CLOCK_TICK_RATE, was: Re: [PATCH V4 3/4] ARM: Xilinx: base header files and assembly macros
From: Arnd Bergmann <arnd@arndb.de>
Date: 2011-02-21 21:08:26
On Monday 21 February 2011, Russell King - ARM Linux wrote:
Eg, LOW_RES_NSEC is exported to userspace via the posix clocks interface. NSEC_PER_SEC and TICK_NSEC are used for cmos clock updates, so probably don't matter too much there. TICK_NSEC is also used by the scheduler, time conversions (timespec/timeval to/from jiffies) and profiling code. NSEC_PER_JIFFY is used by the jiffy clocksource code, which only matters if you don't have your own clocksource. So, I feel very uneasy about saying that CLOCK_TICK_RATE doesn't matter anymore given all the places which reference something that's derived from it.
All the calculations based off of CLOCK_TICK_RATE are derived from ACTHZ, which is either the correct value based on the underlying HW timer tick, or slightly off, when either the HW tick or the value of CLOCK_TICK_RATE is not a true multiple of HZ. In fact, I'm pretty sure that it's off on a lot of machines: arch/frv/include/asm/timex.h:#define CLOCK_TICK_RATE 1193180 /* Underlying HZ */ arch/m68k/include/asm/timex.h:#define CLOCK_TICK_RATE 1193180 /* Underlying HZ */ arch/mips/include/asm/timex.h:#define CLOCK_TICK_RATE 1193182 arch/parisc/include/asm/timex.h:#define CLOCK_TICK_RATE 1193180 /* Underlying HZ */ arch/s390/include/asm/timex.h:#define CLOCK_TICK_RATE 1193180 /* Underlying HZ */ arch/sh/include/asm/timex.h:#define CLOCK_TICK_RATE 1193180 arch/x86/include/asm/timex.h:#define CLOCK_TICK_RATE PIT_TICK_RATE arch/xtensa/include/asm/timex.h:#define CLOCK_TICK_RATE 1193180 /* (everyone is using this value) */ None of these is actually using a PC-style PIT these days, the just copied the definition blindly from old i386. I think a simple #define ACTHZ (HZ << 8) would fix more than it can break, and most likely nobody would ever notice the difference. If we do that, CLOCK_TICK_RATE becomes unused. Arnd