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SubjectRe: [PATCH] Convert alpha to use arch_gettimeoffset()
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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