iio: events.h: add event identifier macros for differential channel

Currently, there are 3 helper macros in iio/events.h to create event
identifiers:
- IIO_EVENT_CODE : create generic event identifier for differential and non
differential channels
- IIO_MOD_EVENT_CODE : create event identifier for modified (non
differential) channels
- IIO_UNMOD_EVENT_CODE : create event identifier for unmodified (non
differential) channels

For differential channels, drivers are expected to use IIO_EVENT_CODE.
However, only one driver in drivers/iio currently uses it correctly,
leading to inconsistent event identifiers for differential channels that
don’t match the intended attributes (such as max1363.c that supports
differential channels, but only uses IIO_UNMOD_EVENT_CODE).

To prevent such issues in future drivers, a new helper macro,
IIO_DIFF_EVENT_CODE, is introduced to specifically create event identifiers
for differential channels. Only one helper is needed for differential
channels since they cannot have modifiers.

Additionally, the descriptions for IIO_MOD_EVENT_CODE and
IIO_UNMOD_EVENT_CODE have been updated to clarify that they are intended
for non-differential channels,

Signed-off-by: Julien Stephan <[e-mail]>
Reviewed-by: David Lechner <[e-mail]>
Link: https://patch.msgid.link/20241028-iio-add-macro[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

include/linux/iio/events.h

···
 
 
 /**
- * IIO_MOD_EVENT_CODE() - create event identifier for modified channels
+ * IIO_MOD_EVENT_CODE() - create event identifier for modified (non
+ * differential) channels
  * @chan_type:	Type of the channel. Should be one of enum iio_chan_type.
  * @number:	Channel number.
  * @modifier:	Modifier for the channel. Should be one of enum iio_modifier.
···
 	IIO_EVENT_CODE(chan_type, 0, modifier, direction, type, number, 0, 0)
 
 /**
- * IIO_UNMOD_EVENT_CODE() - create event identifier for unmodified channels
+ * IIO_UNMOD_EVENT_CODE() - create event identifier for unmodified (non
+ * differential) channels
  * @chan_type:	Type of the channel. Should be one of enum iio_chan_type.
  * @number:	Channel number.
  * @type:	Type of the event. Should be one of enum iio_event_type.
···
 
 #define IIO_UNMOD_EVENT_CODE(chan_type, number, type, direction)	\
 	IIO_EVENT_CODE(chan_type, 0, 0, direction, type, number, 0, 0)
+
+/**
+ * IIO_DIFF_EVENT_CODE() - create event identifier for differential channels
+ * @chan_type:	Type of the channel. Should be one of enum iio_chan_type.
+ * @chan1:	First channel number for differential channels.
+ * @chan2:	Second channel number for differential channels.
+ * @type:	Type of the event. Should be one of enum iio_event_type.
+ * @direction:	Direction of the event. One of enum iio_event_direction.
+ */
+
+#define IIO_DIFF_EVENT_CODE(chan_type, chan1, chan2, type, direction)	\
+	IIO_EVENT_CODE(chan_type, 1, 0, direction, type, 0, chan1, chan2)
 
 #endif