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Date:   Tue, 8 Aug 2017 19:08:00 -0400
From:   Prarit Bhargava <prarit@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     John Stultz <john.stultz@...aro.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Mark Salyzyn <salyzyn@...roid.com>,
        Jonathan Corbet <corbet@....net>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christoffer Dall <cdall@...aro.org>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Joel Fernandes <joelaf@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Olof Johansson <olof@...om.net>,
        Josh Poimboeuf <jpoimboe@...hat.com>, linux-doc@...r.kernel.org
Subject: Re: [PATCH v4] printk: Add monotonic, boottime, and realtime
 timestamps



On 08/08/2017 04:28 AM, Peter Zijlstra wrote:
> On Mon, Aug 07, 2017 at 01:36:39PM -0700, Paul E. McKenney wrote:
>> On Mon, Aug 07, 2017 at 04:06:09PM -0400, Prarit Bhargava wrote:
> 
>>> peterz?  Want to offer a suggestion?  The issue is that I'm changing a bool
>>> config option to an int and that impacts all the arch's defconfigs.  John points
>>> out that this is a lot of churn and we're both wondering if there's a better way
>>> to do the configs.
>>
>> The usual approach is to keep the old bool Kconfig option, and add another
>> int Kconfig option that depends on the original one.  The tests for
>> the int value get a bit more complex, but one way to handle this is to
>> define a cpp macro something like the following:
>>
>> #ifdef CONFIG_OLD_OPTION
>> #define CPP_NEW_OPTION 0
>> #else
>> #define CPP_NEW_OPTION CONFIG_NEW_OPTION
>> #endif
>>
>> Then use CPP_NEW_OPTION, where zero means disabled and other numbers
>> select the available options.
>>
>> Adjust to suit depending on what values mean what.
>>
>> Another approach is to make the range of the new Kconfig option
>> depend on the old option:
>>
>> config NEW_OPTION
>> 	int "your description here"
>> 	range 1 5 if OLD_OPTION
>> 	range 0 0 if !OLD_OPTION
>> 	default 0
>> 	help
>> 	  your help here
>>
>> Again, adjust to suit depending on what values mean what.
> 
> Right this. Except I don't see the !OLD_OPTION working as expected.
> A 'new' config will not include the old one, so the !OLD_OPTION thing
> will 'always' be true.
> 
> So your:
> 
>> @@ -1,8 +1,46 @@
>>  menu "printk and dmesg options"
>>
>> +choice
>> +       prompt "printk default clock"
>> +       config PRINTK_TIME_DISABLE
>> +       bool "Disabled"
>> +       help
>> +        Selecting this option disables the time stamps of printk().
>> +
>> +       config PRINTK_TIME_LOCAL
>> +       bool "Local Clock"
>> +       help
>> +         Selecting this option causes the time stamps of printk() to be
>> +         stamped with the unadjusted hardware clock.
>> +
>> +       config PRINTK_TIME_BOOT
>> +       bool "CLOCK_BOOTTIME"
>> +       help
>> +         Selecting this option causes the time stamps of printk() to be
>> +         stamped with the adjusted boottime clock.
>> +
>> +       config PRINTK_TIME_MONO
>> +       bool "CLOCK_MONOTONIC"
>> +       help
>> +         Selecting this option causes the time stamps of printk() to be
>> +         stamped with the adjusted monotonic clock.
>> +
>> +       config PRINTK_TIME_REAL
>> +       bool "CLOCK_REALTIME"
>> +       help
>> +         Selecting this option causes the time stamps of printk() to be
>> +         stamped with the adjusted realtime clock.
>> +
>> +endchoice
>> +
>>  config PRINTK_TIME
> 
> Change that into something like:
> 
> config PRINTK_CLOCK
> 
> 
>> -       bool "Show timing information on printks"
>> +       int "Show time stamp information on printks"
>>         depends on PRINTK
>> +       default 0 if PRINTK_TIME_DISABLE
>> +       default 1 if PRINTK_TIME_LOCAL
> 
> And that into:
> 
> 	default 1 if PRINTK_TIME_LOCAL || PRINTK_TIME
> 
>> +       default 2 if PRINTK_TIME_BOOT
>> +       default 3 if PRINTK_TIME_MONO
>> +       default 4 if PRINTK_TIME_REAL
>> 	  help
>> 	    Selecting this option causes time stamps of the printk()
> 
> Then the old PRINTK_TIME symbol will auto-convert into the new
> equivalent.
> 

I don't think there's an easy code way around this.  Essentially this Kconfig
code boils down to properly evaluating

config PRINTK_CLOCK
	default 1 if PRINTK_TIME
	default 0

where there is no Kconfig entry for PRINTK_TIME.

If undefined CONFIG_PRINTK_TIME is used in a config, it is immediately
scrubbed by the kconfig script so it doesn't "exist" when CONFIG_PRINTK_CLOCK
is evaluated.  The result of that is CONFIG_PRINT_CLOCK=0.

I tried

config PRINTK_TIME
	bool "old config option"

then I end up with both a CONFIG_PRINTK_CLOCK=1 and a CONFIG_PRINTK_TIME=y in
the resulting config which is confusing.

I've debated using the other suggestion that Paul made but TBH (sorry
Paul) it seems like I'm avoiding the real but noisy solution of

	s/PRINTK_TIME=y/PRINTK_TIME=1/g

I'm obviously open to other suggestions...

P.

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