[PATCH] serial: zynq: disable TX and RX while changing baud rate

[Norbert Braun]

According to the Zynq-7000 TRM (UG585), the UART transmitter and
receiver must be disabled while changing the baud rate. Change
_uart_zynq_serial_setbrg accordingly.
---
 drivers/serial/serial_zynq.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/drivers/serial/serial_zynq.c b/drivers/serial/serial_zynq.c
index 0e71cada1b..4f7bab16fa 100644
--- a/drivers/serial/serial_zynq.c
+++ b/drivers/serial/serial_zynq.c
@@ -23,7 +23,9 @@
 #define ZYNQ_UART_SR_TXFULL	BIT(4) /* TX FIFO full */
 #define ZYNQ_UART_SR_RXEMPTY	BIT(1) /* RX FIFO empty */
 
+#define ZYNQ_UART_CR_TX_DIS	BIT(5) /* TX disable */
 #define ZYNQ_UART_CR_TX_EN	BIT(4) /* TX enabled */
+#define ZYNQ_UART_CR_RX_DIS	BIT(3) /* RX disable */
 #define ZYNQ_UART_CR_RX_EN	BIT(2) /* RX enabled */
 #define ZYNQ_UART_CR_TXRST	BIT(1) /* TX logic reset */
 #define ZYNQ_UART_CR_RXRST	BIT(0) /* RX logic reset */
@@ -82,8 +84,26 @@ static void _uart_zynq_serial_setbrg(struct uart_zynq *regs,
 			break;
 	}
 
-	writel(bdiv, &regs->baud_rate_divider);
-	writel(bgen, &regs->baud_rate_gen);
+	/* Check if baud rate registers actually need to be changed. */
+	if(readl(&regs->baud_rate_divider) != bdiv ||
+	   readl(&regs->baud_rate_gen) != bgen) {
+		/*
+		 * Configure the baud rate.
+		 *
+		 * This follows the procedure from the
+		 * Zynq-7000 SoC Technical Reference Manual,
+		 * UG585 (v1.12.2),
+		 * section 19.3.2, part 2.
+		 */
+
+		writel(ZYNQ_UART_CR_RX_DIS, &regs->control);
+		writel(ZYNQ_UART_CR_TX_DIS, &regs->control);
+		writel(bgen, &regs->baud_rate_gen);
+		writel(bdiv, &regs->baud_rate_divider);
+		writel(ZYNQ_UART_CR_TXRST | ZYNQ_UART_CR_RXRST, &regs->control);
+		writel(ZYNQ_UART_CR_RX_EN, &regs->control);
+		writel(ZYNQ_UART_CR_TX_EN, &regs->control);
+	}
 }
 
 /* Initialize the UART, with...some settings. */
-- 
2.17.1