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SubjectRe: [PATCH] PATCH: add framework for ndelay (nanoseconds)
On Wed, Feb 05, 2003 at 03:34:31PM +0100, Geert Uytterhoeven wrote:
> On Sun, 2 Feb 2003, Linux Kernel Mailing List wrote:
> > ChangeSet 1.953.4.9, 2003/02/02 02:44:15-02:00, alan@lxorguk.ukuu.org.uk
> >
> > [PATCH] PATCH: add framework for ndelay (nanoseconds)
> >
> > +void __ndelay(unsigned long usecs)
> ^^^^^
> > +{
> > + __const_udelay(usecs * 0x00005); /* 2**32 / 1000000000 (rounded up) */
> ^^^^^
> > +}
>
> Wouldn't it make more sense to call the parameter `nsecs' (or `ns')?

There are more serious problems with this implementation:
1) It's *very* imprecise. Even on an 1 GHz CPU and with HZ = 100 precision
is ~86 nsec. With HZ = 1000 it's almost unusable on *any* CPU.
2) Additional delay caused by integer multiplication, cache misses
and so on may be large enough, especially on older processors.
On 233 MHz PII it's 600-700 nsec (perfectly repeatable), on 600 MHz
alpha ev56 - 200-300 nsec.

As of current 2.4, there is the only user of ndelay() - ide_execute_command()
that calls ndelay(400). Why udelay(1) cannot be used instead?

Ivan.
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