[PATCH v5 3/8] serial: max310x: convert RS485 delays from milliseconds to bit-times

[Tapio Reijonen <tapio.reijonen@vaisala.com>]

The HDPIXDELAY register counts the RTS setup and hold delays in
bit-times, four bits per direction, but the driver has been writing the
struct serial_rs485 delay_rts_before_send/delay_rts_after_send values
into it unconverted - and the uapi expresses those in milliseconds. A
requested 9 ms setup delay is programmed as 9 bit-times, which at 9600
baud is 0.94 ms, roughly a tenth of what userspace asked for; the error
grows with the baud rate.

Cache the baud rate in set_termios() and convert the delays to
bit-times at the current rate, rounding up so the delay on the wire is
never shorter than requested, and capping at the 15 bit-times the
4-bit field can hold. Centralize the HDPIXDELAY and MODE1.TRNSCVCTRL
programming in max310x_set_rts_ctl_params(), called from set_termios()
(the conversion depends on the baud rate), the rs485-config worker and
startup(), which each had their own copy.

The rs485-config worker's break guard moves into the helper with the
MODE1 write it protects, and break-off now reapplies the current
configuration through the helper instead of hand-restoring MODE1, so a
reconfigure that arrived during the break takes effect when the break
ends instead of being dropped.

The delays a 4-bit bit-time field can represent still fall well short
of the milliseconds the uapi can express; requests beyond 15 bit-times
are capped, and the -ERANGE rejection of values above 15 ms remains in
place for now.

Fixes: 55367c620aed ("serial: max310x: Add support for RS-485 mode")
Signed-off-by: Tapio Reijonen <tapio.reijonen@vaisala.com>
---
 drivers/tty/serial/max310x.c | 120 +++++++++++++++++++++++++++++--------------
 1 file changed, 82 insertions(+), 38 deletions(-)

diff --git a/drivers/tty/serial/max310x.c b/drivers/tty/serial/max310x.c
index 693decd04de104051b07357973364bd587ab3d91..f8dad37d017afe5c0b1d36d6239ba05fc5e7aff0 100644
--- a/drivers/tty/serial/max310x.c
+++ b/drivers/tty/serial/max310x.c
@@ -165,6 +165,10 @@
 #define MAX310X_IRDA_IRDAEN_BIT		(1 << 0) /* IRDA mode enable */
 #define MAX310X_IRDA_SIR_BIT		(1 << 1) /* SIR mode enable */
 
+/* HDPIXDELAY accessor macros */
+#define MAX310X_HDPIXDELAY_SETUP(val)	(((val) & 0x0f) << 4)
+#define MAX310X_HDPIXDELAY_HOLD(val)	((val) & 0x0f)
+
 /* Flow control trigger level register masks */
 #define MAX310X_FLOWLVL_HALT_MASK	GENMASK(3, 0) /* Flow control halt level */
 #define MAX310X_FLOWLVL_RES_MASK	GENMASK(7, 4) /* Flow control resume level */
@@ -298,6 +302,7 @@ struct max310x_one {
 	struct work_struct	md_work;
 	struct work_struct	rs_work;
 	struct regmap		*regmap;
+	unsigned int		baud;
 	bool			tx_break;	/* break_ctl() owns the transceiver */
 
 	u8 rx_buf[MAX310X_FIFO_SIZE];
@@ -934,6 +939,48 @@ static void max310x_set_mctrl(struct uart_port *port, unsigned int mctrl)
 	schedule_work(&one->md_work);
 }
 
+/*
+ * Program the chip's RS485 RTS timing. The HDPIXDELAY setup and hold fields
+ * count bit-times, four bits per direction, while the uapi expresses the
+ * delays in milliseconds: convert at the current baud rate, rounding up, and
+ * cap at the field maximum.
+ */
+static void max310x_set_rts_ctl_params(struct max310x_one *one)
+{
+	const unsigned int max_bit_dly = 15;
+	struct uart_port *port = &one->port;
+	unsigned int setup = 0, hold = 0;
+	u8 mode1 = 0;
+
+	if (port->rs485.flags & SER_RS485_ENABLED) {
+		/* Convert milliseconds to bit-times, rounding up. */
+		setup = DIV_ROUND_UP(one->baud * port->rs485.delay_rts_before_send,
+				     MSEC_PER_SEC);
+		hold  = DIV_ROUND_UP(one->baud * port->rs485.delay_rts_after_send,
+				     MSEC_PER_SEC);
+		setup = min(setup, max_bit_dly);
+		hold  = min(hold,  max_bit_dly);
+
+		mode1 = MAX310X_MODE1_TRNSCVCTRL_BIT;
+	}
+
+	max310x_port_write(port, MAX310X_HDPIXDELAY_REG,
+			   MAX310X_HDPIXDELAY_SETUP(setup) |
+			   MAX310X_HDPIXDELAY_HOLD(hold));
+
+	/*
+	 * A break owns the transceiver: break_ctl() disabled auto-RTS and
+	 * drives RTS manually, and restores it from the current
+	 * configuration when the break ends. Touching MODE1 here would
+	 * release the transceiver mid-break.
+	 */
+	if (one->tx_break)
+		return;
+
+	max310x_port_update(port, MAX310X_MODE1_REG,
+			    MAX310X_MODE1_TRNSCVCTRL_BIT, mode1);
+}
+
 static void max310x_break_ctl(struct uart_port *port, int break_state)
 {
 	struct max310x_one *one = to_max310x_port(port);
@@ -953,10 +1000,20 @@ static void max310x_break_ctl(struct uart_port *port, int break_state)
 	 * break duration and drive RTS manually so the break reaches the wire;
 	 * restore auto-RTS when the break ends.
 	 */
-	max310x_port_update(port, MAX310X_MODE1_REG,
-			    MAX310X_MODE1_TRNSCVCTRL_BIT,
-			    break_state ? 0 : MAX310X_MODE1_TRNSCVCTRL_BIT);
-	max310x_rts_ctl(port, break_state);
+	if (break_state) {
+		max310x_port_update(port, MAX310X_MODE1_REG,
+				    MAX310X_MODE1_TRNSCVCTRL_BIT, 0);
+		max310x_rts_ctl(port, 1);
+	} else {
+		/*
+		 * Reapply the current configuration: a reconfigure that
+		 * arrived during the break was deferred by the tx_break
+		 * guard. Then release the manual RTS - auto-RTS owns the
+		 * pin again.
+		 */
+		max310x_set_rts_ctl_params(one);
+		max310x_rts_ctl(port, 0);
+	}
 }
 
