[PATCH v6 4/9] serial: max310x: convert RS485 delays from milliseconds to bit-times

[Tapio Reijonen]

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. The conversion lives in
max310x_set_rts_ctl_params(); set_termios() now calls it as well,
since the conversion depends on the baud rate.

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")
Assisted-by: Claude:claude-fable-5
Signed-off-by: Tapio Reijonen <tapio.reijonen@xxxxxxxxxxx>
---
 drivers/tty/serial/max310x.c | 40 +++++++++++++++++++++++++++++++---------
 1 file changed, 31 insertions(+), 9 deletions(-)

diff --git a/drivers/tty/serial/max310x.c b/drivers/tty/serial/max310x.c
index 85353401e09b1481996142432b1279f44fefff1d..2456a2af891f5296cfec833d6406916073d2865f 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];
@@ -935,23 +940,33 @@ static void max310x_set_mctrl(struct uart_port *port, unsigned int mctrl)
 }
 
 /*
- * Program the chip's RS485 RTS timing (HDPIXDELAY) and the auto
- * transceiver control (MODE1) from the current configuration. The
- * rs485-config worker, startup() and break_ctl()'s break-off restore
- * each carried their own copy of this sequence.
+ * 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 delay;
+	unsigned int setup = 0, hold = 0;
 	u8 mode1 = 0;
 
-	delay = (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, delay);
+	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);
 
-	if (port->rs485.flags & SER_RS485_ENABLED)
 		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
@@ -1107,6 +1122,13 @@ 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)

-- 
2.47.3