From: afiskon@devzen.ru (Aleksander Alekseev)
To: kernelnewbies@lists.kernelnewbies.org
Subject: Stupid question regarding bogomips and udelay()
Date: Mon, 1 Aug 2016 08:36:07 +0300 [thread overview]
Message-ID: <20160801083607.02157a64@fujitsu> (raw)
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
next reply other threads:[~2016-08-01 5:36 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-08-01 5:36 Aleksander Alekseev [this message]
2016-08-01 9:47 ` Stupid question regarding bogomips and udelay() François
2016-08-02 8:27 ` Aleksander Alekseev
2016-08-03 1:20 ` Daniel.
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