Messages in this thread | | | Date | Thu, 23 Aug 2001 10:15:04 +0200 (CEST) | From | Gabriel Paubert <> | Subject | Re: adding accuracy to random timers on PPC - new config option or runtime overhead? |
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On Wed, 22 Aug 2001, Chris Friesen wrote:
> > I'm looking at putting in PPC-specific code in add_timer_randomness() that would > be similar to the x86-specific stuff. > > The problem is that the PPC601 uses real time clock registers while the other > PPC chips use a timebase register, so two different versions will be required. > Should I try and identify at runtime which it is (which would be extra > overhead), or should I add another config option to the kernel?
Look at how it's done in do_gettimeofday (expand the macros). Note that the USE_RTC was not yet perfect last time I looked at it: it should only perform a test whenever 601 support is enabled (there are so many problems with 601).
Don't forget that in the case of a 601, you have to shift the least significant word right by 7 bits since the register increments by 128 every 128 nanoseconds. But the upper word changes more often...
However, it would perhaps be better to define the code as a macro in asm-${ARCH}/whatever.h and fall back to jiffies in case the macro is not defined. Arch dependent ifdef'ed code should be avoided.
Gabriel.
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