tty: xuartps: Updating set_baud_rate()

The original algorithm to find the best baud rate dividers does not necessarily
find the best set of dividers. And in the worst case may even write illegal
values to the hardware.
The new function should make better use of the hardware capabilities and be able
to provide valid settings for a wider range of baud rates and also input clocks.
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/xilinx_uartps.c

@@ -156,6 +156,11 @@ MODULE_PARM_DESC(rx_timeout, "Rx timeout, 1-255");
 #define XUARTPS_SR_TXFULL	0x00000010 /* TX FIFO full */
 #define XUARTPS_SR_RXTRIG	0x00000001 /* Rx Trigger */
 
+/* baud dividers min/max values */
+#define XUARTPS_BDIV_MIN	4
+#define XUARTPS_BDIV_MAX	255
+#define XUARTPS_CD_MAX		65535
+
 /**
  * struct xuartps - device data
  * @refclk	Reference clock
@@ -305,59 +310,94 @@ static irqreturn_t xuartps_isr(int irq, void *dev_id)
 }
 
 /**
- * xuartps_set_baud_rate - Calculate and set the baud rate
- * @port: Handle to the uart port structure
- * @baud: Baud rate to set
- *
+ * xuartps_calc_baud_divs - Calculate baud rate divisors
+ * @clk: UART module input clock
+ * @baud: Desired baud rate
+ * @rbdiv: BDIV value (return value)
+ * @rcd: CD value (return value)
+ * @div8: Value for clk_sel bit in mod (return value)
  * Returns baud rate, requested baud when possible, or actual baud when there
- *	was too much error
- **/
-static unsigned int xuartps_set_baud_rate(struct uart_port *port,
-						unsigned int baud)
+ *	was too much error, zero if no valid divisors are found.
+ *
+ * Formula to obtain baud rate is
+ *	baud_tx/rx rate = clk/CD * (BDIV + 1)
+ *	input_clk = (Uart User Defined Clock or Apb Clock)
+ *		depends on UCLKEN in MR Reg
+ *	clk = input_clk or input_clk/8;
+ *		depends on CLKS in MR reg
+ *	CD and BDIV depends on values in
+ *			baud rate generate register
+ *			baud rate clock divisor register
+ */
+static unsigned int xuartps_calc_baud_divs(unsigned int clk, unsigned int baud,
+		u32 *rbdiv, u32 *rcd, int *div8)
 {
-	unsigned int sel_clk;
-	unsigned int calc_baud = 0;
-	unsigned int brgr_val, brdiv_val;
+	u32 cd, bdiv;
+	unsigned int calc_baud;
+	unsigned int bestbaud = 0;
 	unsigned int bauderror;
+	unsigned int besterror = ~0;
 
-	/* Formula to obtain baud rate is
-	 *	baud_tx/rx rate = sel_clk/CD * (BDIV + 1)
-	 *	input_clk = (Uart User Defined Clock or Apb Clock)
-	 *		depends on UCLKEN in MR Reg
-	 *	sel_clk = input_clk or input_clk/8;
-	 *		depends on CLKS in MR reg
-	 *	CD and BDIV depends on values in
-	 *			baud rate generate register
-	 *			baud rate clock divisor register
-	 */
-	sel_clk = port->uartclk;
-	if (xuartps_readl(XUARTPS_MR_OFFSET) & XUARTPS_MR_CLKSEL)
-		sel_clk = sel_clk / 8;
-
-	/* Find the best values for baud generation */
-	for (brdiv_val = 4; brdiv_val < 255; brdiv_val++) {
+	if (baud < clk / ((XUARTPS_BDIV_MAX + 1) * XUARTPS_CD_MAX)) {
+		*div8 = 1;
+		clk /= 8;
+	} else {
+		*div8 = 0;
+	}
 
-		brgr_val = sel_clk / (baud * (brdiv_val + 1));
-		if (brgr_val < 2 || brgr_val > 65535)
+	for (bdiv = XUARTPS_BDIV_MIN; bdiv <= XUARTPS_BDIV_MAX; bdiv++) {
+		cd = DIV_ROUND_CLOSEST(clk, baud * (bdiv + 1));
+		if (cd < 1 || cd > XUARTPS_CD_MAX)
 			continue;
 
-		calc_baud = sel_clk / (brgr_val * (brdiv_val + 1));
+		calc_baud = clk / (cd * (bdiv + 1));
 
 		if (baud > calc_baud)
 			bauderror = baud - calc_baud;
 		else
 			bauderror = calc_baud - baud;
 
