[PATCH v2 3/4] iio: adc: ad_sigma_delta: Add support for reading irq status using a GPIO

["Uwe Kleine-König" <u.kleine-koenig@baylibre.com>]

Some of the ADCs by Analog signal their irq condition on the MISO line.
So typically that line is connected to an SPI controller and a GPIO. The
GPIO is used as input and the respective interrupt is enabled when the
last SPI transfer is completed.

Depending on the GPIO controller the toggling MISO line might make the
interrupt pending even while it's masked. In that case the irq handler
is called immediately after irq_enable() and so before the device
actually pulls that line low which results in non-sense values being
reported to the upper layers.

The only way to find out if the line was actually pulled low is to read
the GPIO. (There is a flag in AD7124's status register that also signals
if an interrupt was asserted, but reading that register toggles the MISO
line and so might trigger another spurious interrupt.)

Add the possibility to specify an interrupt GPIO in the machine
description instead of a plain interrupt. This GPIO is used as interrupt
source and to check if the irq line is actually active in the irq
handler.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
---
 drivers/iio/adc/ad_sigma_delta.c       | 35 ++++++++++++++++++++++----
 include/linux/iio/adc/ad_sigma_delta.h |  1 +
 2 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
index e2bed2d648f2..b5785f2a0abe 100644
--- a/drivers/iio/adc/ad_sigma_delta.c
+++ b/drivers/iio/adc/ad_sigma_delta.c
@@ -539,12 +539,29 @@ static irqreturn_t ad_sd_data_rdy_trig_poll(int irq, void *private)
 {
 	struct ad_sigma_delta *sigma_delta = private;
 
-	complete(&sigma_delta->completion);
-	disable_irq_nosync(irq);
-	sigma_delta->irq_dis = true;
-	iio_trigger_poll(sigma_delta->trig);
+	/*
+	 * AD7124 and a few others use the same physical line for interrupt
+	 * reporting (nRDY) and MISO.
+	 * As MISO toggles when reading a register, this likely results in a
+	 * pending interrupt. This has two consequences: a) The irq might
+	 * trigger immediately after it's enabled even though the conversion
+	 * isn't done yet; and b) checking the STATUS register's nRDY flag is
+	 * off-limits as reading that would trigger another irq event.
+	 *
+	 * So read the MOSI line as GPIO (if available) and only trigger the irq
+	 * if the line is active.
+	 */
 
-	return IRQ_HANDLED;
+	if (!sigma_delta->rdy_gpiod || gpiod_get_value(sigma_delta->rdy_gpiod)) {
+		complete(&sigma_delta->completion);
+		disable_irq_nosync(irq);
+		sigma_delta->irq_dis = true;
+		iio_trigger_poll(sigma_delta->trig);
+
+		return IRQ_HANDLED;
+	} else {
+		return IRQ_NONE;
+	}
 }
 
 /**
@@ -679,6 +696,14 @@ int ad_sd_init(struct ad_sigma_delta *sigma_delta, struct iio_dev *indio_dev,
 	else
 		sigma_delta->irq_line = spi->irq;
 
+	sigma_delta->rdy_gpiod = devm_gpiod_get_optional(&spi->dev, "rdy", GPIOD_IN);
+	if (IS_ERR(sigma_delta->rdy_gpiod))
+		return dev_err_probe(&spi->dev, PTR_ERR(sigma_delta->rdy_gpiod),
+				     "Failed to find rdy gpio\n");
+
+	if (sigma_delta->rdy_gpiod && !sigma_delta->irq_line)
+		sigma_delta->irq_line = gpiod_to_irq(sigma_delta->rdy_gpiod);
+
 	iio_device_set_drvdata(indio_dev, sigma_delta);
 
 	return 0;
diff --git a/include/linux/iio/adc/ad_sigma_delta.h b/include/linux/iio/adc/ad_sigma_delta.h
index f8c1d2505940..866b4c21794b 100644
--- a/include/linux/iio/adc/ad_sigma_delta.h
+++ b/include/linux/iio/adc/ad_sigma_delta.h
@@ -96,6 +96,7 @@ struct ad_sigma_delta {
 	unsigned int		active_slots;
 	unsigned int		current_slot;
 	unsigned int		num_slots;
+	struct gpio_desc	*rdy_gpiod;
 	int		irq_line;
 	bool			status_appended;
 	/* map slots to channels in order to know what to expect from devices */
-- 
2.45.2