Messages in this thread | | | Date | Fri, 12 Jun 2009 00:03:26 -0700 | From | Richard Henderson <> | Subject | Re: [PATCH] Convert alpha to use arch_gettimeoffset() |
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john stultz wrote: > I suspect the alpha arch could even be further improved to provide and > rpcc() based clocksource, but not having the hardware, I don't feel > comfortable attempting the more complicated conversion (but I'd be glad > to help if anyone else is interested).
Unfortunately this isn't possible. The rpcc value is only 32-bits wide, and so rolls over in approx 3 to 4 seconds. Thus all the contortions to try and use the truncated value for sub 1 second adjustments.
> + delta_cycles = rpcc() - state.last_time; > + partial_tick = state.partial_tick; > /* > * usec = cycles * ticks_per_cycle * 2**48 * 1e6 / (2**48 * ticks) > * = cycles * (s_t_p_c) * 1e6 / (2**48 * ticks) > @@ -448,62 +437,9 @@ do_gettimeofday(struct timeval *tv) > delta_usec = ((delta_usec / ((1UL << (FIX_SHIFT-6-1)) * HZ)) + 1) / 2; > #endif ... > + return delta_usec * 1000;
If we're going to be computing nsec now, we might as well adjust the computation here.
nsec = cycles * ticks_per_cycle * 1e9 / HZ = cycles * s_t_p_c * 1e9 / (2**48 * HZ) = cycles * s_t_p_c * 1953125 / (2**39 * HZ)
delta_nsec = delta_cycles*state.scaled_ticks_per_cycle + partial_tick; delta_nsec *= 1953125 delta_nsec /= HZ << (FIX_SHIFT - 9 - 1); delta_nsec = (delta_nsec + 1) / 2;
As far as I can tell, the range of the dividend is about 7e17, which still fits in an unsigned long.
r~
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