 static void max310x_set_termios(struct uart_port *port,
@@ -1073,41 +1130,35 @@ static void max310x_set_termios(struct uart_port *port,
 
 	/* Update timeout according to new baud rate */
 	uart_update_timeout(port, termios->c_cflag, baud);
+
+	/*
+	 * Cache the new baud rate and reprogram the RS485 RTS delays, whose
+	 * millisecond-to-bit-time conversion depends on it.
+	 */
+	to_max310x_port(port)->baud = baud;
+	max310x_set_rts_ctl_params(to_max310x_port(port));
 }
 
 static void max310x_rs_proc(struct work_struct *ws)
 {
 	struct max310x_one *one = container_of(ws, struct max310x_one, rs_work);
-	unsigned int delay, mode1 = 0, mode2 = 0;
+	unsigned int mode2 = 0;
 
 	/*
 	 * Serialize against break_ctl() and set_termios(), which run under
-	 * port->mutex: the tx_break test below and the MODE1 write must not
+	 * port->mutex: the tx_break-guarded register writes must not
 	 * straddle a break starting or ending.
 	 */
 	guard(mutex)(&one->port.state->port.mutex);
 
-	delay = (one->port.rs485.delay_rts_before_send << 4) |
-		one->port.rs485.delay_rts_after_send;
-	max310x_port_write(&one->port, MAX310X_HDPIXDELAY_REG, delay);
+	max310x_set_rts_ctl_params(one);
 
-	if (one->port.rs485.flags & SER_RS485_ENABLED) {
-		mode1 = MAX310X_MODE1_TRNSCVCTRL_BIT;
+	if (one->port.rs485.flags & SER_RS485_ENABLED &&
+	    !(one->port.rs485.flags & SER_RS485_RX_DURING_TX))
+		mode2 = MAX310X_MODE2_ECHOSUPR_BIT;
 
-		if (!(one->port.rs485.flags & SER_RS485_RX_DURING_TX))
-			mode2 = MAX310X_MODE2_ECHOSUPR_BIT;
-	}
-
-	/*
-	 * A break owns the transceiver: break_ctl() disabled auto-RTS and
-	 * drives RTS manually, and restores it when the break ends. Leave
-	 * MODE1 alone meanwhile or the break goes undriven mid-way.
-	 */
-	if (!one->tx_break)
-		max310x_port_update(&one->port, MAX310X_MODE1_REG,
-				    MAX310X_MODE1_TRNSCVCTRL_BIT, mode1);
 	max310x_port_update(&one->port, MAX310X_MODE2_REG,
-			MAX310X_MODE2_ECHOSUPR_BIT, mode2);
+			    MAX310X_MODE2_ECHOSUPR_BIT, mode2);
 }
 
 static int max310x_rs485_config(struct uart_port *port, struct ktermios *termios,
@@ -1151,21 +1202,14 @@ static int max310x_startup(struct uart_port *port)
 	max310x_port_update(port, MAX310X_MODE2_REG,
 			    MAX310X_MODE2_FIFORST_BIT, 0);
 
-	/* Configure mode1/mode2 to have rs485/rs232 enabled at startup */
-	val = (clamp(port->rs485.delay_rts_before_send, 0U, 15U) << 4) |
-		clamp(port->rs485.delay_rts_after_send, 0U, 15U);
-	max310x_port_write(port, MAX310X_HDPIXDELAY_REG, val);
+	/* Configure the RS485 RTS timing and the RS485/RS232 mode bits. */
+	max310x_set_rts_ctl_params(one);
 
-	if (port->rs485.flags & SER_RS485_ENABLED) {
-		max310x_port_update(port, MAX310X_MODE1_REG,
-				    MAX310X_MODE1_TRNSCVCTRL_BIT,
-				    MAX310X_MODE1_TRNSCVCTRL_BIT);
-
-		if (!(port->rs485.flags & SER_RS485_RX_DURING_TX))
-			max310x_port_update(port, MAX310X_MODE2_REG,
-					    MAX310X_MODE2_ECHOSUPR_BIT,
-					    MAX310X_MODE2_ECHOSUPR_BIT);
-	}
+	if (port->rs485.flags & SER_RS485_ENABLED &&
+	    !(port->rs485.flags & SER_RS485_RX_DURING_TX))
+		max310x_port_update(port, MAX310X_MODE2_REG,
+				    MAX310X_MODE2_ECHOSUPR_BIT,
+				    MAX310X_MODE2_ECHOSUPR_BIT);
 
 	/*
 	 * Configure flow control levels:

-- 
2.47.3