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 -- To unsubscribe from this list: send the line "unsubscribe linux-embedded" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
