clk: fractional-divider: Improve fractional divider jitter

Numerator is greater than 4,the clk jitter is better.

Change-Id: I9fda9ddeb7b26c6b8559b4126e2ad1d29bb850d1
Signed-off-by: Elaine Zhang <zhangqing@rock-chips.com>

Author: Elaine Zhang

drivers/clk/clk-fractional-divider.c

@@ -106,6 +106,24 @@ static int clk_fd_set_rate(struct clk_hw *hw, unsigned long rate,
 			GENMASK(fd->mwidth - 1, 0), GENMASK(fd->nwidth - 1, 0),
 			&m, &n);
 
+	/*
+	 * When compensation the fractional divider,
+	 * the [1:0] bits of the numerator register are omitted,
+	 * which will lead to a large deviation in the result.
+	 * Therefore, it is required that the numerator must
+	 * be greater than 4.
+	 */
+	if (m < 4 && m != 0) {
+		val = DIV_ROUND_UP(4, m);
+		n *= val;
+		m *= val;
+		if (n > fd->nmask) {
+			pr_debug("%s n(%ld) is overflow, use mask value\n",
+				 __func__, n);
+			n = fd->nmask;
+		}
+	}
+
 	if (fd->lock)
 		spin_lock_irqsave(fd->lock, flags);
 	else