linux-kernel - Re: [PATCH v11 3/6] time: replace read_boot_clock64() with read_persistent_wall_and_boot

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Date:   Thu, 21 Jun 2018 18:13:16 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        linux@...linux.org.uk, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, john.stultz@...aro.org,
        sboyd@...eaurora.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, hpa@...or.com, douly.fnst@...fujitsu.com,
        peterz@...radead.org, prarit@...hat.com, feng.tang@...el.com,
        pmladek@...e.com, gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH v11 3/6] time: replace read_boot_clock64() with
 read_persistent_wall_and_boot_offset()

On Wed, 20 Jun 2018, Pavel Tatashin wrote:

> If architecture does not support exact boot time, it is challenging to
> estimate boot time without having a reference to the current persistent
> clock value. Yet, we cannot read the persistent clock time again, because
> this may lead to math discrepancies with the caller of read_boot_clock64()
> who have read the persistent clock at a different time.
> 
> This is why it is better to provide two values simultaneously: the
> persistent clock value, and the boot time.
> 
> Thus, we replace read_boot_clock64() with:
> read_persistent_wall_and_boot_offset(wall_time, boot_offset)
> 
> Where wall_time is returned by read_persistent_clock()
> And boot_offset is wall_time - boot time
> 
> We calculate boot_offset using the current value of local_clock() so
> architectures, that do not have a dedicated boot_clock but have early
> sched_clock(), such as SPARCv9, x86, and possibly more will benefit from
> this change by getting a better and more consistent estimate of the boot
> time without need for an arch specific implementation.
> 
> Signed-off-by: Pavel Tatashin <pasha.tatashin@...cle.com>
> ---
>  arch/arm/kernel/time.c      | 12 +-------
>  arch/s390/kernel/time.c     | 11 +++++--
>  include/linux/timekeeping.h |  3 +-
>  kernel/time/timekeeping.c   | 61 +++++++++++++++++++------------------

Please don't make that a wholesale patch. I surely indicated the steps
which are required and the steps can be done as separate patches easily,

Thanks,

	tglx