RE: [PATCH v1 2/4] iio: adc: ad7949: fix spi messages on non 14-bit controllers

-----Original Message-----
From: Liam Beguin <redacted>
Sent: Friday, July 9, 2021 1:56 AM
To: liambeguin@gmail.com; lars@metafoo.de; Hennerich, Michael
[off-list ref]; jic23@kernel.org; charles-
antoine.couret@essensium.com
Cc: linux-kernel@vger.kernel.org; linux-iio@vger.kernel.org;
devicetree@vger.kernel.org; robh+dt@kernel.org
Subject: [PATCH v1 2/4] iio: adc: ad7949: fix spi messages on non 14-bit
controllers

[External]

From: Liam Beguin <redacted>

This driver supports devices with 14-bit and 16-bit sample sizes.
This is not always handled properly by spi controllers and can fail. To
work around this limitation, pad samples to 16-bit and split the sample
into two 8-bit messages in the event that only 8-bit messages are
supported by the controller.

Signed-off-by: Liam Beguin <redacted>
---
 drivers/iio/adc/ad7949.c | 67
++++++++++++++++++++++++++++++++++------
 1 file changed, 58 insertions(+), 9 deletions(-)

diff --git a/drivers/iio/adc/ad7949.c b/drivers/iio/adc/ad7949.c
index 93aacf4f680b..bbc6b56330a3 100644
--- a/drivers/iio/adc/ad7949.c
+++ b/drivers/iio/adc/ad7949.c

@@ -11,6 +11,7 @@
 #include <linux/module.h>
 #include <linux/regulator/consumer.h>
 #include <linux/spi/spi.h>
+#include <linux/bitfield.h>

 #define AD7949_MASK_TOTAL		GENMASK(13, 0)
 #define AD7949_CFG_REG_SIZE_BITS	14

@@ -57,6 +58,7 @@ static const struct ad7949_adc_spec

ad7949_adc_spec[] = {
  * @indio_dev: reference to iio structure
  * @spi: reference to spi structure
  * @resolution: resolution of the chip
+ * @bits_per_word: number of bits per SPI word
  * @cfg: copy of the configuration register
  * @current_channel: current channel in use
  * @buffer: buffer to send / receive data to / from device

@@ -67,28 +69,59 @@ struct ad7949_adc_chip {
 	struct iio_dev *indio_dev;
 	struct spi_device *spi;
 	u8 resolution;
+	u8 bits_per_word;
 	u16 cfg;
 	unsigned int current_channel;
-	u16 buffer ____cacheline_aligned;
+	union {
+		__be16 buffer;
+		u8 buf8[2];
+	} ____cacheline_aligned;
 };

+static void ad7949_set_bits_per_word(struct ad7949_adc_chip

*ad7949_adc)
+{
+	u32 adc_mask = SPI_BPW_MASK(ad7949_adc->resolution);
+	u32 bpw = adc_mask & ad7949_adc->spi->controller-

quoted

bits_per_word_mask;

+
+	if (bpw == adc_mask)
+		ad7949_adc->bits_per_word = ad7949_adc-

quoted

resolution;

+	else if (bpw == SPI_BPW_MASK(16))
+		ad7949_adc->bits_per_word = 16;
+	else
+		ad7949_adc->bits_per_word = 8;
+}
+
 static int ad7949_spi_write_cfg(struct ad7949_adc_chip *ad7949_adc,
u16 val,
 				u16 mask)
 {
 	int ret;
-	int bits_per_word = ad7949_adc->resolution;
-	int shift = bits_per_word - AD7949_CFG_REG_SIZE_BITS;
 	struct spi_message msg;
 	struct spi_transfer tx[] = {
 		{
 			.tx_buf = &ad7949_adc->buffer,
 			.len = 2,
-			.bits_per_word = bits_per_word,
+			.bits_per_word = ad7949_adc->bits_per_word,
 		},
 	};

+	ad7949_adc->buffer = 0;
 	ad7949_adc->cfg = (val & mask) | (ad7949_adc->cfg & ~mask);
-	ad7949_adc->buffer = ad7949_adc->cfg << shift;
+
+	switch (ad7949_adc->bits_per_word) {
+	case 16:
+		ad7949_adc->buffer = ad7949_adc->cfg << 2;
+		break;
+	case 14:
+		ad7949_adc->buffer = ad7949_adc->cfg;
+		break;
+	case 8:
+		/* Pack 14-bit value into 2 bytes, MSB first */
+		ad7949_adc->buf8[0] = FIELD_GET(GENMASK(13, 6),
ad7949_adc->cfg);
+		ad7949_adc->buf8[1] = FIELD_GET(GENMASK(5, 0),
ad7949_adc->cfg);
+		ad7949_adc->buf8[1] = ad7949_adc->buf8[1] << 2;
+		break;
+	}

 	spi_message_init_with_transfers(&msg, tx, 1);
 	ret = spi_sync(ad7949_adc->spi, &msg);

@@ -105,14 +138,12 @@ static int ad7949_spi_read_channel(struct

ad7949_adc_chip *ad7949_adc, int *val,
 {
 	int ret;
 	int i;
-	int bits_per_word = ad7949_adc->resolution;
-	int mask = GENMASK(ad7949_adc->resolution - 1, 0);
 	struct spi_message msg;
 	struct spi_transfer tx[] = {
 		{
 			.rx_buf = &ad7949_adc->buffer,
 			.len = 2,
-			.bits_per_word = bits_per_word,
+			.bits_per_word = ad7949_adc->bits_per_word,
 		},
 	};

@@ -147,7 +178,24 @@ static int ad7949_spi_read_channel(struct

ad7949_adc_chip *ad7949_adc, int *val,

 	ad7949_adc->current_channel = channel;

-	*val = ad7949_adc->buffer & mask;
+	switch (ad7949_adc->bits_per_word) {
+	case 16:
+		*val = ad7949_adc->buffer;
+		/* Shift-out padding bits */
+		if (ad7949_adc->resolution == 14)
+			*val = *val >> 2;
+		break;
+	case 14:
+		*val = ad7949_adc->buffer & GENMASK(13, 0);
+		break;
+	case 8:
+		/* Convert byte array to u16, MSB first */
+		*val = (ad7949_adc->buf8[0] << 8) | ad7949_adc-

quoted

buf8[1];

+		/* Shift-out padding bits */
+		if (ad7949_adc->resolution == 14)
+			*val = *val >> 2;
+		break;
+	}

 	return 0;
 }

@@ -280,6 +328,7 @@ static int ad7949_spi_probe(struct spi_device

*spi)
 	spec = &ad7949_adc_spec[spi_get_device_id(spi)-

quoted

driver_data];

 	indio_dev->num_channels = spec->num_channels;
 	ad7949_adc->resolution = spec->resolution;
+	ad7949_set_bits_per_word(ad7949_adc);

 	ad7949_adc->vref = devm_regulator_get(dev, "vref");
 	if (IS_ERR(ad7949_adc->vref)) {
--
2.30.1.489.g328c10930387