clk: imx: composite-8m: Add imx8m_divider_determine_rate

Currently, certain clocks are derrived as a divider from their
parent clock.  For some clocks, even when CLK_SET_RATE_PARENT
is set, the parent clock is not properly set which can lead
to some relatively inaccurate clock values.

Unlike imx/clk-composite-93 and imx/clk-divider-gate, it
cannot rely on calling a standard determine_rate function,
because the 8m composite clocks have a pre-divider and
post-divider. Because of this, a custom determine_rate
function is necessary to determine the maximum clock
division which is equivalent to pre-divider * the
post-divider.

With this added, the system can attempt to adjust the parent rate
when the proper flags are set which can lead to a more precise clock
value.

On the imx8mplus, no clock changes are present.
On the Mini and Nano, this can help achieve more accurate
lcdif clocks. When trying to get a pixel clock of 31.500MHz
on an imx8m Nano, the clocks divided the 594MHz down, but
left the parent rate untouched which caused a calulation error.

Before:
video_pll              594000000
  video_pll_bypass     594000000
    video_pll_out      594000000
      disp_pixel       31263158
        disp_pixel_clk 31263158

Variance = -236842 Hz

After this patch:
video_pll               31500000
  video_pll_bypass      31500000
    video_pll_out       31500000
      disp_pixel        31500000
        disp_pixel_clk  31500000

Variance = 0 Hz

All other clocks rates and parent were the same.
Similar results on imx8mm were found.

Fixes: 690dccc ("Revert "clk: imx: composite-8m: Add support to determine_rate"")
Signed-off-by: Adam Ford <[email protected]>
Reviewed-by: Abel Vesa <[email protected]>
Tested-by: Fabio Estevam <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Abel Vesa <[email protected]>

Author: aford173

drivers/clk/imx/clk-composite-8m.c

@@ -119,10 +119,41 @@ static int imx8m_clk_composite_divider_set_rate(struct clk_hw *hw,
 	return ret;
 }
 
+static int imx8m_divider_determine_rate(struct clk_hw *hw,
+				      struct clk_rate_request *req)
+{
+	struct clk_divider *divider = to_clk_divider(hw);
+	int prediv_value;
+	int div_value;
+
+	/* if read only, just return current value */
+	if (divider->flags & CLK_DIVIDER_READ_ONLY) {
+		u32 val;
+
+		val = readl(divider->reg);
+		prediv_value = val >> divider->shift;
+		prediv_value &= clk_div_mask(divider->width);
+		prediv_value++;
+
+		div_value = val >> PCG_DIV_SHIFT;
+		div_value &= clk_div_mask(PCG_DIV_WIDTH);
+		div_value++;
+
+		return divider_ro_determine_rate(hw, req, divider->table,
+						 PCG_PREDIV_WIDTH + PCG_DIV_WIDTH,
+						 divider->flags, prediv_value * div_value);
+	}
+
+	return divider_determine_rate(hw, req, divider->table,
+				      PCG_PREDIV_WIDTH + PCG_DIV_WIDTH,
+				      divider->flags);
+}
+
 static const struct clk_ops imx8m_clk_composite_divider_ops = {
 	.recalc_rate = imx8m_clk_composite_divider_recalc_rate,
 	.round_rate = imx8m_clk_composite_divider_round_rate,
 	.set_rate = imx8m_clk_composite_divider_set_rate,
+	.determine_rate = imx8m_divider_determine_rate,
 };
 
 static u8 imx8m_clk_composite_mux_get_parent(struct clk_hw *hw)