[PATCH v5 6/8] serial: max310x: schedule tx_work directly from the IRQ handler

[Tapio Reijonen]

The TXEMPTY interrupt asks for a FIFO refill, and start_tx() does
nothing beyond scheduling tx_work, so going through it makes no
functional difference. It does conflate two distinct events, though:
start_tx() is the serial core starting a new transmission, while
TXEMPTY can only fire for a transmission that is already running -
the bit latches on the FIFO's non-empty to empty transition, and the
IRQ handler's read of IRQSTS consumes the latch, so a stale TXEMPTY
cannot exist on an idle port (if the bit were level-triggered, the
handler's read-until-clear loop would never terminate).

Schedule tx_work directly, keeping the interrupt path a pure FIFO
refill. This is preparation for a following patch that adds
software-timed RS485 RTS control, where start_tx() also starts the
RTS envelope and a refill must not restart it.

No functional change.

Signed-off-by: Tapio Reijonen <tapio.reijonen@xxxxxxxxxxx>
---
 drivers/tty/serial/max310x.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/max310x.c b/drivers/tty/serial/max310x.c
index e07fb87f21f102cfa3fdae5275cb7810be1c3dbd..8f8817b127c891f18af52bd3d24efb269db78211 100644
--- a/drivers/tty/serial/max310x.c
+++ b/drivers/tty/serial/max310x.c
@@ -858,8 +858,12 @@ static irqreturn_t max310x_port_irq(struct max310x_port *s, int portno)
 		}
 		if (rxlen)
 			max310x_handle_rx(port, rxlen);
+		/*
+		 * TXEMPTY latches on the FIFO becoming empty, so a stale
+		 * interrupt cannot pump data during an RTS setup delay.
+		 */
 		if (ists & MAX310X_IRQ_TXEMPTY_BIT)
-			max310x_start_tx(port);
+			schedule_work(&s->p[portno].tx_work);
 	} while (1);
 
 	return res;

-- 
2.47.3