tty/serial/ar933x_uart: fix baud rate calculation

The UART of the AR933x SoC implements a fractional divisor
for generating the desired baud rate.

The current code uses a fixed value for the fractional
part of the divisor, and this leads to improperly
calculated baud rates:

   baud    scale   step  real baud         diff
     300   5207*   8192     17756     17456   5818.66%
     600   2603*   8192     35511     34911   5818.50%
    1200   1301*   8192     71023     69823   5818.58%
    2400    650*   8192     11241      8841    368.37%
    4800    324*   8192     22645     17845    371.77%
    9600    161    8192      9645        45      0.46%
   14400    107    8192     14468        68      0.47%
   19200     80    8192     19290        90      0.46%
   28800     53    8192     28935       135      0.46%
   38400     39    8192     39063       663      1.72%
   57600     26    8192     57870       270      0.46%
  115200     12    8192    120192      4992      4.33%
  230400      5    8192    260417     30017     13.02%
  460800      2    8192    520833     60033     13.02%
  921600      0    8192   1562500    640900     69.93%

After the patch, the integer and fractional parts of the
divisor will be calculated dynamically. This ensures that
the UART will use correct baud rates:

   baud    scale   step  real baud         diff
     300      6      11       300         0      0.00%
     600     54     173       600         0      0.00%
    1200     30     195      1200         0      0.00%
    2400     30     390      2400         0      0.00%
    4800     48    1233      4800         0      0.00%
    9600     78    3976      9600         0      0.00%
   14400     98    7474     14400         0      0.00%
   19200     55    5637     19200         0      0.00%
   28800    130   19780     28800         0      0.00%
   38400     36    7449     38400         0      0.00%
   57600     78   23857     57600         0      0.00%
  115200     43   26575    115200         0      0.00%
  230400     23   28991    230400         0      0.00%
  460800     11   28991    460800         0      0.00%
  921600      5   28991    921599        -1      0.00%

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/ar933x_uart.c

@@ -25,11 +25,19 @@
 #include <linux/io.h>
 #include <linux/irq.h>
 
+#include <asm/div64.h>
+
 #include <asm/mach-ath79/ar933x_uart.h>
 #include <asm/mach-ath79/ar933x_uart_platform.h>
 
 #define DRIVER_NAME "ar933x-uart"
 
+#define AR933X_UART_MAX_SCALE	0xff
+#define AR933X_UART_MAX_STEP	0xffff
+
+#define AR933X_UART_MIN_BAUD	300
+#define AR933X_UART_MAX_BAUD	3000000
+
 #define AR933X_DUMMY_STATUS_RD	0x01
 
 static struct uart_driver ar933x_uart_driver;
@@ -37,6 +45,8 @@ static struct uart_driver ar933x_uart_driver;
 struct ar933x_uart_port {
 	struct uart_port	port;
 	unsigned int		ier;	/* shadow Interrupt Enable Register */
+	unsigned int		min_baud;
+	unsigned int		max_baud;
 };
 
 static inline unsigned int ar933x_uart_read(struct ar933x_uart_port *up,
@@ -162,6 +172,57 @@ static void ar933x_uart_enable_ms(struct uart_port *port)
 {
 }
 
+/*
+ * baudrate = (clk / (scale + 1)) * (step * (1 / 2^17))
+ */
+static unsigned long ar933x_uart_get_baud(unsigned int clk,
+					  unsigned int scale,
+					  unsigned int step)
+{
+	u64 t;
+	u32 div;
+
+	div = (2 << 16) * (scale + 1);
+	t = clk;
+	t *= step;
+	t += (div / 2);
+	do_div(t, div);
+
+	return t;
+}
+
+static void ar933x_uart_get_scale_step(unsigned int clk,
+				       unsigned int baud,
+				       unsigned int *scale,
+				       unsigned int *step)
+{
+	unsigned int tscale;
+	long min_diff;
+
+	*scale = 0;
+	*step = 0;
+
+	min_diff = baud;
+	for (tscale = 0; tscale < AR933X_UART_MAX_SCALE; tscale++) {
+		u64 tstep;
+		int diff;
+
+		tstep = baud * (tscale + 1);
+		tstep *= (2 << 16);
+		do_div(tstep, clk);
+
+		if (tstep > AR933X_UART_MAX_STEP)
+			break;
+
+		diff = abs(ar933x_uart_get_baud(clk, tscale, tstep) - baud);
+		if (diff < min_diff) {
+			min_diff = diff;
+			*scale = tscale;
+			*step = tstep;
+		}
+	}
+}
+
 static void ar933x_uart_set_termios(struct uart_port *port,
 				    struct ktermios *new,
 				    struct ktermios *old)
@@ -169,7 +230,7 @@ static void ar933x_uart_set_termios(struct uart_port *port,
 	struct ar933x_uart_port *up = (struct ar933x_uart_port *) port;
 	unsigned int cs;
 	unsigned long flags;
-	unsigned int baud, scale;
+	unsigned int baud, scale, step;
 
