Input: matrix_keypad - detect change during scan

For a setup where the matrix keypad is connected over a slow interface
(e.g. a gpio-expansion over i2c), the scan can take a longer time to read.

Interrupts need to be disabled during scan. And therefore changes in this
period are not detected.
To improve this situation, scan the matrix again if the row state changed
during interrupts disabled.
The rescan is repeated until no change is detected anymore.

Signed-off-by: Markus Burri <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Torokhov <[e-mail]>

drivers/input/keyboard/matrix_keypad.c

···
 	const unsigned short *keycodes = input_dev->keycode;
 	uint32_t new_state[MATRIX_MAX_COLS];
 	int row, col, code;
+	u32 init_row_state, new_row_state;
+
+	/* read initial row state to detect changes between scan */
+	init_row_state = read_row_state(keypad);
 
 	/* de-activate all columns for scanning */
 	activate_all_cols(keypad, false);
···
 	scoped_guard(spinlock_irq, &keypad->lock) {
 		keypad->scan_pending = false;
 		enable_row_irqs(keypad);
+	}
+
+	/* read new row state and detect if value has changed */
+	new_row_state = read_row_state(keypad);
+	if (init_row_state != new_row_state) {
+		guard(spinlock_irq)(&keypad->lock);
+		if (unlikely(keypad->scan_pending || keypad->stopped))
+			return;
+		disable_row_irqs(keypad);
+		keypad->scan_pending = true;
+		schedule_delayed_work(&keypad->work,
+				      msecs_to_jiffies(keypad->debounce_ms));
 	}
 }