serial: stm32: add KGDB support

Add support for KGDB in stm32 serial driver by implementing characters
polling callbacks (poll_init, poll_get_char and poll_put_char).
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Signed-off-by: Jean Philippe Romain <jean-philippe.romain@foss.st.com>
Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Link: https://lore.kernel.org/r/20220419085330.1178925-3-valentin.caron@foss.st.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/stm32-usart.c

@@ -37,6 +37,7 @@
 
 static void stm32_usart_stop_tx(struct uart_port *port);
 static void stm32_usart_transmit_chars(struct uart_port *port);
+static void __maybe_unused stm32_usart_console_putchar(struct uart_port *port, unsigned char ch);
 
 static inline struct stm32_port *to_stm32_port(struct uart_port *port)
 {
@@ -1217,6 +1218,33 @@ static void stm32_usart_pm(struct uart_port *port, unsigned int state,
 	}
 }
 
+#if defined(CONFIG_CONSOLE_POLL)
+
+ /* Callbacks for characters polling in debug context (i.e. KGDB). */
+static int stm32_usart_poll_init(struct uart_port *port)
+{
+	struct stm32_port *stm32_port = to_stm32_port(port);
+
+	return clk_prepare_enable(stm32_port->clk);
+}
+
+static int stm32_usart_poll_get_char(struct uart_port *port)
+{
+	struct stm32_port *stm32_port = to_stm32_port(port);
+	const struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
+
+	if (!(readl_relaxed(port->membase + ofs->isr) & USART_SR_RXNE))
+		return NO_POLL_CHAR;
+
+	return readl_relaxed(port->membase + ofs->rdr) & stm32_port->rdr_mask;
+}
+
+static void stm32_usart_poll_put_char(struct uart_port *port, unsigned char ch)
+{
+	stm32_usart_console_putchar(port, ch);
+}
+#endif /* CONFIG_CONSOLE_POLL */
+
 static const struct uart_ops stm32_uart_ops = {
 	.tx_empty	= stm32_usart_tx_empty,
 	.set_mctrl	= stm32_usart_set_mctrl,
@@ -1238,6 +1266,11 @@ static const struct uart_ops stm32_uart_ops = {
 	.request_port	= stm32_usart_request_port,
 	.config_port	= stm32_usart_config_port,
 	.verify_port	= stm32_usart_verify_port,
+#if defined(CONFIG_CONSOLE_POLL)
+	.poll_init      = stm32_usart_poll_init,
+	.poll_get_char	= stm32_usart_poll_get_char,
+	.poll_put_char	= stm32_usart_poll_put_char,
+#endif /* CONFIG_CONSOLE_POLL */
 };
 
 /*
@@ -1635,8 +1668,7 @@ static int stm32_usart_serial_remove(struct platform_device *pdev)
 	return 0;
 }
 
-#ifdef CONFIG_SERIAL_STM32_CONSOLE
-static void stm32_usart_console_putchar(struct uart_port *port, unsigned char ch)
+static void __maybe_unused stm32_usart_console_putchar(struct uart_port *port, unsigned char ch)
 {
 	struct stm32_port *stm32_port = to_stm32_port(port);
 	const struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
@@ -1653,6 +1685,7 @@ static void stm32_usart_console_putchar(struct uart_port *port, unsigned char ch
 	writel_relaxed(ch, port->membase + ofs->tdr);
 }
 
+#ifdef CONFIG_SERIAL_STM32_CONSOLE
 static void stm32_usart_console_write(struct console *co, const char *s,
 				      unsigned int cnt)
 {