 	/* Only CS8 is supported */
 	new->c_cflag &= ~CSIZE;
@@ -191,8 +252,8 @@ static void ar933x_uart_set_termios(struct uart_port *port,
 	/* Mark/space parity is not supported */
 	new->c_cflag &= ~CMSPAR;
 
-	baud = uart_get_baud_rate(port, new, old, 0, port->uartclk / 16);
+	baud = uart_get_baud_rate(port, new, old, up->min_baud, up->max_baud);
-	scale = (port->uartclk / (16 * baud)) - 1;
+	ar933x_uart_get_scale_step(port->uartclk, baud, &scale, &step);
 
 	/*
 	 * Ok, we're now changing the port state. Do it with
@@ -200,6 +261,10 @@ static void ar933x_uart_set_termios(struct uart_port *port,
 	 */
 	spin_lock_irqsave(&up->port.lock, flags);
 
+	/* disable the UART */
+	ar933x_uart_rmw_clear(up, AR933X_UART_CS_REG,
+		      AR933X_UART_CS_IF_MODE_M << AR933X_UART_CS_IF_MODE_S);
+
 	/* Update the per-port timeout. */
 	uart_update_timeout(port, new->c_cflag, baud);
 
@@ -210,7 +275,7 @@ static void ar933x_uart_set_termios(struct uart_port *port,
 		up->port.ignore_status_mask |= AR933X_DUMMY_STATUS_RD;
 
 	ar933x_uart_write(up, AR933X_UART_CLOCK_REG,
-			  scale << AR933X_UART_CLOCK_SCALE_S | 8192);
+			  scale << AR933X_UART_CLOCK_SCALE_S | step);
 
 	/* setup configuration register */
 	ar933x_uart_rmw(up, AR933X_UART_CS_REG, AR933X_UART_CS_PARITY_M, cs);
@@ -219,6 +284,11 @@ static void ar933x_uart_set_termios(struct uart_port *port,
 	ar933x_uart_rmw_set(up, AR933X_UART_CS_REG,
 			    AR933X_UART_CS_HOST_INT_EN);
 
+	/* reenable the UART */
+	ar933x_uart_rmw(up, AR933X_UART_CS_REG,
+			AR933X_UART_CS_IF_MODE_M << AR933X_UART_CS_IF_MODE_S,
+			AR933X_UART_CS_IF_MODE_DCE << AR933X_UART_CS_IF_MODE_S);
+
 	spin_unlock_irqrestore(&up->port.lock, flags);
 
 	if (tty_termios_baud_rate(new))
@@ -401,6 +471,8 @@ static void ar933x_uart_config_port(struct uart_port *port, int flags)
 static int ar933x_uart_verify_port(struct uart_port *port,
 				   struct serial_struct *ser)
 {
+	struct ar933x_uart_port *up = (struct ar933x_uart_port *) port;
+
 	if (ser->type != PORT_UNKNOWN &&
 	    ser->type != PORT_AR933X)
 		return -EINVAL;
@@ -408,7 +480,8 @@ static int ar933x_uart_verify_port(struct uart_port *port,
 	if (ser->irq < 0 || ser->irq >= NR_IRQS)
 		return -EINVAL;
 
-	if (ser->baud_base < 28800)
+	if (ser->baud_base < up->min_baud ||
+	    ser->baud_base > up->max_baud)
 		return -EINVAL;
 
 	return 0;
@@ -561,6 +634,7 @@ static int __devinit ar933x_uart_probe(struct platform_device *pdev)
 	struct uart_port *port;
 	struct resource *mem_res;
 	struct resource *irq_res;
+	unsigned int baud;
 	int id;
 	int ret;
 
@@ -611,6 +685,12 @@ static int __devinit ar933x_uart_probe(struct platform_device *pdev)
 	port->fifosize = AR933X_UART_FIFO_SIZE;
 	port->ops = &ar933x_uart_ops;
 
+	baud = ar933x_uart_get_baud(port->uartclk, AR933X_UART_MAX_SCALE, 1);
+	up->min_baud = max_t(unsigned int, baud, AR933X_UART_MIN_BAUD);
+
+	baud = ar933x_uart_get_baud(port->uartclk, 0, AR933X_UART_MAX_STEP);
+	up->max_baud = min_t(unsigned int, baud, AR933X_UART_MAX_BAUD);
+
 	ar933x_uart_add_console_port(up);
 
 	ret = uart_add_one_port(&ar933x_uart_driver, &up->port);