serial: stm32x7: simplify baud rate register calculation

Simplify baud rate register formula and use the oversampling
uart feature.
This code is aligned with what is implemented in kernel driver
drivers/tty/serial/stm32-usart.c since kernel v4.9.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Christophe KERELLO <christophe.kerello@st.com>
Reviewed-by: Patrick DELAUNAY <patrick.delaunay@st.com>
Acked-by: Vikas MANOCHA <vikas.manocha@st.com>

drivers/serial/serial_stm32x7.c

@@ -20,7 +20,7 @@ static int stm32_serial_setbrg(struct udevice *dev, int baudrate)
 {
 	struct stm32x7_serial_platdata *plat = dev->platdata;
 	struct stm32_usart *const usart = plat->base;
-	u32  clock, int_div, frac_div, tmp;
+	u32  clock, int_div, mantissa, fraction, oversampling;
 
 	if (((u32)usart & STM32_BUS_MASK) == APB1_PERIPH_BASE)
 		clock = clock_get(CLOCK_APB1);
@@ -29,11 +29,20 @@ static int stm32_serial_setbrg(struct udevice *dev, int baudrate)
 	else
 		return -EINVAL;
 
-	int_div = (25 * clock) / (4 * baudrate);
-	tmp = ((int_div / 100) << USART_BRR_M_SHIFT) & USART_BRR_M_MASK;
-	frac_div = int_div - (100 * (tmp >> USART_BRR_M_SHIFT));
-	tmp |= (((frac_div * 16) + 50) / 100) & USART_BRR_F_MASK;
-	writel(tmp, &usart->brr);
+	int_div = DIV_ROUND_CLOSEST(clock, baudrate);
+
+	if (int_div < 16) {
+		oversampling = 8;
+		setbits_le32(&usart->cr1, USART_CR1_OVER8);
+	} else {
+		oversampling = 16;
+		clrbits_le32(&usart->cr1, USART_CR1_OVER8);
+	}
+
+	mantissa = (int_div / oversampling) << USART_BRR_M_SHIFT;
+	fraction = int_div % oversampling;
+
+	writel(mantissa | fraction, &usart->brr);
 
 	return 0;
 }

drivers/serial/serial_stm32x7.h

@@ -23,8 +23,9 @@ struct stm32_usart {
 };
 
 
-#define USART_CR1_RE			(1 << 2)
+#define USART_CR1_OVER8			(1 << 15)
 #define USART_CR1_TE			(1 << 3)
+#define USART_CR1_RE			(1 << 2)
 #define USART_CR1_UE			(1 << 0)
 
 #define USART_CR3_OVRDIS		(1 << 12)