clk: ingenic: jz4760: Update M/N/OD calculation algorithm

The previous algorithm was pretty broken.

- The inner loop had a '(m > m_max)' condition, and the value of 'm'
  would increase in each iteration;

- Each iteration would actually multiply 'm' by two, so it is not needed
  to re-compute the whole equation at each iteration;

- It would loop until (m & 1) == 0, which means it would loop at most
  once.

- The outer loop would divide the 'n' value by two at the end of each
  iteration. This meant that for a 12 MHz parent clock and a 1.2 GHz
  requested clock, it would first try n=12, then n=6, then n=3, then
  n=1, none of which would work; the only valid value is n=2 in this
  case.

Simplify this algorithm with a single for loop, which decrements 'n'
after each iteration, addressing all of the above problems.

Fixes: bdbfc029 ("clk: ingenic: Add support for the JZ4760")
Cc: <stable@vger.kernel.org>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20221214123704.7305-1-paul@crapouillou.netSigned-off-by: Stephen Boyd <sboyd@kernel.org>

drivers/clk/ingenic/jz4760-cgu.c

@@ -58,7 +58,7 @@ jz4760_cgu_calc_m_n_od(const struct ingenic_cgu_pll_info *pll_info,
 		       unsigned long rate, unsigned long parent_rate,
 		       unsigned int *pm, unsigned int *pn, unsigned int *pod)
 {
-	unsigned int m, n, od, m_max = (1 << pll_info->m_bits) - 2;
+	unsigned int m, n, od, m_max = (1 << pll_info->m_bits) - 1;
 
 	/* The frequency after the N divider must be between 1 and 50 MHz. */
 	n = parent_rate / (1 * MHZ);
@@ -66,19 +66,17 @@ jz4760_cgu_calc_m_n_od(const struct ingenic_cgu_pll_info *pll_info,
 	/* The N divider must be >= 2. */
 	n = clamp_val(n, 2, 1 << pll_info->n_bits);
 
-	for (;; n >>= 1) {
-		od = (unsigned int)-1;
+	rate /= MHZ;
+	parent_rate /= MHZ;
 
-		do {
-			m = (rate / MHZ) * (1 << ++od) * n / (parent_rate / MHZ);
-		} while ((m > m_max || m & 1) && (od < 4));
-
-		if (od < 4 && m >= 4 && m <= m_max)
-			break;
+	for (m = m_max; m >= m_max && n >= 2; n--) {
+		m = rate * n / parent_rate;
+		od = m & 1;
+		m <<= od;
 	}
 
 	*pm = m;
-	*pn = n;
+	*pn = n + 1;
 	*pod = 1 << od;
 }