 +0200	[thread overview]
Message-ID: <CAK8P3a3P4rDqQaNmBR9C9nF3VCqcQOH06Nq8RyDHsNfc0Ms13g@mail.gmail.com> (raw)
In-Reply-To: <CAHp75VfUU9fO-jnY32caKywf8h3bdst9zBf66Wjz6C5+4z1okA@mail.gmail.com>

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.
>>
>> struct rtc_time is almost the same as struct tm (the former has one extra
>> member), so maybe we can actually define them to be the same and
>> use one format string for both?
>
> The reason I decide to drop struct tm for now due to they are not
> compatible and I have got an interesting bugs.

Ok.

        Arnd