clk: bcm2835: Minimise clock jitter for PCM clock

Fractional clock dividers generate accurate average frequencies but
with jitter, particularly when the integer divisor is small.

Introduce a new metric of clock accuracy to penalise clocks with a good
average but worse jitter compared to clocks with an average which is no
better but with lower jitter. The metric is the ideal rate minus the
worse deviation from that ideal using the nearest integer divisors.

Use this metric for parent selection for clocks requiring low jitter
(currently just PCM).

Signed-off-by: Phil Elwell <[e-mail]>
Reviewed-by: Eric Anholt <[e-mail]>
Acked-by: Stefan Wahren <[e-mail]>
Signed-off-by: Stephen Boyd <[e-mail]>

drivers/clk/bcm/clk-bcm2835.c

···
 
 	bool is_vpu_clock;
 	bool is_mash_clock;
+	bool low_jitter;
 
 	u32 tcnt_mux;
 };
···
 							int parent_idx,
 							unsigned long rate,
 							u32 *div,
-							unsigned long *prate)
+							unsigned long *prate,
+							unsigned long *avgrate)
 {
 	struct bcm2835_clock *clock = bcm2835_clock_from_hw(hw);
 	struct bcm2835_cprman *cprman = clock->cprman;
···
 		*prate = clk_hw_get_rate(parent);
 		*div = bcm2835_clock_choose_div(hw, rate, *prate, true);
 
-		return bcm2835_clock_rate_from_divisor(clock, *prate,
-						       *div);
+		*avgrate = bcm2835_clock_rate_from_divisor(clock, *prate, *div);
+
+		if (data->low_jitter && (*div & CM_DIV_FRAC_MASK)) {
+			unsigned long high, low;
+			u32 int_div = *div & ~CM_DIV_FRAC_MASK;
+
+			high = bcm2835_clock_rate_from_divisor(clock, *prate,
+							       int_div);
+			int_div += CM_DIV_FRAC_MASK + 1;
+			low = bcm2835_clock_rate_from_divisor(clock, *prate,
+							      int_div);
+
+			/*
+			 * Return a value which is the maximum deviation
+			 * below the ideal rate, for use as a metric.
+			 */
+			return *avgrate - max(*avgrate - low, high - *avgrate);
+		}
+		return *avgrate;
 	}
 
 	if (data->frac_bits)
···
 
 	*div = curdiv << CM_DIV_FRAC_BITS;
 	*prate = curdiv * best_rate;
+	*avgrate = best_rate;
 
 	return best_rate;
 }
···
 	bool current_parent_is_pllc;
 	unsigned long rate, best_rate = 0;
 	unsigned long prate, best_prate = 0;
+	unsigned long avgrate, best_avgrate = 0;
 	size_t i;
 	u32 div;
 
···
 			continue;
 
 		rate = bcm2835_clock_choose_div_and_prate(hw, i, req->rate,
-							  &div, &prate);
+							  &div, &prate,
+							  &avgrate);
 		if (rate > best_rate && rate <= req->rate) {
 			best_parent = parent;
 			best_prate = prate;
 			best_rate = rate;
+			best_avgrate = avgrate;
 		}
 	}
 
···
 	req->best_parent_hw = best_parent;
 	req->best_parent_rate = best_prate;
 
-	req->rate = best_rate;
+	req->rate = best_avgrate;
 
 	return 0;
 }
···
 		.int_bits = 12,
 		.frac_bits = 12,
 		.is_mash_clock = true,
+		.low_jitter = true,
 		.tcnt_mux = 23),
 	[BCM2835_CLOCK_PWM]	= REGISTER_PER_CLK(
 		.name = "pwm",