powerpc/5200: improve uart baud rate calculation (reach high baud rates, better accuracy)

On the MPC5200B, make very high baud rates (e.g. 3 MBaud) accessible and
achieve a higher precision for high baud rates in general.  This is done by
selecting the appropriate prescaler (/4 or /32).  As to keep the code clean,
the getuartclk method has been dropped, and all calculations are done in a
new set_baudrate method.

Notes: only "fsl,mpc5200b-psc-uart" compatible devices benefit from these
improvements.

Tested on a custom 5200B based board, from 110 baud up to 3 MBaud, and with
both "fsl,mpc5200b-psc-uart" and "fsl,mpc5200-psc-uart" devices.
Also tested on the mpc5121ads board.

Signed-off-by: Albrecht Dreß <albrecht.dress@arcor.de>
[agust: fixed mpc5121 prescaler comment]
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

1 files changed, 116 insertions, 29 deletions

diff --git a/drivers/serial/mpc52xx_uart.c b/drivers/serial/mpc52xx_uart.c
index 84a35f699016..1a88b363005c 100644
--- a/drivers/serial/mpc52xx_uart.c
+++ b/drivers/serial/mpc52xx_uart.c
@@ -113,7 +113,9 @@ struct psc_ops {
         unsigned char   (*read_char)(struct uart_port *port);
         void            (*cw_disable_ints)(struct uart_port *port);
         void            (*cw_restore_ints)(struct uart_port *port);
-        unsigned long   (*getuartclk)(void *p);
+        unsigned int    (*set_baudrate)(struct uart_port *port,
+                                        struct ktermios *new,
+                                        struct ktermios *old);
         int             (*clock)(struct uart_port *port, int enable);
         int             (*fifoc_init)(void);
         void            (*fifoc_uninit)(void);
@@ -121,6 +123,16 @@ struct psc_ops {
         irqreturn_t     (*handle_irq)(struct uart_port *port);
 };
 
+/* setting the prescaler and divisor reg is common for all chips */
+static inline void mpc52xx_set_divisor(struct mpc52xx_psc __iomem *psc,
+                                       u16 prescaler, unsigned int divisor)
+{
+        /* select prescaler */
+        out_be16(&psc->mpc52xx_psc_clock_select, prescaler);
+        out_8(&psc->ctur, divisor >> 8);
+        out_8(&psc->ctlr, divisor & 0xff);
+}
+
 #ifdef CONFIG_PPC_MPC52xx
 #define FIFO_52xx(port) ((struct mpc52xx_psc_fifo __iomem *)(PSC(port)+1))
 static void mpc52xx_psc_fifo_init(struct uart_port *port)
@@ -128,9 +140,6 @@ static void mpc52xx_psc_fifo_init(struct uart_port *port)
         struct mpc52xx_psc __iomem *psc = PSC(port);
         struct mpc52xx_psc_fifo __iomem *fifo = FIFO_52xx(port);
 
-        /* /32 prescaler */
-        out_be16(&psc->mpc52xx_psc_clock_select, 0xdd00);
-
         out_8(&fifo->rfcntl, 0x00);
         out_be16(&fifo->rfalarm, 0x1ff);
         out_8(&fifo->tfcntl, 0x07);
@@ -219,15 +228,47 @@ static void mpc52xx_psc_cw_restore_ints(struct uart_port *port)
         out_be16(&PSC(port)->mpc52xx_psc_imr, port->read_status_mask);
 }
 
-/* Search for bus-frequency property in this node or a parent */
-static unsigned long mpc52xx_getuartclk(void *p)
+static unsigned int mpc5200_psc_set_baudrate(struct uart_port *port,
+                                             struct ktermios *new,
+                                             struct ktermios *old)
 {
-        /*
-         * 5200 UARTs have a / 32 prescaler
-         * but the generic serial code assumes 16
-         * so return ipb freq / 2
-         */
-        return mpc5xxx_get_bus_frequency(p) / 2;
+        unsigned int baud;
+        unsigned int divisor;
+
+        /* The 5200 has a fixed /32 prescaler, uartclk contains the ipb freq */
+        baud = uart_get_baud_rate(port, new, old,
+                                  port->uartclk / (32 * 0xffff) + 1,
+                                  port->uartclk / 32);
+        divisor = (port->uartclk + 16 * baud) / (32 * baud);
+
+        /* enable the /32 prescaler and set the divisor */
+        mpc52xx_set_divisor(PSC(port), 0xdd00, divisor);
+        return baud;
+}
+
+static unsigned int mpc5200b_psc_set_baudrate(struct uart_port *port,
+                                              struct ktermios *new,
+                                              struct ktermios *old)
+{
+        unsigned int baud;
+        unsigned int divisor;
+        u16 prescaler;
+
+        /* The 5200B has a selectable /4 or /32 prescaler, uartclk contains the
+         * ipb freq */
+        baud = uart_get_baud_rate(port, new, old,
+                                  port->uartclk / (32 * 0xffff) + 1,
+                                  port->uartclk / 4);
+        divisor = (port->uartclk + 2 * baud) / (4 * baud);
+
+        /* select the proper prescaler and set the divisor */
+        if (divisor > 0xffff) {
+                divisor = (divisor + 4) / 8;
+                prescaler = 0xdd00; /* /32 */
+        } else
+                prescaler = 0xff00; /* /4 */
+        mpc52xx_set_divisor(PSC(port), prescaler, divisor);
+        return baud;
 }
 
