linux-kernel - clocksources: order of preference

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Message-ID: <4822D8B6.4090008@gmx.de>
Date:	Thu, 08 May 2008 12:40:54 +0200
From:	Philipp Kohlbecher <xt28@....de>
To:	linux-kernel@...r.kernel.org
CC:	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>
Subject: clocksources: order of preference

Why is the TSC preferred to the HPET as a clocksource for the x86 
architecture?

"Understanding the Linux Kernel" states that the HPET is preferable to 
the TSC due to its richer architecture. Up to version 2.6.17.14, 
arch/i386/kernel/timers/timer.c also contained a comment to that effect 
and accordingly ranked the HPET before the TSC.

This was changed when the new clocksource infrastructure was introduced 
with version 2.6.18. (The HPET clocksource received a rating of 250; the 
TSC, 300.)

Preferring the TSC leads to problems when it is unstable. While this can 
be prevented by setting CONFIG_X86_TSC, certain distribution kernels 
(striving for compatibility) don't, resulting in soft lockups.

Are there better reasons to prefer the TSC or may I submit a patch that 
swaps the respective ratings?

Thanks for reading,
- Philipp Kohlbecher

Please CC me, I am not on the list.
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