[PATCH v7] serial: sc16is7xx: Add polling mode if no IRQ pin is available

[Andre Werner <andre.werner@systec-electronic.com>]

Fall back to polling mode if no interrupt is configured because there
is no possibility to connect the interrupt pin.

If no interrupt pin is available the driver uses a delayed worker to
poll the state of interrupt status registers (IIR).

Signed-off-by: Andre Werner <andre.werner@systec-electronic.com>
Link: https://lore.kernel.org/r/20250110073104.1029633-2-andre.werner@systec-electronic.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
V2:
- Change warning for polling mode to debug log entry
- Correct typo: Resuse -> Reuse
- Format define with missing tabs for SC16IS7XX_POLL_PERIOD
- Format struct declaration sc16is7xx_one_config with missing tabs for polling and shutdown
- Adapt dtbinding with new polling feature
V3:
- Use suffix with units and drop a comment SC16IS7XX_POLL_PERIOD_MS. Sorry for that miss.
- Make Kernel lowercase.
V4:
- Reword commit messages for better understanding.
- Remove 'shutdown' property for canceling delayed worker.
- Rename worker function: sc16is7xx_transmission_poll -> sc16is7xx_poll_proc
- Unify argument for worker functions: kthread_work *work -> kthread_work *ws
V5:
- Replace of_property check with IRQ number check to set polling
  property. This will add support for usage without device tree
  definitions. Thanks for that advice.
- Add blank line es requested.
V6:
- Use polling mode for IRQ numbers <= 0 which encounter no valid IRQ
  were found/defined.
V7:
- Try to improve and unify comments as requested.
- Fix typo in commit message: pull -> poll
---
 drivers/tty/serial/sc16is7xx.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c
index 7b51cdc274fd..348ddc3103cd 100644
--- a/drivers/tty/serial/sc16is7xx.c
+++ b/drivers/tty/serial/sc16is7xx.c
@@ -868,10 +868,12 @@ static void sc16is7xx_poll_proc(struct kthread_work *ws)
 {
 	struct sc16is7xx_port *s = container_of(ws, struct sc16is7xx_port, poll_work.work);
 
-	/* Reuse standard IRQ handler. Interrupt ID is unused in this context. */
+	/*
+	 * Reuse standard IRQ handler. Interrupt ID is unused in this
+	 * context and set to zero.
+	 */
 	sc16is7xx_irq(0, s);
 
-	/* Setup delay based on SC16IS7XX_POLL_PERIOD_MS */
 	kthread_queue_delayed_work(&s->kworker, &s->poll_work,
 				   msecs_to_jiffies(SC16IS7XX_POLL_PERIOD_MS));
 }
@@ -1561,7 +1563,7 @@ int sc16is7xx_probe(struct device *dev, const struct sc16is7xx_devtype *devtype,
 	/* Always ask for fixed clock rate from a property. */
 	device_property_read_u32(dev, "clock-frequency", &uartclk);
 
-	s->polling = !!irq;
+	s->polling = (irq <= 0);
 	if (s->polling)
 		dev_dbg(dev,
 			"No interrupt pin definition, falling back to polling mode\n");
@@ -1694,7 +1696,7 @@ int sc16is7xx_probe(struct device *dev, const struct sc16is7xx_devtype *devtype,
 #endif
 
 	if (s->polling) {
-		/* Initialize kernel thread for polling */
+		/* Initialize a kthread work struct that is dedicated to polling */
 		kthread_init_delayed_work(&s->poll_work, sc16is7xx_poll_proc);
 		return 0;
 	}
-- 
2.48.0