 static void mpc52xx_psc_get_irq(struct uart_port *port, struct device_node *np)
@@ -258,7 +299,28 @@ static struct psc_ops mpc52xx_psc_ops = {
         .read_char = mpc52xx_psc_read_char,
         .cw_disable_ints = mpc52xx_psc_cw_disable_ints,
         .cw_restore_ints = mpc52xx_psc_cw_restore_ints,
-        .getuartclk = mpc52xx_getuartclk,
+        .set_baudrate = mpc5200_psc_set_baudrate,
+        .get_irq = mpc52xx_psc_get_irq,
+        .handle_irq = mpc52xx_psc_handle_irq,
+};
+
+static struct psc_ops mpc5200b_psc_ops = {
+        .fifo_init = mpc52xx_psc_fifo_init,
+        .raw_rx_rdy = mpc52xx_psc_raw_rx_rdy,
+        .raw_tx_rdy = mpc52xx_psc_raw_tx_rdy,
+        .rx_rdy = mpc52xx_psc_rx_rdy,
+        .tx_rdy = mpc52xx_psc_tx_rdy,
+        .tx_empty = mpc52xx_psc_tx_empty,
+        .stop_rx = mpc52xx_psc_stop_rx,
+        .start_tx = mpc52xx_psc_start_tx,
+        .stop_tx = mpc52xx_psc_stop_tx,
+        .rx_clr_irq = mpc52xx_psc_rx_clr_irq,
+        .tx_clr_irq = mpc52xx_psc_tx_clr_irq,
+        .write_char = mpc52xx_psc_write_char,
+        .read_char = mpc52xx_psc_read_char,
+        .cw_disable_ints = mpc52xx_psc_cw_disable_ints,
+        .cw_restore_ints = mpc52xx_psc_cw_restore_ints,
+        .set_baudrate = mpc5200b_psc_set_baudrate,
         .get_irq = mpc52xx_psc_get_irq,
         .handle_irq = mpc52xx_psc_handle_irq,
 };
@@ -392,9 +454,35 @@ static void mpc512x_psc_cw_restore_ints(struct uart_port *port)
         out_be32(&FIFO_512x(port)->rximr, port->read_status_mask & 0x7f);
 }
 
-static unsigned long mpc512x_getuartclk(void *p)
+static unsigned int mpc512x_psc_set_baudrate(struct uart_port *port,
+                                             struct ktermios *new,
+                                             struct ktermios *old)
 {
-        return mpc5xxx_get_bus_frequency(p);
+        unsigned int baud;
+        unsigned int divisor;
+
+        /*
+         * The "MPC5121e Microcontroller Reference Manual, Rev. 3" says on
+         * pg. 30-10 that the chip supports a /32 and a /10 prescaler.
+         * Furthermore, it states that "After reset, the prescaler by 10
+         * for the UART mode is selected", but the reset register value is
+         * 0x0000 which means a /32 prescaler. This is wrong.
+         *
+         * In reality using /32 prescaler doesn't work, as it is not supported!
+         * Use /16 or /10 prescaler, see "MPC5121e Hardware Design Guide",
+         * Chapter 4.1 PSC in UART Mode.
+         * Calculate with a /16 prescaler here.
+         */
+
+        /* uartclk contains the ips freq */
+        baud = uart_get_baud_rate(port, new, old,
+                                  port->uartclk / (16 * 0xffff) + 1,
+                                  port->uartclk / 16);
+        divisor = (port->uartclk + 8 * baud) / (16 * baud);
+
+        /* enable the /16 prescaler and set the divisor */
+        mpc52xx_set_divisor(PSC(port), 0xdd00, divisor);
+        return baud;
 }
 
