serial: sc16is7xx: fix bug in sc16is7xx_set_baud() when using prescaler

When using a high speed clock with a low baud rate, the 4x prescaler is
automatically selected if required. In that case, sc16is7xx_set_baud()
properly configures the chip registers, but returns an incorrect baud
rate by not taking into account the prescaler value. This incorrect baud
rate is then fed to uart_update_timeout().

For example, with an input clock of 80MHz, and a selected baud rate of 50,
sc16is7xx_set_baud() will return 200 instead of 50.

Fix this by first changing the prescaler variable to hold the selected
prescaler value instead of the MCR bitfield. Then properly take into
account the selected prescaler value in the return value computation.

Also add better documentation about the divisor value computation.

Fixes: dfeae619 ("serial: sc16is7xx")
Cc: stable@vger.kernel.org
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20240430200431.4102923-1-hugo@hugovil.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sc16is7xx.c

@@ -554,16 +554,28 @@ static bool sc16is7xx_regmap_noinc(struct device *dev, unsigned int reg)
 	return reg == SC16IS7XX_RHR_REG;
 }
 
+/*
+ * Configure programmable baud rate generator (divisor) according to the
+ * desired baud rate.
+ *
+ * From the datasheet, the divisor is computed according to:
+ *
+ *              XTAL1 input frequency
+ *             -----------------------
+ *                    prescaler
+ * divisor = ---------------------------
+ *            baud-rate x sampling-rate
+ */
 static int sc16is7xx_set_baud(struct uart_port *port, int baud)
 {
 	struct sc16is7xx_one *one = to_sc16is7xx_one(port, port);
 	u8 lcr;
-	u8 prescaler = 0;
+	unsigned int prescaler = 1;
 	unsigned long clk = port->uartclk, div = clk / 16 / baud;
 
 	if (div >= BIT(16)) {
-		prescaler = SC16IS7XX_MCR_CLKSEL_BIT;
-		div /= 4;
+		prescaler = 4;
+		div /= prescaler;
 	}
 
 	/* Enable enhanced features */
@@ -573,9 +585,10 @@ static int sc16is7xx_set_baud(struct uart_port *port, int baud)
 			      SC16IS7XX_EFR_ENABLE_BIT);
 	sc16is7xx_efr_unlock(port);
 
+	/* If bit MCR_CLKSEL is set, the divide by 4 prescaler is activated. */
 	sc16is7xx_port_update(port, SC16IS7XX_MCR_REG,
 			      SC16IS7XX_MCR_CLKSEL_BIT,
-			      prescaler);
+			      prescaler == 1 ? 0 : SC16IS7XX_MCR_CLKSEL_BIT);
 
 	/* Backup LCR and access special register set (DLL/DLH) */
 	lcr = sc16is7xx_port_read(port, SC16IS7XX_LCR_REG);
@@ -591,7 +604,7 @@ static int sc16is7xx_set_baud(struct uart_port *port, int baud)
 	/* Restore LCR and access to general register set */
 	sc16is7xx_port_write(port, SC16IS7XX_LCR_REG, lcr);
 
-	return DIV_ROUND_CLOSEST(clk / 16, div);
+	return DIV_ROUND_CLOSEST((clk / prescaler) / 16, div);
 }
 
 static void sc16is7xx_handle_rx(struct uart_port *port, unsigned int rxlen,