[PATCH 5/6] iio: ad7949: Fix SPI interfacing for 14-bit messages

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

The AD7949 (but not the other two models supported by this driver) uses
samples 14 bits wide. When attempting to communicate through certain SPI
controllers that require power-of-2 word widths, this fails. Adding
logic to pack the 14-bit messages into the most-significant bits of a
16-bit message for communicating over byte-based SPI bus.

Only able to test with AD7949 part, but should support the 16-bit
samples of the AD7682 and AD7689, as well.

Signed-off-by: Adam Michaelis <adam.michaelis@rockwellcollins.com>
---
 drivers/iio/adc/ad7949.c | 97 ++++++++++++++++++++++++------------------------
 1 file changed, 48 insertions(+), 49 deletions(-)

diff --git a/drivers/iio/adc/ad7949.c b/drivers/iio/adc/ad7949.c
index 1d75fff698d1..dc1ae4c143b0 100644
--- a/drivers/iio/adc/ad7949.c
+++ b/drivers/iio/adc/ad7949.c
@@ -18,6 +18,7 @@
 #define AD7949_OFFSET_CHANNEL_SEL	7
 #define AD7949_CFG_READ_BACK		0x1
 #define AD7949_CFG_REG_SIZE_BITS	14
+#define AD7949_BUFFER_LEN 4
 
 enum {
 	ID_AD7949 = 0,
@@ -71,7 +72,7 @@ struct ad7949_adc_chip {
 	u8 resolution;
 	u16 cfg;
 	unsigned int current_channel;
-	u32 buffer ____cacheline_aligned;
+	u8 buffer[AD7949_BUFFER_LEN] ____cacheline_aligned;
 };
 
 static bool ad7949_spi_cfg_is_read_back(struct ad7949_adc_chip *ad7949_adc)
@@ -82,41 +83,40 @@ static bool ad7949_spi_cfg_is_read_back(struct ad7949_adc_chip *ad7949_adc)
 	return false;
 }
 
-static int ad7949_spi_bits_per_word(struct ad7949_adc_chip *ad7949_adc)
-{
-	int ret = ad7949_adc->resolution;
-
-	if (ad7949_spi_cfg_is_read_back(ad7949_adc))
-		ret += AD7949_CFG_REG_SIZE_BITS;
-
-	return ret;
-}
-
 static int ad7949_spi_write_cfg(struct ad7949_adc_chip *ad7949_adc, u16 val,
 				u16 mask)
 {
-	int ret;
-	int bits_per_word = ad7949_spi_bits_per_word(ad7949_adc);
-	int shift = bits_per_word - AD7949_CFG_REG_SIZE_BITS;
+	int ret = 0;
 	struct spi_message msg;
-	struct spi_transfer tx[] = {
-		{
-			.tx_buf = &ad7949_adc->buffer,
-			.len = 4,
-			.bits_per_word = bits_per_word,
-		},
-	};
-
-	ad7949_adc->cfg = (val & mask) | (ad7949_adc->cfg & ~mask);
-	ad7949_adc->buffer = ad7949_adc->cfg << shift;
-	spi_message_init_with_transfers(&msg, tx, 1);
-	ret = spi_sync(ad7949_adc->spi, &msg);
+	struct spi_transfer tx;
+	u16 tmp_cfg = 0;
+
+	(void)memset(&tx, 0, sizeof(tx));
+	(void)memset(ad7949_adc->buffer, 0, AD7949_BUFFER_LEN);
+
+	tmp_cfg = (val & mask) | (ad7949_adc->cfg & ~mask);
+
+	if (tmp_cfg != ad7949_adc->cfg) {
+		ad7949_adc->cfg = tmp_cfg;
+
+		/* Pack 14-bit value into 2 bytes, MSB first */
+		ad7949_adc->buffer[0] = ((ad7949_adc->cfg & 0x7f00) >> 6) |
+			((ad7949_adc->cfg & 0xc0) >> 6);
+		ad7949_adc->buffer[1] = (ad7949_adc->cfg & 0x007f) << 2;
+
+		tx.tx_buf = ad7949_adc->buffer;
+		tx.len = ad7949_spi_cfg_is_read_back(ad7949_adc) ? 4 : 2;
+
+		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);
+	}
 
-	/*
-	 * This delay is to avoid a new request before the required time to
-	 * send a new command to the device
-	 */
-	udelay(2);
 	return ret;
 }
 
@@ -124,16 +124,10 @@ static int ad7949_spi_read_channel(struct ad7949_adc_chip *ad7949_adc, int *val,
 				   unsigned int channel)
 {
 	int ret;
-	int bits_per_word = ad7949_spi_bits_per_word(ad7949_adc);
-	int mask = GENMASK(ad7949_adc->resolution, 0);
 	struct spi_message msg;
-	struct spi_transfer tx[] = {
-		{
-			.rx_buf = &ad7949_adc->buffer,
-			.len = 4,
-			.bits_per_word = bits_per_word,
-		},
-	};
+	struct spi_transfer tx;
+
+	ad7949_adc->current_channel = channel;
 
 	ret = ad7949_spi_write_cfg(ad7949_adc,
 				   channel << AD7949_OFFSET_CHANNEL_SEL,
@@ -141,24 +135,29 @@ static int ad7949_spi_read_channel(struct ad7949_adc_chip *ad7949_adc, int *val,
 	if (ret)
 		return ret;
 
-	ad7949_adc->buffer = 0;
-	spi_message_init_with_transfers(&msg, tx, 1);
+	(void)memset(&tx, 0, sizeof(tx));
+	(void)memset(ad7949_adc->buffer, 0, AD7949_BUFFER_LEN);
+
+	tx.rx_buf = ad7949_adc->buffer;
+	tx.len = ad7949_spi_cfg_is_read_back(ad7949_adc) ? 4 : 2;
+
+	spi_message_init_with_transfers(&msg, &tx, 1);
 	ret = spi_sync(ad7949_adc->spi, &msg);
 	if (ret)
 		return ret;
 
 	/*
-	 * This delay is to avoid a new request before the required time to
-	 * send a new command to the device
+	 * This delay is to avoid a new request before the required time
+	 * to send a new command to the device.
 	 */
 	udelay(2);
 
-	ad7949_adc->current_channel = channel;
+	*val = (ad7949_adc->buffer[0] << 8) | ad7949_adc->buffer[1];
 
-	if (ad7949_spi_cfg_is_read_back(ad7949_adc))
-		*val = (ad7949_adc->buffer >> AD7949_CFG_REG_SIZE_BITS) & mask;
-	else
-		*val = ad7949_adc->buffer & mask;
+	if (ad7949_adc->resolution == 14) {
+		/* 14-bit value in 16-bit buffer */
+		*val = *val >> 2;
+	}
 
 	return 0;
 }
-- 
1.9.1