It is based on the work done by Ken Ma [off-list ref]

Cc: stable at vger.kernel.org
Fixes: 2089dc33ea0e ("clk: mvebu: armada-37xx-periph: add DVFS support for cpu clocks")
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
---
 drivers/clk/mvebu/armada-37xx-periph.c | 38 ++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/drivers/clk/mvebu/armada-37xx-periph.c b/drivers/clk/mvebu/armada-37xx-periph.c
index 6860bd5a37c5..44e4e27eddad 100644
--- a/drivers/clk/mvebu/armada-37xx-periph.c
+++ b/drivers/clk/mvebu/armada-37xx-periph.c

@@ -35,6 +35,7 @@
 #define CLK_SEL		0x10
 #define CLK_DIS		0x14
 
+#define  ARMADA_37XX_DVFS_LOAD_1 1
 #define LOAD_LEVEL_NR	4
 
 #define ARMADA_37XX_NB_L0L1	0x18

@@ -507,6 +508,40 @@ static long clk_pm_cpu_round_rate(struct clk_hw *hw, unsigned long rate,
 	return -EINVAL;
 }
 
+/*
+ * Switching the CPU from the L2 or L3 frequencies (300 and 200 Mhz
+ * respectively) to L0 frequency (1.2 Ghz) requires a significant
+ * amount of time to let VDD stabilize to the appropriate
+ * voltage. This amount of time is large enough that it cannot be
+ * covered by the hardware countdown register. Due to this, the CPU
+ * might start operating at L0 before the voltage is stabilized,
+ * leading to CPU stalls.
+ *
+ * To work around this problem, we prevent switching directly from the
+ * L2/L3 frequencies to the L0 frequency, and instead switch to the L1
+ * frequency in-between. The sequence therefore becomes:
+ * 1. First switch from L2/L3(200/300MHz) to L1(600MHZ)
+ * 2. Sleep 20ms for stabling VDD voltage
+ * 3. Then switch from L1(600MHZ) to L0(1200Mhz).
+ */
+static void clk_pm_cpu_set_rate_wa(unsigned long rate, struct regmap *base)
+{
+	unsigned int cur_level;
+
+	if (rate != 1200 * 1000 * 1000)
+		return;
+
+	regmap_read(base, ARMADA_37XX_NB_CPU_LOAD, &cur_level);
+	cur_level &= ARMADA_37XX_NB_CPU_LOAD_MASK;
+	if (cur_level <= ARMADA_37XX_DVFS_LOAD_1)
+		return;
+
+	regmap_update_bits(base, ARMADA_37XX_NB_CPU_LOAD,
+			   ARMADA_37XX_NB_CPU_LOAD_MASK,
+			   ARMADA_37XX_DVFS_LOAD_1);
+	msleep(20);
+}
+
 static int clk_pm_cpu_set_rate(struct clk_hw *hw, unsigned long rate,
 			       unsigned long parent_rate)
 {

@@ -537,6 +572,9 @@ static int clk_pm_cpu_set_rate(struct clk_hw *hw, unsigned long rate,
 			 */
 			reg = ARMADA_37XX_NB_CPU_LOAD;
 			mask = ARMADA_37XX_NB_CPU_LOAD_MASK;
+
+			clk_pm_cpu_set_rate_wa(rate, base);
+
 			regmap_update_bits(base, reg, mask, load_level);
 
 			return rate;

-- 
2.17.1


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