sys/stdio_uart: add stdio_uart_onlcr (pseudo-) module

Add USE_MODULE += "stdio_uart_onlcr" to enable it.
This is named after the "onlcr" stty flag, which does the same thing.

Co-authored-by: Marian Buschsieweke <marian.buschsieweke@ovgu.de>

makefiles/pseudomodules.inc.mk

@@ -371,8 +371,15 @@ PSEUDOMODULES += stdio_available
 PSEUDOMODULES += stdio_cdc_acm
 PSEUDOMODULES += stdio_ethos
 PSEUDOMODULES += stdio_nimble_debug
-PSEUDOMODULES += stdio_uart_rx
 PSEUDOMODULES += stdio_telnet
+## @defgroup sys_stdio_uart_onlcr   Support for DOS line endings in STDIO-UART
+## @ingroup sys_stdio_uart
+## @{
+## Enable this (pseudo-) module to emit DOS style line endings (`\r\n`) instead
+## of UNIX style line endings (`\n`) via STDIO over UART.
+PSEUDOMODULES += stdio_uart_onlcr
+## @}
+PSEUDOMODULES += stdio_uart_rx
 PSEUDOMODULES += stm32_eth
 PSEUDOMODULES += stm32_eth_auto
 PSEUDOMODULES += stm32_eth_link_up

sys/Kconfig.stdio

@@ -34,6 +34,10 @@ config MODULE_STDIO_UART
     depends on HAS_PERIPH_UART
     select MODULE_PERIPH_UART
 
+config MODULE_STDIO_UART_ONLCR
+    bool "Emit DOS line endings for STDIO output via UART"
+    depends on MODULE_STDIO_UART
+
 config MODULE_STDIO_NATIVE
     bool "Native"
     depends on CPU_ARCH_NATIVE

sys/include/stdio_uart.h

@@ -13,12 +13,37 @@
  *
  * @brief       Standard input/output backend using UART
  *
+ * ## Input
+ *
  * @warning Standard input is disabled by default on UART. To enable it, load
  *          the `stdin` module in your application:
  * ```
  * USEMODULE += stdin
  * ```
  *
+ * ## UART Configuration
+ *
+ * @note    Running `make BOARD=<your_board> term` will launch `pyterm` with the
+ *          correct parameters, so mostly you do not really need to care about
+ *          the UART configuration.
+ *
+ * By convention RIOT boards used 8N1 encoding with a symbol rate of 115200 Bd
+ * for the UART used as stdio. However, some boards may have a different
+ * configuration due to hardware limitations. Most notably, many AVR boards use
+ * 9600 Bd as symbol rate instead, as they otherwise frequently loose an input
+ * character due to losing interrupts.
+ *
+ * By default UNIX style line endings (`\n`) are used. However, some terminal
+ * programs default to DOS style line endings (`\r\n`). It usually is better to
+ * configure the terminal program on the host to use UNIX style line endings.
+ * In scenarios this is not possible/desired, you can enable the (pseudo-)
+ * module @ref sys_stdio_uart_onlcr to emit DOS style line endings instead.
+ *
+ * RIOT's shell happily accepts both DOS and UNIX style line endings in any
+ * case, so typically no line ending conversion is needed on the input.
+ *
+ * ## STDIO from ISR
+ *
  * @attention   Using STDIO over UART from interrupt context should be avoided,
  *              except for debugging purposes
  *

sys/stdio_uart/stdio_uart.c

@@ -28,6 +28,7 @@
  */
 
 #include <errno.h>
+#include <string.h>
 
 #include "stdio_uart.h"
 
@@ -85,8 +86,29 @@ ssize_t stdio_read(void* buffer, size_t count)
 #endif
 }
 
-ssize_t stdio_write(const void* buffer, size_t len)
+ssize_t stdio_write(const void *buffer, size_t len)
 {
-    uart_write(STDIO_UART_DEV, (const uint8_t *)buffer, len);
-    return len;
+    ssize_t result = len;
+    if (IS_USED(MODULE_STDIO_UART_ONLCR)) {
+        static const uint8_t crlf[2] = { (uint8_t)'\r', (uint8_t)'\n' };
+        const uint8_t *buf = buffer;
+        while (len) {
+            const uint8_t *pos = memchr(buf, '\n', len);
+            size_t chunk_len = (pos != NULL)
+                             ? (uintptr_t)pos - (uintptr_t)buf
+                             : len;
+            uart_write(STDIO_UART_DEV, buf, chunk_len);
+            buf += chunk_len;
+            len -= chunk_len;
+            if (len) {
+                uart_write(STDIO_UART_DEV, crlf, sizeof(crlf));
+                buf++;
+                len--;
+            }
+        }
+    }
+    else {
+        uart_write(STDIO_UART_DEV, (const uint8_t *)buffer, len);
+    }
+    return result;
 }