[PATCH 4/7] tty: max310x: Fix invalid baudrate divisors calculator

[Serge Semin]

Current calculator doesn't do it' job quite correct. First of all the
max310x baud-rates generator supports the divisor being less than 16.
In this case the x2/x4 modes can be used to double or quadruple
the reference frequency. But the current baud-rate setter function
just filters all these modes out by the first condition and setups
these modes only if there is a clocks-baud division remainder. The former
doesn't seem right at all, since enabling the x2/x4 modes causes the line
noise tolerance reduction and should be only used as a last resort to
enable a requested too high baud-rate.

Finally the fraction is supposed to be calculated from D = Fref/(c*baud)
formulae, but not from D % 16, which causes the precision loss. So to speak
the current baud-rate calculator code works well only if the baud perfectly
fits to the uart reference input frequency.

Lets fix the calculator by implementing the algo fully compliant with
the fractional baud-rate generator described in the datasheet:
D = Fref / (c*baud), where c={16,8,4} is the x1/x2/x4 rate mode
respectively, Fref - reference input frequency. The divisor fraction is
calculated from the same formulae, but making sure it is found with a
resolution of 0.0625 (four bits).

Signed-off-by: Serge Semin <fancer.lancer@xxxxxxxxx>
---
 drivers/tty/serial/max310x.c | 51 ++++++++++++++++++++++--------------
 1 file changed, 31 insertions(+), 20 deletions(-)

diff --git a/drivers/tty/serial/max310x.c b/drivers/tty/serial/max310x.c
index 0e3dc89c459b..ca044f96c5cc 100644
--- a/drivers/tty/serial/max310x.c
+++ b/drivers/tty/serial/max310x.c
@@ -501,37 +501,48 @@ static bool max310x_reg_precious(struct device *dev, unsigned int reg)
 
 static int max310x_set_baud(struct uart_port *port, int baud)
 {
-	unsigned int mode = 0, clk = port->uartclk, div = clk / baud;
+	unsigned int mode = 0, div = 0, frac = 0, c = 0, F = 0;
 
-	/* Check for minimal value for divider */
-	if (div < 16)
-		div = 16;
-
-	if (clk % baud && (div / 16) < 0x8000) {
+	/*
+	 * Calculate the integer divisor first. Select a proper mode
+	 * in case if the requested baud is too high for the pre-defined
+	 * clocks frequency.
+	 */
+	div = port->uartclk / baud;
+	if (div < 8) {
+		/* Mode x4 */
+		c = 4;
+		mode = MAX310X_BRGCFG_4XMODE_BIT;
+	} else if (div < 16) {
 		/* Mode x2 */
+		c = 8;
 		mode = MAX310X_BRGCFG_2XMODE_BIT;
-		clk = port->uartclk * 2;
-		div = clk / baud;
-
-		if (clk % baud && (div / 16) < 0x8000) {
-			/* Mode x4 */
-			mode = MAX310X_BRGCFG_4XMODE_BIT;
-			clk = port->uartclk * 4;
-			div = clk / baud;
-		}
+	} else {
+		c = 16;
 	}
 
-	max310x_port_write(port, MAX310X_BRGDIVMSB_REG, (div / 16) >> 8);
-	max310x_port_write(port, MAX310X_BRGDIVLSB_REG, div / 16);
-	max310x_port_write(port, MAX310X_BRGCFG_REG, (div % 16) | mode);
+	/* Calculate the divisor in accordance with the fraction coefficient */
+	div /= c;
+	F = c*baud;
+
+	/* Calculate the baud rate fraction */
+	if (div > 0)
+		frac = (16*(port->uartclk % F)) / F;
+	else
+		div = 1;
+
+	max310x_port_write(port, MAX310X_BRGDIVMSB_REG, div >> 8);
+	max310x_port_write(port, MAX310X_BRGDIVLSB_REG, div);
+	max310x_port_write(port, MAX310X_BRGCFG_REG, frac | mode);
 
-	return DIV_ROUND_CLOSEST(clk, div);
+	/* Return the actual baud rate we just programmed */
+	return (16*port->uartclk) / (c*(16*div + frac));
 }
 
 static int max310x_update_best_err(unsigned long f, long *besterr)
 {
 	/* Use baudrate 115200 for calculate error */
-	long err = f % (115200 * 16);
+	long err = f % (460800 * 16);
 
 	if ((*besterr < 0) || (*besterr > err)) {
 		*besterr = err;
-- 
2.21.0