-		/* use the values when percent error is acceptable */
-		if (((bauderror * 100) / baud) < 3) {
-			calc_baud = baud;
-			break;
+		if (besterror > bauderror) {
+			*rbdiv = bdiv;
+			*rcd = cd;
+			bestbaud = calc_baud;
+			besterror = bauderror;
 		}
 	}
+	/* use the values when percent error is acceptable */
+	if (((besterror * 100) / baud) < 3)
+		bestbaud = baud;
+
+	return bestbaud;
+}
 
-	/* Set the values for the new baud rate */
-	xuartps_writel(brgr_val, XUARTPS_BAUDGEN_OFFSET);
-	xuartps_writel(brdiv_val, XUARTPS_BAUDDIV_OFFSET);
+/**
+ * xuartps_set_baud_rate - Calculate and set the baud rate
+ * @port: Handle to the uart port structure
+ * @baud: Baud rate to set
+ * Returns baud rate, requested baud when possible, or actual baud when there
+ *	   was too much error, zero if no valid divisors are found.
+ */
+static unsigned int xuartps_set_baud_rate(struct uart_port *port,
+		unsigned int baud)
+{
+	unsigned int calc_baud;
+	u32 cd, bdiv;
+	u32 mreg;
+	int div8;
+
+	calc_baud = xuartps_calc_baud_divs(port->uartclk, baud, &bdiv, &cd,
+			&div8);
+
+	/* Write new divisors to hardware */
+	mreg = xuartps_readl(XUARTPS_MR_OFFSET);
+	if (div8)
+		mreg |= XUARTPS_MR_CLKSEL;
+	else
+		mreg &= ~XUARTPS_MR_CLKSEL;
+	xuartps_writel(mreg, XUARTPS_MR_OFFSET);
+	xuartps_writel(cd, XUARTPS_BAUDGEN_OFFSET);
+	xuartps_writel(bdiv, XUARTPS_BAUDDIV_OFFSET);
 
 	return calc_baud;
 }
@@ -495,7 +535,7 @@ static void xuartps_set_termios(struct uart_port *port,
 				struct ktermios *termios, struct ktermios *old)
 {
 	unsigned int cval = 0;
-	unsigned int baud;
+	unsigned int baud, minbaud, maxbaud;
 	unsigned long flags;
 	unsigned int ctrl_reg, mode_reg;
 
@@ -512,8 +552,14 @@ static void xuartps_set_termios(struct uart_port *port,
 			(XUARTPS_CR_TX_DIS | XUARTPS_CR_RX_DIS),
 			XUARTPS_CR_OFFSET);
 
-	/* Min baud rate = 6bps and Max Baud Rate is 10Mbps for 100Mhz clk */
-	baud = uart_get_baud_rate(port, termios, old, 0, 10000000);
+	/*
+	 * Min baud rate = 6bps and Max Baud Rate is 10Mbps for 100Mhz clk
+	 * min and max baud should be calculated here based on port->uartclk.
+	 * this way we get a valid baud and can safely call set_baud()
+	 */
+	minbaud = port->uartclk / ((XUARTPS_BDIV_MAX + 1) * XUARTPS_CD_MAX * 8);
+	maxbaud = port->uartclk / (XUARTPS_BDIV_MIN + 1);
+	baud = uart_get_baud_rate(port, termios, old, minbaud, maxbaud);
 	baud = xuartps_set_baud_rate(port, baud);
 	if (tty_termios_baud_rate(termios))
 		tty_termios_encode_baud_rate(termios, baud, baud);
@@ -589,13 +635,17 @@ static void xuartps_set_termios(struct uart_port *port,
 				cval |= XUARTPS_MR_PARITY_MARK;
 			else
 				cval |= XUARTPS_MR_PARITY_SPACE;
-		} else if (termios->c_cflag & PARODD)
+		} else {
+			if (termios->c_cflag & PARODD)
 				cval |= XUARTPS_MR_PARITY_ODD;
 			else
 				cval |= XUARTPS_MR_PARITY_EVEN;
-	} else
+		}
+	} else {
 		cval |= XUARTPS_MR_PARITY_NONE;
-	xuartps_writel(cval , XUARTPS_MR_OFFSET);
+	}
+	cval |= mode_reg & 1;
+	xuartps_writel(cval, XUARTPS_MR_OFFSET);
 
 	spin_unlock_irqrestore(&port->lock, flags);
 }