clk: qcom: clk-rcg2: Update logic to calculate D value for RCG

The display pixel clock has a requirement on certain newer platforms to
support M/N as (2/3) and the final D value calculated results in
underflow errors.
As the current implementation does not check for D value is within
the accepted range for a given M & N value. Update the logic to
calculate the final D value based on the range.

Fixes: 99cbd06 ("clk: qcom: Support display RCG clocks")
Signed-off-by: Taniya Das <tdas@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220227175536.3131-1-tdas@codeaurora.org

Author: Taniya Das

drivers/clk/qcom/clk-rcg2.c

@@ -264,7 +264,7 @@ static int clk_rcg2_determine_floor_rate(struct clk_hw *hw,
 
 static int __clk_rcg2_configure(struct clk_rcg2 *rcg, const struct freq_tbl *f)
 {
-	u32 cfg, mask;
+	u32 cfg, mask, d_val, not2d_val, n_minus_m;
 	struct clk_hw *hw = &rcg->clkr.hw;
 	int ret, index = qcom_find_src_index(hw, rcg->parent_map, f->src);
 
@@ -283,8 +283,17 @@ static int __clk_rcg2_configure(struct clk_rcg2 *rcg, const struct freq_tbl *f)
 		if (ret)
 			return ret;
 
+		/* Calculate 2d value */
+		d_val = f->n;
+
+		n_minus_m = f->n - f->m;
+		n_minus_m *= 2;
+
+		d_val = clamp_t(u32, d_val, f->m, n_minus_m);
+		not2d_val = ~d_val & mask;
+
 		ret = regmap_update_bits(rcg->clkr.regmap,
-				RCG_D_OFFSET(rcg), mask, ~f->n);
+				RCG_D_OFFSET(rcg), mask, not2d_val);
 		if (ret)
 			return ret;
 	}