clk: fractional-divider: Improve approximation when zero based and export

Consider the CLK_FRAC_DIVIDER_ZERO_BASED flag when finding the best
approximation for m and n. By doing so, increase the range of valid
values for the numerator and denominator by 1.

Furthermore, export the approximation function so that users of this
function can be compiled as modules.

Cc: A.s. Dong <[e-mail]>
Signed-off-by: Frank Oltmanns <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Stephen Boyd <[e-mail]>

drivers/clk/clk-fractional-divider.c

···
 						  unsigned long *m, unsigned long *n)
 {
 	struct clk_fractional_divider *fd = to_clk_fd(hw);
+	unsigned long max_m, max_n;
 
 	/*
 	 * Get rate closer to *parent_rate to guarantee there is no overflow
···
 			rate <<= scale - fd->nwidth;
 	}
 
-	rational_best_approximation(rate, *parent_rate,
-			GENMASK(fd->mwidth - 1, 0), GENMASK(fd->nwidth - 1, 0),
-			m, n);
+	if (fd->flags & CLK_FRAC_DIVIDER_ZERO_BASED) {
+		max_m = 1 << fd->mwidth;
+		max_n = 1 << fd->nwidth;
+	} else {
+		max_m = GENMASK(fd->mwidth - 1, 0);
+		max_n = GENMASK(fd->nwidth - 1, 0);
+	}
+
+	rational_best_approximation(rate, *parent_rate, max_m, max_n, m, n);
 }
+EXPORT_SYMBOL_GPL(clk_fractional_divider_general_approximation);
 
 static long clk_fd_round_rate(struct clk_hw *hw, unsigned long rate,
 			      unsigned long *parent_rate)
···
 {
 	struct clk_fractional_divider *fd = to_clk_fd(hw);
 	unsigned long flags = 0;
-	unsigned long m, n;
+	unsigned long m, n, max_m, max_n;
 	u32 mmask, nmask;
 	u32 val;
 
-	rational_best_approximation(rate, parent_rate,
-			GENMASK(fd->mwidth - 1, 0), GENMASK(fd->nwidth - 1, 0),
-			&m, &n);
+	if (fd->flags & CLK_FRAC_DIVIDER_ZERO_BASED) {
+		max_m = 1 << fd->mwidth;
+		max_n = 1 << fd->nwidth;
+	} else {
+		max_m = GENMASK(fd->mwidth - 1, 0);
+		max_n = GENMASK(fd->nwidth - 1, 0);
+	}
+	rational_best_approximation(rate, parent_rate, max_m, max_n, &m, &n);
 
 	if (fd->flags & CLK_FRAC_DIVIDER_ZERO_BASED) {
 		m--;