serial: pl011: Add configurable bounded polling for RS485 TX drain

The hrtimer-based TX drain introduced in commit 2c1fd53 ("serial:
amba-pl011: Fix RTS handling in RS485 mode") can cause RX errors in
fast request/response RS485 protocols. The hrtimer callback latency
(often 50-100µs per iteration) delays RX re-enable, causing the first
bytes of slave responses to be lost.

Add a configurable bounded polling mode that spins for TX completion
with a timeout, falling back to hrtimer only if the timeout is exceeded.
This is controlled by two new device tree properties:

  rs485-tx-drain-poll    - enable bounded polling mode
  rs485-tx-drain-timeout-us - explicit timeout in microseconds (optional)

When rs485-tx-drain-poll is set without an explicit timeout, the timeout
is auto-calculated as (fifo_size + 1) * character_time, accounting for
the FIFO depth plus one character in the shift register. This adapts
automatically to baud rate changes.

The default behavior (no DT properties) remains unchanged, preserving
the pure hrtimer approach for systems where shared IRQ latency is the
primary concern.

Signed-off-by: Nicolai Buchwitz <n.buchwitz@kunbus.com>

Author: nbuchwitz

drivers/tty/serial/amba-pl011.c

@@ -286,6 +286,8 @@ struct uart_amba_port {
 	enum pl011_rs485_tx_state	rs485_tx_state;
 	struct hrtimer		trigger_start_tx;
 	struct hrtimer		trigger_stop_tx;
+	bool rs485_tx_drain_poll;		/* use bounded polling */
+	unsigned int rs485_tx_drain_timeout_us; /* explicit override, 0 = auto */
 #ifdef CONFIG_DMA_ENGINE
 	/* DMA stuff */
 	unsigned int		dmacr;		/* dma control reg */
@@ -1288,6 +1290,19 @@ static inline bool pl011_dma_rx_running(struct uart_amba_port *uap)
 #define pl011_dma_flush_buffer	NULL
 #endif
 
+static unsigned int pl011_rs485_tx_drain_timeout(struct uart_amba_port *uap)
+{
+	if (uap->rs485_tx_drain_timeout_us > 0)
+		return uap->rs485_tx_drain_timeout_us;
+
+	/*
+	 * Auto-calculate based on FIFO depth plus one character
+	 * in the shift register.
+	 */
+	return div_u64(ktime_to_ns(uap->rs485_tx_drain_interval) *
+		       (uap->fifosize + 1), NSEC_PER_USEC);
+}
+
 static void pl011_rs485_tx_stop(struct uart_amba_port *uap)
 {
 	struct uart_port *port = &uap->port;
@@ -1297,6 +1312,19 @@ static void pl011_rs485_tx_stop(struct uart_amba_port *uap)
 		uap->rs485_tx_state = WAIT_AFTER_SEND;
 
 	if (uap->rs485_tx_state == WAIT_AFTER_SEND) {
+		if (uap->rs485_tx_drain_poll) {
+			/* Bounded spin with auto-calculated or explicit timeout */
+			unsigned int timeout_us = pl011_rs485_tx_drain_timeout(uap);
+			ktime_t deadline = ktime_add_us(ktime_get(), timeout_us);
+
+			while (!pl011_tx_empty(port)) {
+				if (ktime_after(ktime_get(), deadline)) {
+					/* Timeout - fall back to hrtimer */
+					break;
+				}
+				cpu_relax();
+			}
+		}
 		/* Schedule hrtimer if tx queue not empty */
 		if (!pl011_tx_empty(port)) {
 			hrtimer_start(&uap->trigger_stop_tx,
@@ -2904,6 +2932,11 @@ static int pl011_probe(struct amba_device *dev, const struct amba_id *id)
 	uap->trigger_start_tx.function = pl011_trigger_start_tx;
 	uap->trigger_stop_tx.function = pl011_trigger_stop_tx;
 
+	uap->rs485_tx_drain_poll = device_property_read_bool(&dev->dev,
+			"rs485-tx-drain-poll");
+	device_property_read_u32(&dev->dev, "rs485-tx-drain-timeout-us",
+			&uap->rs485_tx_drain_timeout_us);
+
 	ret = pl011_setup_port(&dev->dev, uap, &dev->res, portnr);
 	if (ret)
 		return ret;
@@ -3103,6 +3136,11 @@ static int pl011_axi_probe(struct platform_device *pdev)
 	uap->trigger_start_tx.function = pl011_trigger_start_tx;
 	uap->trigger_stop_tx.function = pl011_trigger_stop_tx;
 
+	uap->rs485_tx_drain_poll = device_property_read_bool(&pdev->dev,
+			"rs485-tx-drain-poll");
+	device_property_read_u32(&pdev->dev, "rs485-tx-drain-timeout-us",
+			&uap->rs485_tx_drain_timeout_us);
+
 	ret = pl011_setup_port(&pdev->dev, uap, r, portnr);
 	if (ret)
 		return ret;