iio: adc: ad7606: remove frstdata check for serial mode

The current implementation attempts to recover from an eventual glitch
in the clock by checking frstdata state after reading the first
channel's sample: If frstdata is low, it will reset the chip and
return -EIO.

This will only work in parallel mode, where frstdata pin is set low
after the 2nd sample read starts.

For the serial mode, according to the datasheet, "The FRSTDATA output
returns to a logic low following the 16th SCLK falling edge.", thus
after the Xth pulse, X being the number of bits in a sample, the check
will always be true, and the driver will not work at all in serial
mode if frstdata(optional) is defined in the devicetree as it will
reset the chip, and return -EIO every time read_sample is called.

Hence, this check must be removed for serial mode.

Fixes: b9618c0 ("staging: IIO: ADC: New driver for AD7606/AD7606-6/AD7606-4")
Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240702-cleanup-ad7606-v3-1-18d5ea18770e@baylibre.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: gstols

drivers/iio/adc/ad7606.c

@@ -49,7 +49,7 @@ static const unsigned int ad7616_oversampling_avail[8] = {
 	1, 2, 4, 8, 16, 32, 64, 128,
 };
 
-static int ad7606_reset(struct ad7606_state *st)
+int ad7606_reset(struct ad7606_state *st)
 {
 	if (st->gpio_reset) {
 		gpiod_set_value(st->gpio_reset, 1);
@@ -60,6 +60,7 @@ static int ad7606_reset(struct ad7606_state *st)
 
 	return -ENODEV;
 }
+EXPORT_SYMBOL_NS_GPL(ad7606_reset, IIO_AD7606);
 
 static int ad7606_reg_access(struct iio_dev *indio_dev,
 			     unsigned int reg,
@@ -88,31 +89,6 @@ static int ad7606_read_samples(struct ad7606_state *st)
 {
 	unsigned int num = st->chip_info->num_channels - 1;
 	u16 *data = st->data;
-	int ret;
-
-	/*
-	 * The frstdata signal is set to high while and after reading the sample
-	 * of the first channel and low for all other channels. This can be used
-	 * to check that the incoming data is correctly aligned. During normal
-	 * operation the data should never become unaligned, but some glitch or
-	 * electrostatic discharge might cause an extra read or clock cycle.
-	 * Monitoring the frstdata signal allows to recover from such failure
-	 * situations.
-	 */
-
-	if (st->gpio_frstdata) {
-		ret = st->bops->read_block(st->dev, 1, data);
-		if (ret)
-			return ret;
-
-		if (!gpiod_get_value(st->gpio_frstdata)) {
-			ad7606_reset(st);
-			return -EIO;
-		}
-
-		data++;
-		num--;
-	}
 
 	return st->bops->read_block(st->dev, num, data);
 }

drivers/iio/adc/ad7606.h

@@ -151,6 +151,8 @@ int ad7606_probe(struct device *dev, int irq, void __iomem *base_address,
 		 const char *name, unsigned int id,
 		 const struct ad7606_bus_ops *bops);
 
+int ad7606_reset(struct ad7606_state *st);
+
 enum ad7606_supported_device_ids {
 	ID_AD7605_4,
 	ID_AD7606_8,

drivers/iio/adc/ad7606_par.c

@@ -7,6 +7,7 @@
 
 #include <linux/mod_devicetable.h>
 #include <linux/module.h>
+#include <linux/gpio/consumer.h>
 #include <linux/platform_device.h>
 #include <linux/types.h>
 #include <linux/err.h>
@@ -21,8 +22,29 @@ static int ad7606_par16_read_block(struct device *dev,
 	struct iio_dev *indio_dev = dev_get_drvdata(dev);
 	struct ad7606_state *st = iio_priv(indio_dev);
 
-	insw((unsigned long)st->base_address, buf, count);
 
+	/*
+	 * On the parallel interface, the frstdata signal is set to high while
+	 * and after reading the sample of the first channel and low for all
+	 * other channels.  This can be used to check that the incoming data is
+	 * correctly aligned.  During normal operation the data should never
+	 * become unaligned, but some glitch or electrostatic discharge might
+	 * cause an extra read or clock cycle.  Monitoring the frstdata signal
+	 * allows to recover from such failure situations.
+	 */
+	int num = count;
+	u16 *_buf = buf;
+
+	if (st->gpio_frstdata) {
+		insw((unsigned long)st->base_address, _buf, 1);
+		if (!gpiod_get_value(st->gpio_frstdata)) {
+			ad7606_reset(st);
+			return -EIO;
+		}
+		_buf++;
+		num--;
+	}
+	insw((unsigned long)st->base_address, _buf, num);
 	return 0;
 }
 
@@ -35,8 +57,28 @@ static int ad7606_par8_read_block(struct device *dev,
 {
 	struct iio_dev *indio_dev = dev_get_drvdata(dev);
 	struct ad7606_state *st = iio_priv(indio_dev);
-
-	insb((unsigned long)st->base_address, buf, count * 2);
+	/*
+	 * On the parallel interface, the frstdata signal is set to high while
+	 * and after reading the sample of the first channel and low for all
+	 * other channels.  This can be used to check that the incoming data is
+	 * correctly aligned.  During normal operation the data should never
+	 * become unaligned, but some glitch or electrostatic discharge might
+	 * cause an extra read or clock cycle.  Monitoring the frstdata signal
+	 * allows to recover from such failure situations.
+	 */
+	int num = count;
+	u16 *_buf = buf;
+
+	if (st->gpio_frstdata) {
+		insb((unsigned long)st->base_address, _buf, 2);
+		if (!gpiod_get_value(st->gpio_frstdata)) {
+			ad7606_reset(st);
+			return -EIO;
+		}
+		_buf++;
+		num--;
+	}
+	insb((unsigned long)st->base_address, _buf, num * 2);
 
 	return 0;
 }