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Commit 85a5f78d authored by Paul Walmsley's avatar Paul Walmsley Committed by Russell King
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[ARM] 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.

linux-omap source commit is 6f6d82bb2f80fa20a841ac3e95a6f44a5a156188.

Signed-off-by: default avatarPaul Walmsley <paul@pwsan.com>
Cc: Peter de Schrijver <peter.de-schrijver@nokia.com>
Signed-off-by: default avatarTony Lindgren <tony@atomide.com>
Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
parent b3245040

arch/arm/mach-omap2/clock.c

+18 −17
Original line number Diff line number Diff line
@@ -45,7 +45,7 @@ 
#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.
@@ -826,7 +826,7 @@ 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)
@@ -847,13 +847,13 @@ 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;

}



/**
@@ -892,21 +892,27 @@ 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);
@@ -918,17 +924,12 @@ 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)

			/* We found good settings -- bail out now */

			if (min_e <= clk->dpll_data->rate_tolerance)

				break;

		}

	}



	if (min_e < 0) {

		pr_debug("clock: error: target rate or tolerance too low\n");