[PATCH v3 1/5] iio: ad7949: Fix dummy read cycle placement

[Adam Michaelis <adam.michaelis@rockwellcollins.com>]

The AD7949 requires two conversion cycles following the first
configuration change, and one extra cycle following any other
configuration change (including changing the analog channel being
sampled). Therefore, adding a dummy read cycle when config is changed
and removing the extra cycle at initial configuration (the first dummy
cycle is now performed as part of applying the configuration change).

Signed-off-by: Adam Michaelis <adam.michaelis@rockwellcollins.com>
---
	V2:
	- Add some defines to reduce use of magic numbers.
	V3:
	- Switch back to using a u32 data buffer.
	- Add-back the second dummy cycle on initialization.
	- Move to first patch in series.
---
 drivers/iio/adc/ad7949.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/drivers/iio/adc/ad7949.c b/drivers/iio/adc/ad7949.c
index ac0ffff6c5ae..c7fe27aa2519 100644
--- a/drivers/iio/adc/ad7949.c
+++ b/drivers/iio/adc/ad7949.c
@@ -100,6 +100,23 @@ static int ad7949_spi_write_cfg(struct ad7949_adc_chip *ad7949_adc, u16 val,
 	 * send a new command to the device
 	 */
 	udelay(2);
+
+	/*
+	 * Perform extra read cycle to allow configuration, acquisition,
+	 * and conversion sequences to complete for new configuration.
+	 */
+	ad7949_adc->buffer = 0;
+
+	spi_message_init_with_transfers(&msg, tx, 1);
+
+	ret = spi_sync(ad7949_adc->spi, &msg);
+
+	/*
+	 * This delay is to avoid a new request before the required time
+	 * to send a new command to the device.
+	 */
+	udelay(2);
+
 	return ret;
 }
 
@@ -229,11 +246,10 @@ static int ad7949_spi_init(struct ad7949_adc_chip *ad7949_adc)
 	ret = ad7949_spi_write_cfg(ad7949_adc, 0x3C79, AD7949_MASK_TOTAL);
 
 	/*
-	 * Do two dummy conversions to apply the first configuration setting.
+	 * Do a dummy conversion to apply the first configuration setting.
 	 * Required only after the start up of the device.
 	 */
 	ad7949_spi_read_channel(ad7949_adc, &val, ad7949_adc->current_channel);
-	ad7949_spi_read_channel(ad7949_adc, &val, ad7949_adc->current_channel);
 
 	return ret;
 }
-- 
1.9.1