iio: adc: stm32: add trigger polarity extended attribute

Define extended attribute so that user may choose rising, falling or both
edges for external trigger sources.
Default to rising edge in case it isn't set.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio-adc-stm32

+What:		/sys/bus/iio/devices/triggerX/trigger_polarity
+KernelVersion:	4.11
+Contact:	fabrice.gasnier@st.com
+Description:
+		The STM32 ADC can be configured to use external trigger sources
+		(e.g. timers, pwm or exti gpio). Then, it can be tuned to start
+		conversions on external trigger by either:
+		- "rising-edge"
+		- "falling-edge"
+		- "both-edges".
+		Reading returns current trigger polarity.
+		Writing value before enabling conversions sets trigger polarity.
+
+What:		/sys/bus/iio/devices/triggerX/trigger_polarity_available
+KernelVersion:	4.11
+Contact:	fabrice.gasnier@st.com
+Description:
+		List all available trigger_polarity settings.

drivers/iio/adc/stm32-adc.c

@@ -135,6 +135,7 @@ struct stm32_adc_regs {
  * @lock:		spinlock
  * @bufi:		data buffer index
  * @num_conv:		expected number of scan conversions
+ * @trigger_polarity:	external trigger polarity (e.g. exten)
  */
 struct stm32_adc {
 	struct stm32_adc_common	*common;
@@ -146,6 +147,7 @@ struct stm32_adc {
 	spinlock_t		lock;		/* interrupt lock */
 	unsigned int		bufi;
 	unsigned int		num_conv;
+	u32			trigger_polarity;
 };
 
 /**
@@ -410,7 +412,7 @@ static int stm32_adc_set_trig(struct iio_dev *indio_dev,
 
 		/* set trigger source and polarity (default to rising edge) */
 		extsel = ret;
-		exten = STM32_EXTEN_HWTRIG_RISING_EDGE;
+		exten = adc->trigger_polarity + STM32_EXTEN_HWTRIG_RISING_EDGE;
 	}
 
 	spin_lock_irqsave(&adc->lock, flags);
@@ -424,6 +426,36 @@ static int stm32_adc_set_trig(struct iio_dev *indio_dev,
 	return 0;
 }
 
+static int stm32_adc_set_trig_pol(struct iio_dev *indio_dev,
+				  const struct iio_chan_spec *chan,
+				  unsigned int type)
+{
+	struct stm32_adc *adc = iio_priv(indio_dev);
+
+	adc->trigger_polarity = type;
+
+	return 0;
+}
+
+static int stm32_adc_get_trig_pol(struct iio_dev *indio_dev,
+				  const struct iio_chan_spec *chan)
+{
+	struct stm32_adc *adc = iio_priv(indio_dev);
+
+	return adc->trigger_polarity;
+}
+
+static const char * const stm32_trig_pol_items[] = {
+	"rising-edge", "falling-edge", "both-edges",
+};
+
+const struct iio_enum stm32_adc_trig_pol = {
+	.items = stm32_trig_pol_items,
+	.num_items = ARRAY_SIZE(stm32_trig_pol_items),
+	.get = stm32_adc_get_trig_pol,
+	.set = stm32_adc_set_trig_pol,
+};
+
 /**
  * stm32_adc_single_conv() - Performs a single conversion
  * @indio_dev: IIO device
@@ -682,6 +714,17 @@ static irqreturn_t stm32_adc_trigger_handler(int irq, void *p)
 	return IRQ_HANDLED;
 }
 
+static const struct iio_chan_spec_ext_info stm32_adc_ext_info[] = {
+	IIO_ENUM("trigger_polarity", IIO_SHARED_BY_ALL, &stm32_adc_trig_pol),
+	{
+		.name = "trigger_polarity_available",
+		.shared = IIO_SHARED_BY_ALL,
+		.read = iio_enum_available_read,
+		.private = (uintptr_t)&stm32_adc_trig_pol,
+	},
+	{},
+};
+
 static void stm32_adc_chan_init_one(struct iio_dev *indio_dev,
 				    struct iio_chan_spec *chan,
 				    const struct stm32_adc_chan_spec *channel,
@@ -697,6 +740,7 @@ static void stm32_adc_chan_init_one(struct iio_dev *indio_dev,
 	chan->scan_type.sign = 'u';
 	chan->scan_type.realbits = 12;
 	chan->scan_type.storagebits = 16;
+	chan->ext_info = stm32_adc_ext_info;
 }
 
 static int stm32_adc_chan_of_init(struct iio_dev *indio_dev)