[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>

src/arch/x86/core/x86_uart.c

  * @ret rc		Return status code
  */
 int uart_select ( struct uart *uart, unsigned int port ) {
-
-	/* Clear UART base */
-	uart->base = NULL;
+	int rc;
 
 	/* Set new UART base */
-	if ( port < ( sizeof ( uart_base ) / sizeof ( uart_base[0] ) ) ) {
-		uart->base = ( ( void * ) ( intptr_t ) uart_base[port] );
-		return 0;
-	} else {
-		return -ENODEV;
+	if ( port >= ( sizeof ( uart_base ) / sizeof ( uart_base[0] ) ) ) {
+		rc = -ENODEV;
+		goto err;
 	}
+	uart->base = ( ( void * ) ( intptr_t ) uart_base[port] );
+
+	/* Check that UART exists */
+	if ( ( rc = uart_exists ( uart ) ) != 0 )
+		goto err;
+
+	return 0;
+
+ err:
+	uart->base = NULL;
+	return rc;
 }

src/core/uart.c

 }
 
 /**
- * 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;
 	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 );

src/include/ipxe/uart.h

 
 extern void uart_transmit ( struct uart *uart, uint8_t data );
 extern void uart_flush ( struct uart *uart );
+extern int uart_exists ( struct uart *uart );
 extern int uart_init ( struct uart *uart, unsigned int baud, uint8_t lcr );
 
 #endif /* _IPXE_UART_H */