[PATCH] clk: imx: composite-8m: avoid glitches when set_rate would be a no-op

[Ahmad Fatoum]

Reconfiguring the clock divider to the exact same value is observed
on an i.MX8MN to often cause a short clock pause, probably because
the divider restarts counting from the time the register is written.

This issue doesn't show up normally, because the clock framework will
take care to not call set_rate when the clock rate is the same.
However, when we configure an upstream clock (e.g. an audio_pll), the
common code will call set_rate with the newly calculated rate on all
children. As the new rate is different, we enter set_rate and compute
the same divider values, write them back and cause the glitch (e.g.
on a SAI's MCLK).

To avoid the glitch, we skip writing the same value back again.

Fixes: d3ff9728134e ("clk: imx: Add imx composite clock")
Signed-off-by: Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx>
---
 drivers/clk/imx/clk-composite-8m.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/clk/imx/clk-composite-8m.c b/drivers/clk/imx/clk-composite-8m.c
index cbf0d7955a00..3e9a092e136c 100644
--- a/drivers/clk/imx/clk-composite-8m.c
+++ b/drivers/clk/imx/clk-composite-8m.c
@@ -97,7 +97,7 @@ static int imx8m_clk_composite_divider_set_rate(struct clk_hw *hw,
 	int prediv_value;
 	int div_value;
 	int ret;
-	u32 val;
+	u32 orig, val;
 
 	ret = imx8m_clk_composite_compute_dividers(rate, parent_rate,
 						&prediv_value, &div_value);
@@ -106,13 +106,15 @@ static int imx8m_clk_composite_divider_set_rate(struct clk_hw *hw,
 
 	spin_lock_irqsave(divider->lock, flags);
 
-	val = readl(divider->reg);
-	val &= ~((clk_div_mask(divider->width) << divider->shift) |
-			(clk_div_mask(PCG_DIV_WIDTH) << PCG_DIV_SHIFT));
+	orig = readl(divider->reg);
+	val = orig & ~((clk_div_mask(divider->width) << divider->shift) |
+		       (clk_div_mask(PCG_DIV_WIDTH) << PCG_DIV_SHIFT));
 
 	val |= (u32)(prediv_value  - 1) << divider->shift;
 	val |= (u32)(div_value - 1) << PCG_DIV_SHIFT;
-	writel(val, divider->reg);
+
+	if (val != orig)
+		writel(val, divider->reg);
 
 	spin_unlock_irqrestore(divider->lock, flags);
 
-- 
2.39.2