[PATCH v2] counter: microchip-tcb-capture: Add watch validation support

[Dharma Balasubiramani <dharma.b@microchip.com>]

The Timer Counter Block (TCB) exposes several kinds of events to the
Counter framework, but not every event is meaningful on every hardware
channel. Add a `watch_validate()` callback so userspace may register only
the combinations actually supported:

* Channel 0 (COUNTER_MCHP_EVCHN_CV, COUNTER_MCHP_EVCHN_RA)
   - COUNTER_EVENT_CAPTURE
   - COUNTER_EVENT_CHANGE_OF_STATE
   - COUNTER_EVENT_OVERFLOW

* Channel 1 (COUNTER_MCHP_EVCHN_RB)
   - COUNTER_EVENT_CAPTURE

* Channel 2 (COUNTER_MCHP_EVCHN_RC)
   - COUNTER_EVENT_THRESHOLD

Any other request is rejected with `-EINVAL`, preventing undefined
behaviour in userspace.

Signed-off-by: Dharma Balasubiramani <dharma.b@microchip.com>
---
Tested the code on target (sam9x60_curiosity) using the following commands

valid ones:
./counter_watch_events -d -wevt_change_of_state,chan=0
./counter_watch_events -d -wevt_ovf,chan=0
./counter_watch_events -d -wevt_capture,chan=0
./counter_watch_events -d -wevt_capture,chan=1
./counter_watch_events -d -wevt_threshold,chan=2

invalid ones:
./counter_watch_events -d -wevt_threshold,chan=0
./counter_watch_events -d -wevt_threshold,chan=1
Error adding watches[0]: Invalid argument
---
Changes in v2:
- Include COUNTER_MCHP_EVCHN_CV as well for the sake of completeness.
- Adjust the code to ensure channel limitations.
- Drop sorting in this patch, will be taken care seperately.
- Link to v1: https://lore.kernel.org/r/20250515-counter-tcb-v1-1-e547061ed80f@microchip.com
---
 drivers/counter/microchip-tcb-capture.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/drivers/counter/microchip-tcb-capture.c b/drivers/counter/microchip-tcb-capture.c
index 1de3c50b9804..fe817f4f1edc 100644
--- a/drivers/counter/microchip-tcb-capture.c
+++ b/drivers/counter/microchip-tcb-capture.c
@@ -337,6 +337,27 @@ static struct counter_comp mchp_tc_count_ext[] = {
 	COUNTER_COMP_COMPARE(mchp_tc_count_compare_read, mchp_tc_count_compare_write),
 };
 
+static int mchp_tc_watch_validate(struct counter_device *counter,
+				  const struct counter_watch *watch)
+{
+	if (watch->channel == COUNTER_MCHP_EVCHN_CV || watch->channel == COUNTER_MCHP_EVCHN_RA) {
+		switch (watch->event) {
+		case COUNTER_EVENT_CHANGE_OF_STATE:
+		case COUNTER_EVENT_OVERFLOW:
+		case COUNTER_EVENT_CAPTURE:
+			return 0;
+		default:
+			return -EINVAL;
+		}
+	} else if (watch->channel == COUNTER_MCHP_EVCHN_RB) {
+		return (watch->event == COUNTER_EVENT_CAPTURE) ? 0 : -EINVAL;
+	} else if (watch->channel == COUNTER_MCHP_EVCHN_RC) {
+		return (watch->event == COUNTER_EVENT_THRESHOLD) ? 0 : -EINVAL;
+	} else {
+		return -EINVAL;
+	}
+}
+
 static struct counter_count mchp_tc_counts[] = {
 	{
 		.id = 0,
@@ -356,7 +377,8 @@ static const struct counter_ops mchp_tc_ops = {
 	.function_read  = mchp_tc_count_function_read,
 	.function_write = mchp_tc_count_function_write,
 	.action_read    = mchp_tc_count_action_read,
-	.action_write   = mchp_tc_count_action_write
+	.action_write   = mchp_tc_count_action_write,
+	.watch_validate = mchp_tc_watch_validate,
 };
 
 static const struct atmel_tcb_config tcb_rm9200_config = {

---
base-commit: 8566fc3b96539e3235909d6bdda198e1282beaed
change-id: 20250515-counter-tcb-b6ae1945210b

Best regards,
-- 
Dharma Balasubiramani <dharma.b@microchip.com>