Re: [PATCH v4] printk: Add monotonic, boottime, and realtime timestamps

[Prarit Bhargava <prarit@redhat.com>]

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.