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

Reduce the calculation error to 1ps.

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

diff --git a/arch/powerpc/cpu/mpc8xxx/ddr/util.c b/arch/powerpc/cpu/mpc8xxx/ddr/util.c
index 104d360..c644131 100644 (file)
--- a/arch/powerpc/cpu/mpc8xxx/ddr/util.c
+++ b/arch/powerpc/cpu/mpc8xxx/ddr/util.c
@@ -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 */