linux-kernel - Re: DIfferent BogoMIPS value after bbaa06702719 ("clocksource/drivers/arm_global

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Message-ID: <20170302141334.GM21222@n2100.armlinux.org.uk>
Date:   Thu, 2 Mar 2017 14:13:34 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Rafał Miłecki <rafal@...ecki.pl>
Cc:     Rabin Vincent <rabin.vincent@...s.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Srinivas Kandagatla <srinivas.kandagatla@...il.com>,
        tglx@...utronix.de, Patrice Chotard <patrice.chotard@...com>,
        Hauke Mehrtens <hauke@...ke-m.de>,
        Maxime Coquelin <maxime.coquelin@...com>
Subject: Re: DIfferent BogoMIPS value after bbaa06702719
 ("clocksource/drivers/arm_global_timer: Register delay timer")

On Thu, Mar 02, 2017 at 02:36:23PM +0100, Rafał Miłecki wrote:
> Hi,
> 
> I just updated kernel on my SmartRG SR400ac (bcm4708-smartrg-sr400ac.dts) from
> 4.4 to 4.9 and noticed that this part of the log:
> [    0.020820] Calibrating delay loop... 1594.16 BogoMIPS (lpj=7970816)
> (...)
> [    0.190806] Brought up 2 CPUs
> [    0.200022] SMP: Total of 2 processors activated (3188.32 BogoMIPS).
> 
> became:
> [    0.027082] Calibrating delay loop (skipped), value calculated using timer frequency.. 800.00 BogoMIPS (lpj=4000000)
> (...)
> [    0.078858] Brought up 2 CPUs
> [    0.088254] SMP: Total of 2 processors activated (1600.00 BogoMIPS).
> 
> This is caused by commit bbaa06702719 ("clocksource/drivers/arm_global_timer:
> Register delay timer").
> 
> Is this something expected?

Yes.

> Or should I be worried about this? I'm aware it's just BogoMIPS, but I
> still prefer to ask :)

It's *bogus*.  Bogomips is the calibration value that the kernel uses
to achieve delays.  It has nothing to do with the speed of the CPU on
modern systems that use a timer instead.  (In that case, it's a
calibration value for the timer, rather than a calibration value for
the number of software loops required to achieve a delay.)

-- 
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