sys/usb_cdc_acm_stdio: only submit and flush for non-empty buffer

[mguetschow]

Contribution description

There seems to be some problem the implementation of stdio over usbus_cdc_acm where printing an empty string sometimes leads to the next character being missed in the printout. I've managed to pin the problem down to the call to usbus_cdc_acm_flush(&cdcacm); in sys/usb/usbus/cdc/acm/cdc_acm_stdio.c and, more specifically, to _ep_xmit in cpu/nrf5x_common/periph/usbdev.c, but since I have no non-nRF hardware at hand, I cannot tell whether it is a nRF specific issue.

In any case, skipping the flushing (and submitting) in the first place seems to fix the issue for now. If anyone has an idea how to investigate this further and maybe fix the root cause somewhere deeper in the usb driver, please tell me, I'm happy to test more.

Testing procedure

1. apply the following diff:

diff --git a/tests/sys/usbus_cdc_acm_stdio/main.c b/tests/sys/usbus_cdc_acm_stdio/main.c
index c603882121..50c0b9a9d8 100644
--- a/tests/sys/usbus_cdc_acm_stdio/main.c
+++ b/tests/sys/usbus_cdc_acm_stdio/main.c
@@ -21,6 +21,7 @@
 #include <stdlib.h>
 
 #include "shell.h"
+#include "fmt.h"
 
 static int cmd_text(int argc, char **argv)
 {
@@ -30,7 +31,7 @@ static int cmd_text(int argc, char **argv)
         return -1;
     }
     int n = atoi(argv[1]);
-    if (n <= 0) {
+    if (n < 0) {
         puts(usage);
         return -1;
     }
@@ -40,7 +41,9 @@ static int cmd_text(int argc, char **argv)
         }
         putc('0' + (i % 10), stdout);
     }
-    puts("");
+    print_str("");
+    print_str("x");
+    print_str("\n");
     return 0;
 }

2. pick any (nrf-based?) board using stdio_cdc_acm such as feather-nrf52840-sense
3. make -C tests/sys/usbus_cdc_acm_stdio BOARD=feather-nrf52840-sense flash term
4. repeatedly enter text 0

On master, this sometimes skips over the x in the printout. With this PR, I never managed to reproduce the issue.

Issues/PRs references

Found while investigating failing tests for #20980

[benpicco]

Now that's a very straightforward fix, neat!

[riot-ci]

Murdock results

✔️ PASSED

67edd6c sys/usb_cdc_acm_stdio: only submit and flush for non-empty buffer

Success	Failures	Total	Runtime
10251	0	10251	19m:06s

Artifacts

• Documentation preview

[mguetschow]

Thanks!

[benpicco]

Should we also backport this to the 2024.10 release?

[mguetschow]

May be good, if its not too much hassle :)