 /* Init PSC FIFO Controller */
@@ -498,7 +586,7 @@ static struct psc_ops mpc512x_psc_ops = {
         .read_char = mpc512x_psc_read_char,
         .cw_disable_ints = mpc512x_psc_cw_disable_ints,
         .cw_restore_ints = mpc512x_psc_cw_restore_ints,
-        .getuartclk = mpc512x_getuartclk,
+        .set_baudrate = mpc512x_psc_set_baudrate,
         .clock = mpc512x_psc_clock,
         .fifoc_init = mpc512x_psc_fifoc_init,
         .fifoc_uninit = mpc512x_psc_fifoc_uninit,
@@ -666,8 +754,8 @@ mpc52xx_uart_set_termios(struct uart_port *port, struct ktermios *new,
         struct mpc52xx_psc __iomem *psc = PSC(port);
         unsigned long flags;
         unsigned char mr1, mr2;
-        unsigned short ctr;
-        unsigned int j, baud, quot;
+        unsigned int j;
+        unsigned int baud;
 
         /* Prepare what we're gonna write */
         mr1 = 0;
@@ -704,16 +792,9 @@ mpc52xx_uart_set_termios(struct uart_port *port, struct ktermios *new,
                 mr2 |= MPC52xx_PSC_MODE_TXCTS;
         }
 
-        baud = uart_get_baud_rate(port, new, old, 0, port->uartclk/16);
-        quot = uart_get_divisor(port, baud);
-        ctr = quot & 0xffff;
-
         /* Get the lock */
         spin_lock_irqsave(&port->lock, flags);
 
-        /* Update the per-port timeout */
-        uart_update_timeout(port, new->c_cflag, baud);
-
         /* Do our best to flush TX & RX, so we don't lose anything */
         /* But we don't wait indefinitely ! */
         j = 5000000;    /* Maximum wait */
@@ -737,8 +818,10 @@ mpc52xx_uart_set_termios(struct uart_port *port, struct ktermios *new,
         out_8(&psc->command, MPC52xx_PSC_SEL_MODE_REG_1);
         out_8(&psc->mode, mr1);
         out_8(&psc->mode, mr2);
-        out_8(&psc->ctur, ctr >> 8);
-        out_8(&psc->ctlr, ctr & 0xff);
+        baud = psc_ops->set_baudrate(port, new, old);
+
+        /* Update the per-port timeout */
+        uart_update_timeout(port, new->c_cflag, baud);
 
         if (UART_ENABLE_MS(port, new->c_cflag))
                 mpc52xx_uart_enable_ms(port);
@@ -1118,7 +1201,7 @@ mpc52xx_console_setup(struct console *co, char *options)
                 return ret;
         }
 
-        uartclk = psc_ops->getuartclk(np);
+        uartclk = mpc5xxx_get_bus_frequency(np);
         if (uartclk == 0) {
                 pr_debug("Could not find uart clock frequency!\n");
                 return -EINVAL;
@@ -1201,6 +1284,7 @@ static struct uart_driver mpc52xx_uart_driver = {
 
 static struct of_device_id mpc52xx_uart_of_match[] = {
 #ifdef CONFIG_PPC_MPC52xx
+        { .compatible = "fsl,mpc5200b-psc-uart", .data = &mpc5200b_psc_ops, },
         { .compatible = "fsl,mpc5200-psc-uart", .data = &mpc52xx_psc_ops, },
         /* binding used by old lite5200 device trees: */
         { .compatible = "mpc5200-psc-uart", .data = &mpc52xx_psc_ops, },
@@ -1233,7 +1317,10 @@ mpc52xx_uart_of_probe(struct of_device *op, const struct of_device_id *match)
         pr_debug("Found %s assigned to ttyPSC%x\n",
                  mpc52xx_uart_nodes[idx]->full_name, idx);
 
-        uartclk = psc_ops->getuartclk(op->dev.of_node);
+        /* set the uart clock to the input clock of the psc, the different
+         * prescalers are taken into account in the set_baudrate() methods
+         * of the respective chip */
+        uartclk = mpc5xxx_get_bus_frequency(op->dev.of_node);
         if (uartclk == 0) {
                 dev_dbg(&op->dev, "Could not find uart clock frequency!\n");
                 return -EINVAL;