serial: stm32: fix TX and RX FIFO thresholds

TX and RX FIFO thresholds may be cleared after suspend/resume, depending
on the low power mode.

Those configurations (done in startup) are not effective for UART console,
as:
- the reference manual indicates that FIFOEN bit can only be written when
  the USART is disabled (UE=0)
- a set_termios (where UE is set) is requested firstly for console
  enabling, before the startup.

Fixes: 84872dc4 ("serial: stm32: add RX and TX FIFO flush")
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Link: https://lore.kernel.org/r/20210304162308.8984-5-erwan.leray@foss.st.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/stm32-usart.c

@@ -649,19 +649,8 @@ static int stm32_usart_startup(struct uart_port *port)
 	if (ofs->rqr != UNDEF_REG)
 		stm32_usart_set_bits(port, ofs->rqr, USART_RQR_RXFRQ);
 
-	/* Tx and RX FIFO configuration */
-	if (stm32_port->fifoen) {
-		val = readl_relaxed(port->membase + ofs->cr3);
-		val &= ~(USART_CR3_TXFTCFG_MASK | USART_CR3_RXFTCFG_MASK);
-		val |= USART_CR3_TXFTCFG_HALF << USART_CR3_TXFTCFG_SHIFT;
-		val |= USART_CR3_RXFTCFG_HALF << USART_CR3_RXFTCFG_SHIFT;
-		writel_relaxed(val, port->membase + ofs->cr3);
-	}
-
-	/* RX FIFO enabling */
+	/* RX enabling */
 	val = stm32_port->cr1_irq | USART_CR1_RE | BIT(cfg->uart_enable_bit);
-	if (stm32_port->fifoen)
-		val |= USART_CR1_FIFOEN;
 	stm32_usart_set_bits(port, ofs->cr1, val);
 
 	return 0;
@@ -770,9 +759,15 @@ static void stm32_usart_set_termios(struct uart_port *port,
 	if (stm32_port->fifoen)
 		cr1 |= USART_CR1_FIFOEN;
 	cr2 = 0;
+
+	/* Tx and RX FIFO configuration */
 	cr3 = readl_relaxed(port->membase + ofs->cr3);
-	cr3 &= USART_CR3_TXFTIE | USART_CR3_RXFTCFG_MASK | USART_CR3_RXFTIE
-		| USART_CR3_TXFTCFG_MASK;
+	cr3 &= USART_CR3_TXFTIE | USART_CR3_RXFTIE;
+	if (stm32_port->fifoen) {
+		cr3 &= ~(USART_CR3_TXFTCFG_MASK | USART_CR3_RXFTCFG_MASK);
+		cr3 |= USART_CR3_TXFTCFG_HALF << USART_CR3_TXFTCFG_SHIFT;
+		cr3 |= USART_CR3_RXFTCFG_HALF << USART_CR3_RXFTCFG_SHIFT;
+	}
 
 	if (cflag & CSTOPB)
 		cr2 |= USART_CR2_STOP_2B;