[PATCH v2] serial: max310x: rework RX interrupt handling

[Thomas Petazzoni <thomas.petazzoni@bootlin.com>]

Currently, the RX interrupt logic uses the RXEMPTY interrupt, with the
RXEMPTYINV bit set, which means we get an RX interrupt as soon as the
RX FIFO is non-empty.

However, with the MAX310X having a FIFO of 128 bytes, this makes very
poor use of the FIFO: we trigger an interrupt as soon as the RX FIFO
has one byte, which means a lot of interrupts, each only collecting a
few bytes from the FIFO, causing a significant CPU load.

Instead this commit relies on two other RX interrupt events:

 - MAX310X_IRQ_RXFIFO_BIT, which triggers when the RX FIFO has reached
   a certain threshold, which we define to be half of the FIFO
   size. This ensure we get an interrupt before the RX FIFO fills up.

 - MAX310X_LSR_RXTO_BIT, which triggers when the RX FIFO has received
   some bytes, and then no more bytes are received for a certain
   time. Arbitrarily, this time is defined to the time is takes to
   receive 4 characters.

On a Microchip SAMA5D3 platform that is receiving 20 bytes every 16ms
over one MAX310X UART, this patch has allowed to reduce the CPU
consumption of the interrupt handler thread from ~25% to 6-7%.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
---
Changes since v1:
- Fix missing space when closing a comment
---
 drivers/tty/serial/max310x.c | 29 ++++++++++++++++++++++++-----
 1 file changed, 24 insertions(+), 5 deletions(-)

diff --git a/drivers/tty/serial/max310x.c b/drivers/tty/serial/max310x.c
index 8434bd5a8ec7..21130af106bb 100644
--- a/drivers/tty/serial/max310x.c
+++ b/drivers/tty/serial/max310x.c
@@ -1056,9 +1056,9 @@ static int max310x_startup(struct uart_port *port)
 	max310x_port_update(port, MAX310X_MODE1_REG,
 			    MAX310X_MODE1_TRNSCVCTRL_BIT, 0);
 
-	/* Configure MODE2 register & Reset FIFOs*/
-	val = MAX310X_MODE2_RXEMPTINV_BIT | MAX310X_MODE2_FIFORST_BIT;
-	max310x_port_write(port, MAX310X_MODE2_REG, val);
+	/* Reset FIFOs */
+	max310x_port_write(port, MAX310X_MODE2_REG,
+			   MAX310X_MODE2_FIFORST_BIT);
 	max310x_port_update(port, MAX310X_MODE2_REG,
 			    MAX310X_MODE2_FIFORST_BIT, 0);
 
@@ -1086,8 +1086,27 @@ static int max310x_startup(struct uart_port *port)
 	/* Clear IRQ status register */
 	max310x_port_read(port, MAX310X_IRQSTS_REG);
 
-	/* Enable RX, TX, CTS change interrupts */
-	val = MAX310X_IRQ_RXEMPTY_BIT | MAX310X_IRQ_TXEMPTY_BIT;
+	/*
+	 * Let's ask for an interrupt after a timeout equivalent to
+	 * the receiving time of 4 characters after the last character
+	 * has been received.
+	 */
+	max310x_port_write(port, MAX310X_RXTO_REG, 4);
+
+	/*
+	 * Make sure we also get RX interrupts when the RX FIFO is
+	 * filling up quickly, so get an interrupt when half of the RX
+	 * FIFO has been filled in.
+	 */
+	max310x_port_write(port, MAX310X_FIFOTRIGLVL_REG,
+			   MAX310X_FIFOTRIGLVL_RX(MAX310X_FIFO_SIZE / 2));
+
+	/* Enable RX timeout interrupt in LSR */
+	max310x_port_write(port, MAX310X_LSR_IRQEN_REG,
+			   MAX310X_LSR_RXTO_BIT);
+
+	/* Enable LSR, RX FIFO trigger, CTS change interrupts */
+	val = MAX310X_IRQ_LSR_BIT  | MAX310X_IRQ_RXFIFO_BIT | MAX310X_IRQ_TXEMPTY_BIT;
 	max310x_port_write(port, MAX310X_IRQEN_REG, val | MAX310X_IRQ_CTS_BIT);
 
 	return 0;
-- 
2.26.2