linux-kernel - Re: PATCH [0/3]: Simplify the kernel build by removing perl.

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Message-Id: <200901031739.28517.david-b@pacbell.net>
Date:	Sat, 3 Jan 2009 17:39:27 -0800
From:	David Brownell <david-b@...bell.net>
To:	Robert Hancock <hancockr@...w.ca>
Cc:	Rob Landley <rob@...dley.net>,
	Matthieu CASTET <matthieu.castet@...rot.com>,
	Arkadiusz Miskiewicz <a.miskiewicz@...il.com>,
	linux-kernel@...r.kernel.org,
	Embedded Linux mailing list <linux-embedded@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Sam Ravnborg <sam@...nborg.org>
Subject: Re: PATCH [0/3]: Simplify the kernel build by removing perl.

On Saturday 03 January 2009, Robert Hancock wrote:
> Rob Landley wrote:
> > ... some architectures (arm omap and and arm at91) 
> > allow you to enter arbitrary HZ values in kconfig.  (Their help text says that 
> > in many cases values that aren't powers of two won't work, but nothing 
> > enforces this.)
> 
> Is there a good reason that these archs allow you enter arbitrary HZ 
> values?

Power-of-two can be desirable when using a 32 KiHz oscillator, because
other values accumulate rounding errors ... you can't make 100 Hz, or
250 Hz, or 300 Hz, or 1000 Hz, by a binary division of 32 KiHz.

Other values were supported to help work around stupid software making
bad assumptions about HZ.  IMO, enforcing power-of-two would be better;
that software breaks with dyntick anyway, and needs fixing.

> The use case for using custom HZ values at all nowadays seems  
> fairly low now that dynticks is around (if that arch supports it 
> anyway),

A better argument would be that GENERIC_TIME exists (and works
on OMAP and AT91), which avoids some flavors of rounding error.
ISTR those CONFIG_HZ options predate GENERIC_TIME support.

However, the issue remains that most kernel times are measured in
jiffies not ktime_t -- they're easier and more efficient, all
those 64-bit multiplies can hurt on ARM (32-bit, non-GHz) -- so
it's still good to be able to ensure that jiffies-centric logic
won't always be inserting easily avoidable errors.

- Dave
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