From mboxrd@z Thu Jan  1 00:00:00 1970
From: afiskon@devzen.ru (Aleksander Alekseev)
Date: Mon, 1 Aug 2016 08:36:07 +0300
Subject: Stupid question regarding bogomips and udelay()
Message-ID: <20160801083607.02157a64@fujitsu>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

Hello

Currently I'm reading Linux Kernel Development, 3rd Edition by Robert
Love. Chapter 11 describes timers and udelay() procedure in
particular. According to the book (if I didn't get anything wrong):

* bogomips value (see `cat /proc/cpuinfo`) is determined only once
  during system boot
* udelay() is implemented as busy loop where number of loops is
  calculated based on bogomips value [1][2]
* udelay() guarantees that delay will be _at_least_ given number of
  usecs

It's my understanding that it shouldn't work. CPU frequency actually
changes manually (see cpufrequtils) and/or automatically depending for
instance on laptop's current battery charge. There is also such a nasty
thing as Intel Turbo Boost. In other words actual MIPS could be higher
or lower than measured MIPS. So udealy() can't guarantee anything.

Clearly I don't understand something. Could you please explain why
udelay() supposed to work?

[1]
https://github.com/torvalds/linux/blob/master/arch/x86/lib/delay.c#L171
[2]
https://github.com/torvalds/linux/blob/master/include/asm-generic/delay.h#L19

-- 
Best regards,
Aleksander Alekseev