sys/usb/cdc_acm: Change API to report errors

- Change return type of `usbus_cdc_acm_submit` from `size_t` to `ssize_t`
  to be able to report errors
- Change `stdio_cdc_acm` to make use of the new API
- Change `stdio_cdc_acm` to not loop until all data is flushed, but just
  report what could be added to the buffer now and post the flush event
    - The layers above should already loop until all data is written
    - The layers above could opt to not loop when in IRQ context. This
      would allow using `stdio_usb_cdc_acm` from IRQ context.

Author: maribu

sys/include/usb/usbus/cdc/acm.h

@@ -41,6 +41,8 @@
  */
 
 #include <stdint.h>
+#include <unistd.h>
+
 #include "usb/cdc.h"
 #include "usb/usbus.h"
 #include "tsrb.h"
@@ -198,9 +200,10 @@ void usbus_cdc_acm_init(usbus_t *usbus, usbus_cdcacm_device_t *cdcacm,
  * @param[in]   buf         buffer to submit
  * @param[in]   len         length of the submitted buffer
  *
- * @return                  Number of bytes added to the CDC ACM ring buffer
+ * @retval      -ECONNRESET Failed to sub data
+ * @retval      >=0         Number of bytes added to the CDC ACM ring buffer
  */
-size_t usbus_cdc_acm_submit(usbus_cdcacm_device_t *cdcacm,
+ssize_t usbus_cdc_acm_submit(usbus_cdcacm_device_t *cdcacm,
                            const uint8_t *buf, size_t len);
 
 /**

sys/usb/usbus/cdc/acm/cdc_acm.c

@@ -19,6 +19,7 @@
 #define USB_H_USER_IS_RIOT_INTERNAL
 
 #include <assert.h>
+#include <errno.h>
 #include <string.h>
 
 #include "tsrb.h"
@@ -148,12 +149,12 @@ static size_t _gen_full_acm_descriptor(usbus_t *usbus, void *arg)
 }
 
 /* Submit (ACM interface in) */
-size_t usbus_cdc_acm_submit(usbus_cdcacm_device_t *cdcacm, const uint8_t *buf, size_t len)
+ssize_t usbus_cdc_acm_submit(usbus_cdcacm_device_t *cdcacm, const uint8_t *buf, size_t len)
 {
-    size_t n;
+    ssize_t n;
     unsigned old;
     if (cdcacm->state == USBUS_CDC_ACM_LINE_STATE_DISCONNECTED) {
-        return len;
+        return -ECONNRESET;
     }
 
     old = irq_disable();

sys/usb/usbus/cdc/acm/cdc_acm_stdio.c

@@ -24,7 +24,6 @@
 #include <stdio.h>
 #include <sys/types.h>
 
-#include "log.h"
 #include "isrpipe.h"
 #include "stdio_base.h"
 
@@ -36,15 +35,9 @@ static uint8_t _cdc_tx_buf_mem[CONFIG_USBUS_CDC_ACM_STDIO_BUF_SIZE];
 
 static ssize_t _write(const void* buffer, size_t len)
 {
-    const char *start = buffer;
-    while (len) {
-        size_t n = usbus_cdc_acm_submit(&cdcacm, buffer, len);
-        usbus_cdc_acm_flush(&cdcacm);
-        /* Use tsrb and flush */
-        buffer = (char *)buffer + n;
-        len -= n;
-    }
-    return (char *)buffer - start;
+    ssize_t retval = usbus_cdc_acm_submit(&cdcacm, buffer, len);
+    usbus_cdc_acm_flush(&cdcacm);
+    return retval;
 }
 
 static void _cdc_acm_rx_pipe(usbus_cdcacm_device_t *cdcacm,