[PATCH] power: regulator: stm32: vrefbuf: fix a possible overshoot when re-enabling

[Patrick Delaunay]

From: Fabrice Gasnier <fabrice.gasnier at st.com>

There maybe an overshoot:
- when disabling, then re-enabling vrefbuf too quickly
- or upon platform reset as external capacitor maybe slow
  discharging (VREFBUF is HiZ at reset by default).
VREFBUF is used by ADC/DAC on some boards. An overshoot on the reference
voltage make the conversions inaccurate for a short period of time. So:
- Don't put the VREFBUF in HiZ when disabling, to force an active
  discharge.
- Enforce a 1ms OFF/ON delay, also upon reset

Penalty is a 1ms delay is applied (even for a cold boot), when enabling
VREFBUF.

Fixes: 93cf0ae7758d ("power: regulator: Add support for stm32-vrefbuf")

Signed-off-by: Fabrice Gasnier <fabrice.gasnier at st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay at st.com>
---

 drivers/power/regulator/stm32-vrefbuf.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/power/regulator/stm32-vrefbuf.c b/drivers/power/regulator/stm32-vrefbuf.c
index 250773514f..92136961c2 100644
--- a/drivers/power/regulator/stm32-vrefbuf.c
+++ b/drivers/power/regulator/stm32-vrefbuf.c
@@ -43,8 +43,20 @@ static int stm32_vrefbuf_set_enable(struct udevice *dev, bool enable)
 	u32 val;
 	int ret;
 
-	clrsetbits_le32(priv->base + STM32_VREFBUF_CSR, STM32_HIZ | STM32_ENVR,
-			enable ? STM32_ENVR : STM32_HIZ);
+	if (enable && !(readl(priv->base + STM32_VREFBUF_CSR) & STM32_ENVR)) {
+		/*
+		 * There maybe an overshoot:
+		 * - when disabling, then re-enabling vrefbuf too quickly
+		 * - or upon platform reset as external capacitor maybe slow
+		 *   discharging (VREFBUF is HiZ at reset by default).
+		 * So force active discharge (HiZ=0) for 1ms before enabling.
+		 */
+		clrbits_le32(priv->base + STM32_VREFBUF_CSR, STM32_HIZ);
+		udelay(1000);
+	}
+
+	clrsetbits_le32(priv->base + STM32_VREFBUF_CSR, STM32_ENVR,
+			enable ? STM32_ENVR : 0);
 	if (!enable)
 		return 0;
 
-- 
2.17.1