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powerpc/mpc8xxx: Fix picos_to_mclk() and get_memory_clk_period_ps()

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Reduce the calculation error to 1ps.

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
This commit is contained in:
York Sun 2011-08-26 11:32:42 -07:00 committed by Kumar Gala
parent 639f330f5f
commit 905acde21a
1 changed files with 11 additions and 15 deletions
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26
arch/powerpc/cpu/mpc8xxx/ddr/util.c
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@ -20,7 +20,8 @@
#define ULL_8FS 0xFFFFFFFFULL
/*
* Round mclk_ps to nearest 10 ps in memory controller code.
* Round up mclk_ps to nearest 1 ps in memory controller code
* if the error is 0.5ps or more.
*
* If an imprecise data rate is too high due to rounding error
* propagation, compute a suitably rounded mclk_ps to compute
@ -32,42 +33,37 @@ unsigned int get_memory_clk_period_ps(void)
unsigned int result;
/* Round to nearest 10ps, being careful about 64-bit multiply/divide */
unsigned long long mclk_ps = ULL_2E12;
/* Add 5*data_rate, for rounding */
mclk_ps += 5*(unsigned long long)data_rate;
unsigned long long rem, mclk_ps = ULL_2E12;
/* Now perform the big divide, the result fits in 32-bits */
do_div(mclk_ps, data_rate);
result = mclk_ps;
rem = do_div(mclk_ps, data_rate);
result = (rem >= (data_rate >> 1)) ? mclk_ps + 1 : mclk_ps;
/* We still need to round to 10ps */
return 10 * (result/10);
return result;
}
/* Convert picoseconds into DRAM clock cycles (rounding up if needed). */
unsigned int picos_to_mclk(unsigned int picos)
{
unsigned long long clks, clks_rem;
unsigned long data_rate = get_ddr_freq(0);
/* Short circuit for zero picos */
if (!picos)
return 0;
/* First multiply the time by the data rate (32x32 => 64) */
clks = picos * (unsigned long long)get_ddr_freq(0);
clks = picos * (unsigned long long)data_rate;
/*
* Now divide by 5^12 and track the 32-bit remainder, then divide
* by 2*(2^12) using shifts (and updating the remainder).
*/
clks_rem = do_div(clks, UL_5POW12);
clks_rem <<= 13;
clks_rem |= clks & (UL_2POW13-1);
clks_rem += (clks & (UL_2POW13-1)) * UL_5POW12;
clks >>= 13;
/* If we had a remainder, then round up */
if (clks_rem)
/* If we had a remainder greater than the 1ps error, then round up */
if (clks_rem > data_rate)
clks++;
/* Clamp to the maximum representable value */