clk: sunxi-ng: Add helper function to find closest rate

The default behaviour of clocks in the sunxi-ng driver is to select a
clock rate that is closest to but less than the requested rate.

Add the ccu_is_better_rate() helper function that - depending on the
fact if thc CCU_FEATURE_CLOSEST_RATE flag is set - decides if a rate is
closer than another rate.

Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Frank Oltmanns <frank@oltmanns.dev>
Link: https://lore.kernel.org/r/20230807-pll-mipi_set_rate_parent-v6-5-f173239a4b59@oltmanns.dev
Signed-off-by: Chen-Yu Tsai <wens@csie.org>

Author: Frank Oltmanns

drivers/clk/sunxi-ng/ccu_common.c

@@ -39,6 +39,18 @@ void ccu_helper_wait_for_lock(struct ccu_common *common, u32 lock)
 }
 EXPORT_SYMBOL_NS_GPL(ccu_helper_wait_for_lock, SUNXI_CCU);
 
+bool ccu_is_better_rate(struct ccu_common *common,
+			unsigned long target_rate,
+			unsigned long current_rate,
+			unsigned long best_rate)
+{
+	if (common->features & CCU_FEATURE_CLOSEST_RATE)
+		return abs(current_rate - target_rate) < abs(best_rate - target_rate);
+
+	return current_rate <= target_rate && current_rate > best_rate;
+}
+EXPORT_SYMBOL_NS_GPL(ccu_is_better_rate, SUNXI_CCU);
+
 /*
  * This clock notifier is called when the frequency of a PLL clock is
  * changed. In common PLL designs, changes to the dividers take effect

drivers/clk/sunxi-ng/ccu_common.h

@@ -53,6 +53,11 @@ struct sunxi_ccu_desc {
 
 void ccu_helper_wait_for_lock(struct ccu_common *common, u32 lock);
 
+bool ccu_is_better_rate(struct ccu_common *common,
+			unsigned long target_rate,
+			unsigned long current_rate,
+			unsigned long best_rate);
+
 struct ccu_pll_nb {
 	struct notifier_block	clk_nb;
 	struct ccu_common	*common;