tty: serial: atmel: Only divide Clock Divisor if the IP is USART

Make sure that the driver only divides the clock divisor if the
IP handled at that point is USART, since UART IP's do not support
implicit peripheral clock division. Instead, in the case of UART,
go with the highest possible clock divisor.

Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220922113347.144383-8-sergiu.moga@microchip.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/atmel_serial.c

@@ -150,6 +150,7 @@ struct atmel_uart_port {
 	u32			rts_low;
 	bool			ms_irq_enabled;
 	u32			rtor;	/* address of receiver timeout register if it exists */
+	bool			is_usart;
 	bool			has_frac_baudrate;
 	bool			has_hw_timer;
 	struct timer_list	uart_timer;
@@ -1825,6 +1826,7 @@ static void atmel_get_ip_name(struct uart_port *port)
 	 */
 	atmel_port->has_frac_baudrate = false;
 	atmel_port->has_hw_timer = false;
+	atmel_port->is_usart = false;
 
 	if (name == new_uart) {
 		dev_dbg(port->dev, "Uart with hw timer");
@@ -1834,6 +1836,7 @@ static void atmel_get_ip_name(struct uart_port *port)
 		dev_dbg(port->dev, "Usart\n");
 		atmel_port->has_frac_baudrate = true;
 		atmel_port->has_hw_timer = true;
+		atmel_port->is_usart = true;
 		atmel_port->rtor = ATMEL_US_RTOR;
 		version = atmel_uart_readl(port, ATMEL_US_VERSION);
 		switch (version) {
@@ -1863,6 +1866,7 @@ static void atmel_get_ip_name(struct uart_port *port)
 			dev_dbg(port->dev, "This version is usart\n");
 			atmel_port->has_frac_baudrate = true;
 			atmel_port->has_hw_timer = true;
+			atmel_port->is_usart = true;
 			atmel_port->rtor = ATMEL_US_RTOR;
 			break;
 		case 0x203:
@@ -2286,10 +2290,21 @@ static void atmel_set_termios(struct uart_port *port,
 		cd = uart_get_divisor(port, baud);
 	}
 
-	if (cd > 65535) {	/* BRGR is 16-bit, so switch to slower clock */
+	/*
+	 * If the current value of the Clock Divisor surpasses the 16 bit
+	 * ATMEL_US_CD mask and the IP is USART, switch to the Peripheral
+	 * Clock implicitly divided by 8.
+	 * If the IP is UART however, keep the highest possible value for
+	 * the CD and avoid needless division of CD, since UART IP's do not
+	 * support implicit division of the Peripheral Clock.
+	 */
+	if (atmel_port->is_usart && cd > ATMEL_US_CD) {
 		cd /= 8;
 		mode |= ATMEL_US_USCLKS_MCK_DIV8;
+	} else {
+		cd = min_t(unsigned int, cd, ATMEL_US_CD);
 	}
+
 	quot = cd | fp << ATMEL_US_FP_OFFSET;
 
 	if (!(port->iso7816.flags & SER_ISO7816_ENABLED))