iio: adc: ad7476: Generate CONVST signal internally

Compared to the other supported parts, AD7091R are dependent of
a CONVST signal that initiates the conversion. At this moment, only
sampling in buffered mode is supported for AD7091R and the only
option until now was to generate this signal externally using an
IIO trigger. This patch adds the option of generating it internally,
more compatible triggers being available in this case.

Also, it is an intermediate step of adding support more devices.
Signed-off-by: Dragos Bogdan <dragos.bogdan@analog.com>
Signed-off-by: Beniamin Bia <beniamin.bia@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ad7476.c

@@ -12,9 +12,11 @@
 #include <linux/sysfs.h>
 #include <linux/spi/spi.h>
 #include <linux/regulator/consumer.h>
+#include <linux/gpio/consumer.h>
 #include <linux/err.h>
 #include <linux/module.h>
 #include <linux/bitops.h>
+#include <linux/delay.h>
 
 #include <linux/iio/iio.h>
 #include <linux/iio/sysfs.h>
@@ -34,6 +36,7 @@ struct ad7476_state {
 	struct spi_device		*spi;
 	const struct ad7476_chip_info	*chip_info;
 	struct regulator		*reg;
+	struct gpio_desc		*convst_gpio;
 	struct spi_transfer		xfer;
 	struct spi_message		msg;
 	/*
@@ -64,6 +67,17 @@ enum ad7476_supported_device_ids {
 	ID_ADS7868,
 };
 
+static void ad7091_convst(struct ad7476_state *st)
+{
+	if (!st->convst_gpio)
+		return;
+
+	gpiod_set_value(st->convst_gpio, 0);
+	udelay(1); /* CONVST pulse width: 10 ns min */
+	gpiod_set_value(st->convst_gpio, 1);
+	udelay(1); /* Conversion time: 650 ns max */
+}
+
 static irqreturn_t ad7476_trigger_handler(int irq, void  *p)
 {
 	struct iio_poll_func *pf = p;
@@ -71,6 +85,8 @@ static irqreturn_t ad7476_trigger_handler(int irq, void  *p)
 	struct ad7476_state *st = iio_priv(indio_dev);
 	int b_sent;
 
+	ad7091_convst(st);
+
 	b_sent = spi_sync(st->spi, &st->msg);
 	if (b_sent < 0)
 		goto done;
@@ -254,6 +270,12 @@ static int ad7476_probe(struct spi_device *spi)
 	if (ret)
 		return ret;
 
+	st->convst_gpio = devm_gpiod_get_optional(&spi->dev,
+						  "adi,conversion-start",
+						  GPIOD_OUT_LOW);
+	if (IS_ERR(st->convst_gpio))
+		return PTR_ERR(st->convst_gpio);
+
 	spi_set_drvdata(spi, indio_dev);
 
 	st->spi = spi;