serial: stm32: implement prescaler tuning to compute low baudrate

In the case of high USART input clock and low baud rate, BRR value
is not enough to get correct baud rate. So here we use USART prescaler to
divide USART input clock to get the correct baud rate.

PRESC register is only available since stm32h7.

Signed-off-by: Valentin Caron <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/stm32-usart.c

···
  * Inspired by st-asc.c from STMicroelectronics (c)
  */
 
+#include <linux/bitfield.h>
 #include <linux/clk.h>
 #include <linux/console.h>
 #include <linux/delay.h>
···
 		.rtor	= UNDEF_REG,
 		.rqr	= UNDEF_REG,
 		.icr	= UNDEF_REG,
+		.presc	= UNDEF_REG,
 	},
 	.cfg = {
 		.uart_enable_bit = 13,
···
 		.icr	= 0x20,
 		.rdr	= 0x24,
 		.tdr	= 0x28,
+		.presc	= UNDEF_REG,
 	},
 	.cfg = {
 		.uart_enable_bit = 0,
···
 		.icr	= 0x20,
 		.rdr	= 0x24,
 		.tdr	= 0x28,
+		.presc	= 0x2c,
 	},
 	.cfg = {
 		.uart_enable_bit = 0,
···
 	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)
···
 	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);
 
···
 		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);
 

drivers/tty/serial/stm32-usart.h

···
 	u8 icr;
 	u8 rdr;
 	u8 tdr;
+	u8 presc;
 };
 
 struct stm32_usart_config {
···
 #define USART_BRR_DIV_M_MASK	GENMASK(15, 4)
 #define USART_BRR_DIV_M_SHIFT	4
 #define USART_BRR_04_R_SHIFT	1
+#define USART_BRR_MASK		(USART_BRR_DIV_M_MASK | USART_BRR_DIV_F_MASK)
 
 /* USART_CR1 */
 #define USART_CR1_SBK		BIT(0)
···
 #define USART_ICR_EOBCF		BIT(12)		/* F7 */
 #define USART_ICR_CMCF		BIT(17)		/* F7 */
 #define USART_ICR_WUCF		BIT(20)		/* H7 */
+
+/* USART_PRESC */
+#define USART_PRESC		GENMASK(3, 0)	/* H7 */
+#define USART_PRESC_MAX		0b1011
 
 #define STM32_SERIAL_NAME "ttySTM"
 #define STM32_MAX_PORTS 8