Ensure that conversion mode is successfully exited when the command is
issued by adding an extra transfer beforehand, matching the minimum CNV
high and low times from the AD4695 datasheet. The AD4695 has a quirk
where the exit command only works during a conversion, so guarantee this
happens by triggering a conversion in ad4695_exit_conversion_mode().
Then make this even more robust by ensuring that the exit command is run
at AD4695_REG_ACCESS_SCLK_HZ rather than the bus maximum.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
---
 drivers/iio/adc/ad4695.c | 34 ++++++++++++++++++++++++++++------
 1 file changed, 28 insertions(+), 6 deletions(-)

diff --git a/drivers/iio/adc/ad4695.c b/drivers/iio/adc/ad4695.c
index 82e930b21c69..f36c1a1db886 100644
--- a/drivers/iio/adc/ad4695.c
+++ b/drivers/iio/adc/ad4695.c
@@ -92,6 +92,8 @@
 #define AD4695_T_REFBUF_MS		100
 #define AD4695_T_REGCONFIG_NS		20
 #define AD4695_T_SCK_CNV_DELAY_NS	80
+#define AD4695_T_CNVL_NS		80
+#define AD4695_T_CNVH_NS		10
 #define AD4695_REG_ACCESS_SCLK_HZ	(10 * MEGA)
 
 /* Max number of voltage input channels. */
@@ -364,11 +366,31 @@ static int ad4695_enter_advanced_sequencer_mode(struct ad4695_state *st, u32 n)
  */
 static int ad4695_exit_conversion_mode(struct ad4695_state *st)
 {
-	struct spi_transfer xfer = {
-		.tx_buf = &st->cnv_cmd2,
-		.len = 1,
-		.delay.value = AD4695_T_REGCONFIG_NS,
-		.delay.unit = SPI_DELAY_UNIT_NSECS,
+	/*
+	 * An extra transfer is needed to trigger a conversion here so
+	 * that we can be 100% sure the command will be processed by the
+	 * ADC, rather than relying on it to be in the correct state
+	 * when this function is called (this chip has a quirk where the
+	 * command only works when reading a conversion, and if the
+	 * previous conversion was already read then it won't work). The
+	 * actual conversion command is then run at the slower
+	 * AD4695_REG_ACCESS_SCLK_HZ speed to guarantee this works.
+	 */
+	struct spi_transfer xfers[] = {
+		{
+			.delay.value = AD4695_T_CNVL_NS,
+			.delay.unit = SPI_DELAY_UNIT_NSECS,
+			.cs_change = 1,
+			.cs_change_delay.value = AD4695_T_CNVH_NS,
+			.cs_change_delay.unit = SPI_DELAY_UNIT_NSECS,
+		},
+		{
+			.speed_hz = AD4695_REG_ACCESS_SCLK_HZ,
+			.tx_buf = &st->cnv_cmd2,
+			.len = 1,
+			.delay.value = AD4695_T_REGCONFIG_NS,
+			.delay.unit = SPI_DELAY_UNIT_NSECS,
+		},
 	};
 
 	/*
@@ -377,7 +399,7 @@ static int ad4695_exit_conversion_mode(struct ad4695_state *st)
 	 */
 	st->cnv_cmd2 = AD4695_CMD_EXIT_CNV_MODE << 3;
 
-	return spi_sync_transfer(st->spi, &xfer, 1);
+	return spi_sync_transfer(st->spi, xfers, ARRAY_SIZE(xfers));
 }
 
 static int ad4695_set_ref_voltage(struct ad4695_state *st, int vref_mv)

-- 
2.39.5