OMAP3 clock: optimize DPLL rate rounding algorithm

The previous DPLL rate rounding algorithm counted the divider (N) down
from the maximum to 1.  Since we currently use a broad DPLL rate
tolerance, and lower N values are more power-efficient, we can often
bypass several iterations through the loop by counting N upwards from
1.

Peter de Schrijver <peter.de-schrijver@nokia.com> put up with several
test cycles of this patch - thanks Peter.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Peter de Schrijver <peter.de-schrijver@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

diff --git a/arch/arm/mach-omap2/clock.c b/arch/arm/mach-omap2/clock.c
index 7aefabb6cf221a3d7b87f452e1c734fdb1561c8c..bd3be45917d6a0ceaa35321bd011964468e5eb9e 100644 (file)
--- a/arch/arm/mach-omap2/clock.c
+++ b/arch/arm/mach-omap2/clock.c
@@ -48,7 +48,7 @@
 #define DPLL_MIN_DIVIDER               1
 
 /* Possible error results from _dpll_test_mult */
-#define DPLL_MULT_UNDERFLOW            (1 << 0)
+#define DPLL_MULT_UNDERFLOW            -1
 
 /*
  * Scale factor to mitigate roundoff errors in DPLL rate rounding.
@@ -877,7 +877,7 @@ static int _dpll_test_mult(int *m, int n, unsigned long *new_rate,
                           unsigned long target_rate,
                           unsigned long parent_rate)
 {
-       int flags = 0, carry = 0;
+       int r = 0, carry = 0;
 
        /* Unscale m and round if necessary */
        if (*m % DPLL_SCALE_FACTOR >= DPLL_ROUNDING_VAL)
@@ -898,13 +898,13 @@ static int _dpll_test_mult(int *m, int n, unsigned long *new_rate,
        if (*m < DPLL_MIN_MULTIPLIER) {
                *m = DPLL_MIN_MULTIPLIER;
                *new_rate = 0;
-               flags = DPLL_MULT_UNDERFLOW;
+               r = DPLL_MULT_UNDERFLOW;
        }
 
        if (*new_rate == 0)
                *new_rate = _dpll_compute_new_rate(parent_rate, *m, n);
 
-       return flags;
+       return r;
 }
 
 /**
@@ -943,21 +943,27 @@ long omap2_dpll_round_rate(struct clk *clk, unsigned long target_rate)
 
        dd->last_rounded_rate = 0;
 
-       for (n = dd->max_divider; n >= DPLL_MIN_DIVIDER; n--) {
+       for (n = DPLL_MIN_DIVIDER; n <= dd->max_divider; n++) {
 
                /* Compute the scaled DPLL multiplier, based on the divider */
                m = scaled_rt_rp * n;
 
                /*
-                * Since we're counting n down, a m overflow means we can
-                * can immediately skip to the next n
+                * Since we're counting n up, a m overflow means we
+                * can bail out completely (since as n increases in
+                * the next iteration, there's no way that m can
+                * increase beyond the current m)
                 */
                if (m > scaled_max_m)
-                       continue;
+                       break;
 
                r = _dpll_test_mult(&m, n, &new_rate, target_rate,
                                    clk->parent->rate);
 
+               /* m can't be set low enough for this n - try with a larger n */
+               if (r == DPLL_MULT_UNDERFLOW)
+                       continue;
+
                e = target_rate - new_rate;
                pr_debug("clock: n = %d: m = %d: rate error is %d "
                         "(new_rate = %ld)\n", n, m, e, new_rate);
@@ -969,16 +975,11 @@ long omap2_dpll_round_rate(struct clk *clk, unsigned long target_rate)
                        min_e_n = n;
 
                        pr_debug("clock: found new least error %d\n", min_e);
-               }
 
-               /*
-                * Since we're counting n down, a m underflow means we
-                * can bail out completely (since as n decreases in
-                * the next iteration, there's no way that m can
-                * increase beyond the current m)
-                */
-               if (r & DPLL_MULT_UNDERFLOW)
-                       break;
+                       /* We found good settings -- bail out now */
+                       if (min_e <= clk->dpll_data->rate_tolerance)
+                               break;
+               }
        }
 
        if (min_e < 0) {