1 files changed, 50 insertions, 21 deletions

diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
index 794b77512740..19a45e1b7102 100644
--- a/drivers/tty/serial/stm32-usart.c
+++ b/drivers/tty/serial/stm32-usart.c
@@ -9,6 +9,7 @@
  * Inspired by st-asc.c from STMicroelectronics (c)
  */
 
+#include <linux/bitfield.h>
 #include <linux/clk.h>
 #include <linux/console.h>
 #include <linux/delay.h>
@@ -50,6 +51,7 @@ static struct stm32_usart_info __maybe_unused stm32f4_info = {
 		.rtor	= UNDEF_REG,
 		.rqr	= UNDEF_REG,
 		.icr	= UNDEF_REG,
+		.presc	= UNDEF_REG,
 	},
 	.cfg = {
 		.uart_enable_bit = 13,
@@ -71,6 +73,7 @@ static struct stm32_usart_info __maybe_unused stm32f7_info = {
 		.icr	= 0x20,
 		.rdr	= 0x24,
 		.tdr	= 0x28,
+		.presc	= UNDEF_REG,
 	},
 	.cfg = {
 		.uart_enable_bit = 0,
@@ -93,6 +96,7 @@ static struct stm32_usart_info __maybe_unused stm32h7_info = {
 		.icr	= 0x20,
 		.rdr	= 0x24,
 		.tdr	= 0x28,
+		.presc	= 0x2c,
 	},
 	.cfg = {
 		.uart_enable_bit = 0,
@@ -1145,6 +1149,8 @@ static void stm32_usart_shutdown(struct uart_port *port)
 	free_irq(port->irq, port);
 }
 
+static const unsigned int stm32_usart_presc_val[] = {1, 2, 4, 6, 8, 10, 12, 16, 32, 64, 128, 256};
+
 static void stm32_usart_set_termios(struct uart_port *port,
 				    struct ktermios *termios,
 				    const struct ktermios *old)
@@ -1153,17 +1159,19 @@ static void stm32_usart_set_termios(struct uart_port *port,
 	const struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
 	const struct stm32_usart_config *cfg = &stm32_port->info->cfg;
 	struct serial_rs485 *rs485conf = &port->rs485;
-	unsigned int baud, bits;
+	unsigned int baud, bits, uart_clk, uart_clk_pres;
 	u32 usartdiv, mantissa, fraction, oversampling;
 	tcflag_t cflag = termios->c_cflag;
-	u32 cr1, cr2, cr3, isr;
+	u32 cr1, cr2, cr3, isr, brr, presc;
 	unsigned long flags;
 	int ret;
 
 	if (!stm32_port->hw_flow_control)
 		cflag &= ~CRTSCTS;
 
-	baud = uart_get_baud_rate(port, termios, old, 0, port->uartclk / 8);
+	uart_clk = clk_get_rate(stm32_port->clk);
+
+	baud = uart_get_baud_rate(port, termios, old, 0, uart_clk / 8);
 
 	uart_port_lock_irqsave(port, &flags);
 
@@ -1265,27 +1273,48 @@ static void stm32_usart_set_termios(struct uart_port *port,
 		cr3 |= USART_CR3_CTSE | USART_CR3_RTSE;
 	}
 
-	usartdiv = DIV_ROUND_CLOSEST(port->uartclk, baud);
+	for (presc = 0; presc <= USART_PRESC_MAX; presc++) {
+		uart_clk_pres = DIV_ROUND_CLOSEST(uart_clk, stm32_usart_presc_val[presc]);
+		usartdiv = DIV_ROUND_CLOSEST(uart_clk_pres, baud);
 
-	/*
-	 * The USART supports 16 or 8 times oversampling.
-	 * By default we prefer 16 times oversampling, so that the receiver
-	 * has a better tolerance to clock deviations.
-	 * 8 times oversampling is only used to achieve higher speeds.
-	 */
-	if (usartdiv < 16) {
-		oversampling = 8;
-		cr1 |= USART_CR1_OVER8;
-		stm32_usart_set_bits(port, ofs->cr1, USART_CR1_OVER8);
-	} else {
-		oversampling = 16;
-		cr1 &= ~USART_CR1_OVER8;
-		stm32_usart_clr_bits(port, ofs->cr1, USART_CR1_OVER8);
+		/*
+		 * The USART supports 16 or 8 times oversampling.
+		 * By default we prefer 16 times oversampling, so that the receiver
+		 * has a better tolerance to clock deviations.
+		 * 8 times oversampling is only used to achieve higher speeds.
+		 */
+		if (usartdiv < 16) {
+			oversampling = 8;
+			cr1 |= USART_CR1_OVER8;
+			stm32_usart_set_bits(port, ofs->cr1, USART_CR1_OVER8);
+		} else {
+			oversampling = 16;
+			cr1 &= ~USART_CR1_OVER8;
+			stm32_usart_clr_bits(port, ofs->cr1, USART_CR1_OVER8);
+		}
+
+		mantissa = (usartdiv / oversampling) << USART_BRR_DIV_M_SHIFT;
+		fraction = usartdiv % oversampling;
+		brr = mantissa | fraction;
+
+		if (FIELD_FIT(USART_BRR_MASK, brr)) {
+			if (ofs->presc != UNDEF_REG) {
+				port->uartclk = uart_clk_pres;
+				writel_relaxed(presc, port->membase + ofs->presc);
+			} else if (presc) {
+				/* We need a prescaler but we don't have it (STM32F4, STM32F7) */
+				dev_err(port->dev,
+					"unable to set baudrate, input clock is too high");
+			}
+			break;
+		} else if (presc == USART_PRESC_MAX) {
+			/* Even with prescaler and brr at max value we can't set baudrate */
+			dev_err(port->dev, "unable to set baudrate, input clock is too high");
+			break;
+		}
 	}
 
-	mantissa = (usartdiv / oversampling) << USART_BRR_DIV_M_SHIFT;
-	fraction = usartdiv % oversampling;
-	writel_relaxed(mantissa | fraction, port->membase + ofs->brr);
+	writel_relaxed(brr, port->membase + ofs->brr);
 
 	uart_update_timeout(port, cflag, baud);