udelay and nanosleep questions

From: Jos Hulzink (josh@stack.nl)
Date: Sat Sep 21 2002 - 06:23:10 EST


Talking about kernel driver programming:

1) Can I rely on udelay(1) ? i.e. is the resolution high enough to wait at
least 1 microsecond given it returns normally ? I know the actual
implementation is platform / cpu dependant, so maybe I should ask: Should
I be able to rely on udelay(1) ?

2) With the highspeed CPUs these days, the implementation of sys_nanosleep
(in kernel/timer.c) for realtime processes:

sys_nanosleep {udelay ((nsec+999)/1000}

is rather low-res. Time for something new ? sys_nanosleep seems not the
call to make for in-kernel accurate delays, for it schedules a timeout
instead of doing a busy wait. My driver needs 250 ns delays, is there a
more accurate way than udelay(1) ? It is a pity to waste 4x more
clockcycli than needed.

3) Usleep and friends seem not to care about speedstepping technologies.
Shouldn't we care, at least for in-kernel and realtime process waits ?
True, you are an idiot when running realtime processes on a speedstep
enabled CPU, but still...


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