periph/uart: Fix doc, use errno for return values

Some terminology issues are fixed. (The speed of an UART interface is
the symbol rate. The unit it is measured in is baud, which is symbols
per second. There is no such thing as baudrate or even baud/s.)

The return codes are changed to use negative errno constants on error
to match the driver design rules. For backward compatibility, the
enum was updated to match the error codes. Unless an application
depends on the exact numerical value (which would be insane), this is
not breaking any code.

drivers/include/periph/uart.h

@@ -21,7 +21,7 @@
  * data, as well as writing data to the UART port, which means transmitting
  * data. The UART device and the corresponding pins need to be mapped in
  * `RIOT/boards/ * /include/periph_conf.h`. Furthermore, you need to select the
- * baudrate for initialization which is typically {9600, 19200, 38400, 57600,
+ * symbol rate for initialization which is typically {9600, 19200, 38400, 57600,
  * 115200} baud. Additionally, you should register a callback function that is
  * executed in interrupt context when data is being received. The driver will
  * then read the received data byte, call the registered callback function and
@@ -58,9 +58,10 @@
 #ifndef PERIPH_UART_H
 #define PERIPH_UART_H
 
+#include <errno.h>
+#include <limits.h>
 #include <stddef.h>
 #include <stdint.h>
-#include <limits.h>
 
 #include "periph_cpu.h"
 #include "periph_conf.h"
@@ -109,14 +110,16 @@ typedef struct {
 #endif
 
 /**
- * @brief   Possible UART return values
+ * @brief       Possible UART return values
+ *
+ * @deprecated  Use the errno constants directly instead.
  */
 enum {
-    UART_OK         =  0,   /**< everything in order */
-    UART_NODEV      = -1,   /**< invalid UART device given */
-    UART_NOBAUD     = -2,   /**< given baudrate is not applicable */
-    UART_INTERR     = -3,   /**< all other internal errors */
-    UART_NOMODE     = -4    /**< given mode is not applicable */
+    UART_OK         =  0,           /**< everything in order */
+    UART_NODEV      = -ENODEV,      /**< invalid UART device given */
+    UART_NOBAUD     = -ENOTSUP,     /**< given symbol rate is not applicable */
+    UART_NOMODE     = -ENOTSUP,     /**< given mode is not applicable */
+    UART_INTERR     = -EIO,         /**< all other internal errors */
 };
 
 /**
@@ -161,24 +164,24 @@ typedef enum {
  * - 8 data bits
  * - no parity
  * - 1 stop bit
- * - baudrate as given
+ * - symbol rate as given
  *
  * If no callback parameter is given (rx_cb := NULL), the UART will be
  * initialized in TX only mode.
  *
  * @param[in] uart          UART device to initialize
- * @param[in] baudrate      desired baudrate in baud/s
+ * @param[in] baud          desired symbol rate in baud
  * @param[in] rx_cb         receive callback, executed in interrupt context once
  *                          for every byte that is received (RX buffer filled),
  *                          set to NULL for TX only mode
  * @param[in] arg           optional context passed to the callback functions
  *
- * @return                  UART_OK on success
- * @return                  UART_NODEV on invalid UART device
- * @return                  UART_NOBAUD on inapplicable baudrate
- * @return                  UART_INTERR on other errors
+ * @retval  0               Success
+ * @retval  -ENODEV         Invalid UART device
+ * @retval  -ENOTSUP        Unsupported symbol rate
+ * @retval  <0              On other errors
  */
-int uart_init(uart_t uart, uint32_t baudrate, uart_rx_cb_t rx_cb, void *arg);
+int uart_init(uart_t uart, uint32_t baud, uart_rx_cb_t rx_cb, void *arg);
 
 #if defined(MODULE_PERIPH_UART_RECONFIGURE) || DOXYGEN
 /**
@@ -251,8 +254,9 @@ gpio_t uart_pin_tx(uart_t uart);
  * @param[in] parity        parity mode
  * @param[in] stop_bits     number of stop bits in a UART frame
  *
- * @return                  UART_OK on success
- * @return                  UART_NOMODE on other errors
+ * @retval  0               Success
+ * @retval  -ENOTSUP        Given configuration not supported
+ * @retval  <0              Other error
  */
 int uart_mode(uart_t uart, uart_data_bits_t data_bits, uart_parity_t parity,
               uart_stop_bits_t stop_bits);