[serial] Check for UART existence in uart_select()

Check for existence of the UART in uart_select(), not just in
uart_init().  This allows uart_select() to refuse to set a non-working
address in uart->base, which in turns means that the serial console
code will not attempt to use a non-existent UART.

Reported-by: Torgeir Wulfsberg <Torgeir.Wulfsberg@kongsberg.com>
Reported-by: Ján ONDREJ (SAL) <ondrejj@salstar.sk>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

1 files changed, 25 insertions, 7 deletions

diff --git a/src/core/uart.c b/src/core/uart.c
index 77721484..b85fe076 100644
--- a/src/core/uart.c
+++ b/src/core/uart.c
@@ -80,20 +80,18 @@ void uart_flush ( struct uart *uart ) {
 }
 
 /**
- * Initialise UART
+ * Check for existence of UART
  *
  * @v uart		UART
- * @v baud		Baud rate, or zero to leave unchanged
- * @v lcr		Line control register value, or zero to leave unchanged
  * @ret rc		Return status code
  */
-int uart_init ( struct uart *uart, unsigned int baud, uint8_t lcr ) {
-	uint8_t dlm;
-	uint8_t dll;
+int uart_exists ( struct uart *uart ) {
 
-	/* Check for existence of UART */
+	/* Fail if no UART port is defined */
 	if ( ! uart->base )
 		return -ENODEV;
+
+	/* Fail if UART scratch register seems not to be present */
 	uart_write ( uart, UART_SCR, 0x18 );
 	if ( uart_read ( uart, UART_SCR ) != 0x18 )
 		return -ENODEV;
@@ -101,6 +99,26 @@ int uart_init ( struct uart *uart, unsigned int baud, uint8_t lcr ) {
 	if ( uart_read ( uart, UART_SCR ) != 0xae )
 		return -ENODEV;
 
+	return 0;
+}
+
+/**
+ * Initialise UART
+ *
+ * @v uart		UART
+ * @v baud		Baud rate, or zero to leave unchanged
+ * @v lcr		Line control register value, or zero to leave unchanged
+ * @ret rc		Return status code
+ */
+int uart_init ( struct uart *uart, unsigned int baud, uint8_t lcr ) {
+	uint8_t dlm;
+	uint8_t dll;
+	int rc;
+
+	/* Check for existence of UART */
+	if ( ( rc = uart_exists ( uart ) ) != 0 )
+		return rc;
+
 	/* Configure divisor and line control register, if applicable */
 	if ( ! lcr )
 		lcr = uart_read ( uart, UART_LCR );