linux-kernel - Re: [PATCH v5 1/2] sched/clock: interface to allow timestamps early in boot

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Date:   Mon, 28 Aug 2017 13:59:29 -0400
From:   Pasha Tatashin <pasha.tatashin@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org, hpa@...or.com, douly.fnst@...fujitsu.com
Subject: Re: [PATCH v5 1/2] sched/clock: interface to allow timestamps early
 in boot

>> void __init timekeeping_init(void)
>> {
>> 	/*
>> 	 * We must determine boot timestamp before getting current  	
>> 	 * persistent clock value, because implementation of
>> 	 * read_boot_clock64() might also call the persistent
>> 	 * clock, and a leap second may occur.
>> 	 */
>>
>> 	read_boot_clock64(&boot);
>> 	...
>> 	read_persistent_clock64(&now);
> 
> No. That's the same crap just the other way round.
> 
> s390 can do that, because the boot timestamp is correlated with the
> persistent clock. Your's not so much.
> 

OK, how about reading the persistent clock only once, and send it's 
value to use for calculation of boot stamp to read_boot_clock64() via a 
new argument:

read_boot_clock64(&now, &boot);

Does this sound alright or is there a better way?

I would need to update read_boot_clock64() everywhere it is declared to 
add the __init macro, so this extra argument is not going to increase 
number of line changes.

Thank you,
Pasha