[PATCH 1/2] serial: sc16is7xx: fix TX gap caused by kfifo circular buffer wrap-around

[Paul Mbewe]

kfifo_out_linear_ptr() returns only one contiguous linear segment of the
circular kfifo buffer. When transmit data wraps around the end of the
buffer, only the first segment (up to the buffer end) is sent. The
remaining data at the start of the buffer is not sent until the next TX
interrupt fires, resulting in a visible inter-frame gap on the wire.

This gap violates the Modbus RTU 1.5 character-time inter-character
silence limit. Receivers interpret any silence exceeding 1.5 character
times as an end-of-frame marker, splitting a single valid frame into
two malformed fragments and corrupting communication on the bus.

The incomplete transfer also causes unnecessary TX interrupts: instead
of draining the full available FIFO space in one pass, the driver fires
an extra interrupt per wrap-around just to send the remaining bytes.

The pre-kfifo code handled wrap-around by copying bytes one at a time
from the circ_buf into a linear staging buffer. The conversion to kfifo
replaced this with a single kfifo_out_linear_ptr() call, losing the
wrap-around handling. The max310x driver (a similar SPI UART) correctly
handles this with a while loop.

Fix this by calling kfifo_out_linear_ptr() in a loop, advancing through
all contiguous segments until the available TX FIFO space is exhausted
or the kfifo is empty.

Tested on SC16IS752 (SPI) driving RS-485 at 115200 baud 8N1 on an
i.MX6ULL based board. Oscilloscope confirmed mid-frame breaks at the
kfifo wrap-around boundary before the fix; no breaks observed after.

Fixes: 1788cf6a91d9 ("tty: serial: switch from circ_buf to kfifo")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Tobias Gannert <tobias.gannert@xxxxxxxxxxxxxx>
Tested-by: Tobias Gannert <tobias.gannert@xxxxxxxxxxxxxx>
Reviewed-by: Joachim Knorr <joachim.knorr@xxxxxxxxxxxxxx>
Signed-off-by: Paul Mbewe <paultyson.mbewe@xxxxxxxxxxxxxx>
---
 drivers/tty/serial/sc16is7xx.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c
index 1a2c4c14f6aa..395a219280be 100644
--- a/drivers/tty/serial/sc16is7xx.c
+++ b/drivers/tty/serial/sc16is7xx.c
@@ -730,9 +730,17 @@ static void sc16is7xx_handle_tx(struct uart_port *port)
 		txlen = 0;
 	}
 
-	txlen = kfifo_out_linear_ptr(&tport->xmit_fifo, &tail, txlen);
-	sc16is7xx_fifo_write(port, tail, txlen);
-	uart_xmit_advance(port, txlen);
+	/* Handle circular buffer wrap-around by sending in contiguous segments */
+	while (txlen > 0 && !kfifo_is_empty(&tport->xmit_fifo)) {
+		unsigned int to_send;
+
+		to_send = kfifo_out_linear_ptr(&tport->xmit_fifo, &tail, txlen);
+		if (!to_send)
+			break;
+		sc16is7xx_fifo_write(port, tail, to_send);
+		uart_xmit_advance(port, to_send);
+		txlen -= to_send;
+	}
 
 	uart_port_lock_irqsave(port, &flags);
 	if (kfifo_len(&tport->xmit_fifo) < WAKEUP_CHARS)
-- 
2.43.0