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Subject[PATCH] tty/serial: atmel: add fractional baud rate support
Date
The USART device provides a fractional baud rate generator to get a more
accurate baud rate. It can be used only when the USART is configured in
'normal mode' and this feature is not available on AT91RM9200 SoC.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
---
drivers/tty/serial/atmel_serial.c | 41 +++++++++++++++++++++++++++++++--------
include/linux/atmel_serial.h | 1 +
2 files changed, 34 insertions(+), 8 deletions(-)

diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
index 2eaa18d..1759239 100644
--- a/drivers/tty/serial/atmel_serial.c
+++ b/drivers/tty/serial/atmel_serial.c
@@ -166,6 +166,7 @@ struct atmel_uart_port {
u32 rts_low;
bool ms_irq_enabled;
u32 rtor; /* address of receiver timeout register if it exists */
+ bool has_frac_baudrate;
bool has_hw_timer;
struct timer_list uart_timer;

@@ -1745,6 +1746,11 @@ static void atmel_get_ip_name(struct uart_port *port)
dbgu_uart = 0x44424755; /* DBGU */
new_uart = 0x55415254; /* UART */

+ /*
+ * Only USART devices from at91sam9260 SOC implement fractional
+ * baudrate.
+ */
+ atmel_port->has_frac_baudrate = false;
atmel_port->has_hw_timer = false;

if (name == new_uart) {
@@ -1753,6 +1759,7 @@ static void atmel_get_ip_name(struct uart_port *port)
atmel_port->rtor = ATMEL_UA_RTOR;
} else if (name == usart) {
dev_dbg(port->dev, "Usart\n");
+ atmel_port->has_frac_baudrate = true;
atmel_port->has_hw_timer = true;
atmel_port->rtor = ATMEL_US_RTOR;
} else if (name == dbgu_uart) {
@@ -1764,6 +1771,7 @@ static void atmel_get_ip_name(struct uart_port *port)
case 0x302:
case 0x10213:
dev_dbg(port->dev, "This version is usart\n");
+ atmel_port->has_frac_baudrate = true;
atmel_port->has_hw_timer = true;
atmel_port->rtor = ATMEL_US_RTOR;
break;
@@ -2025,8 +2033,9 @@ static void atmel_serial_pm(struct uart_port *port, unsigned int state,
static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,
struct ktermios *old)
{
+ struct atmel_uart_port *atmel_port = to_atmel_uart_port(port);
unsigned long flags;
- unsigned int old_mode, mode, imr, quot, baud;
+ unsigned int old_mode, mode, imr, quot, baud, div, cd, fp = 0;

/* save the current mode register */
mode = old_mode = atmel_uart_readl(port, ATMEL_US_MR);
@@ -2036,12 +2045,6 @@ static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,
ATMEL_US_PAR | ATMEL_US_USMODE);

baud = uart_get_baud_rate(port, termios, old, 0, port->uartclk / 16);
- quot = uart_get_divisor(port, baud);
-
- if (quot > 65535) { /* BRGR is 16-bit, so switch to slower clock */
- quot /= 8;
- mode |= ATMEL_US_USCLKS_MCK_DIV8;
- }

/* byte size */
switch (termios->c_cflag & CSIZE) {
@@ -2160,7 +2163,29 @@ static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,
atmel_uart_writel(port, ATMEL_US_CR, rts_state);
}

- /* set the baud rate */
+ /*
+ * Set the baud rate:
+ * Fractional baudrate allows to setup output frequency more
+ * accurately. This feature is enabled only when using normal mode.
+ * baudrate = selected clock / (8 * (2 - OVER) * (CD + FP / 8))
+ * Currently, OVER is always set to 0 so we get
+ * baudrate = selected clock (16 * (CD + FP / 8))
+ */
+ if (atmel_port->has_frac_baudrate &&
+ (mode & ATMEL_US_USMODE) == ATMEL_US_USMODE_NORMAL) {
+ div = DIV_ROUND_CLOSEST(port->uartclk, baud);
+ cd = div / 16;
+ fp = DIV_ROUND_CLOSEST(div % 16, 2);
+ } else {
+ cd = uart_get_divisor(port, baud);
+ }
+
+ if (cd > 65535) { /* BRGR is 16-bit, so switch to slower clock */
+ cd /= 8;
+ mode |= ATMEL_US_USCLKS_MCK_DIV8;
+ }
+ quot = cd | fp << ATMEL_US_FP_OFFSET;
+
atmel_uart_writel(port, ATMEL_US_BRGR, quot);
atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_RSTSTA | ATMEL_US_RSTRX);
atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_TXEN | ATMEL_US_RXEN);
diff --git a/include/linux/atmel_serial.h b/include/linux/atmel_serial.h
index 5a4d664..f8e452a 100644
--- a/include/linux/atmel_serial.h
+++ b/include/linux/atmel_serial.h
@@ -118,6 +118,7 @@

#define ATMEL_US_BRGR 0x20 /* Baud Rate Generator Register */
#define ATMEL_US_CD GENMASK(15, 0) /* Clock Divider */
+#define ATMEL_US_FP_OFFSET 16 /* Fractional Part */

#define ATMEL_US_RTOR 0x24 /* Receiver Time-out Register for USART */
#define ATMEL_UA_RTOR 0x28 /* Receiver Time-out Register for UART */
--
2.9.0
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