counter: Introduce the compare component

Compare registers are used in devices to compare a counter channel
against a particular count value (e.g. to check if a threshold has been
reached). A macro COUNTER_COMP_COMPARE() is introduced to facilitate the
creation of compare components as Count extensions.

Link: https://lore.kernel.org/r/20250306-introduce-compare-component-v1-1-93993b3dca9c@kernel.org
Signed-off-by: William Breathitt Gray <wbg@kernel.org>

Author: William Breathitt Gray

Documentation/ABI/testing/sysfs-bus-counter

@@ -34,6 +34,14 @@ Contact:	linux-iio@vger.kernel.org
 Description:
 		Count data of Count Y represented as a string.
 
+What:		/sys/bus/counter/devices/counterX/countY/compare
+KernelVersion:	6.15
+Contact:	linux-iio@vger.kernel.org
+Description:
+		If the counter device supports compare registers -- registers
+		used to compare counter channels against a particular count --
+		the compare count for channel Y is provided by this attribute.
+
 What:		/sys/bus/counter/devices/counterX/countY/capture
 KernelVersion:	6.1
 Contact:	linux-iio@vger.kernel.org
@@ -301,6 +309,7 @@ Description:
 
 What:		/sys/bus/counter/devices/counterX/cascade_counts_enable_component_id
 What:		/sys/bus/counter/devices/counterX/external_input_phase_clock_select_component_id
+What:		/sys/bus/counter/devices/counterX/countY/compare_component_id
 What:		/sys/bus/counter/devices/counterX/countY/capture_component_id
 What:		/sys/bus/counter/devices/counterX/countY/ceiling_component_id
 What:		/sys/bus/counter/devices/counterX/countY/floor_component_id

include/linux/counter.h

@@ -580,6 +580,9 @@ struct counter_array {
 #define COUNTER_COMP_CEILING(_read, _write) \
 	COUNTER_COMP_COUNT_U64("ceiling", _read, _write)
 
+#define COUNTER_COMP_COMPARE(_read, _write) \
+	COUNTER_COMP_COUNT_U64("compare", _read, _write)
+
 #define COUNTER_COMP_COUNT_MODE(_read, _write, _available) \
 { \
 	.type = COUNTER_COMP_COUNT_MODE, \