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Description
Hi,
I'm working on a USB MIDI library using thePluggableUSBModule class. During my tests I've had sporadic failures due to the received USB packets reporting an incorrect length (the number returned byread_finish() is often incorrect). I've printed the packet lengths I get in the USB callback on the Arduino and compared them to the lengths of the packets sent by my computer (using Wireshark), and the received sizes are often incorrect. It's most noticeable at the end of a transmission, where the packet size is less than the maximum size. The strange thing is that the second to last packet often reports the same length as the last packet.
For example, when sending 196 bytes in total, the packet sizes are64, 64, 64, 4, but the reported sizes on the Arduino are sometimes64, 64, 4, 4.
The standard Arduino USBSerial port seems to exhibit similar issues. When using simple loop-back code (see below) and sending long strings in the serial monitor, the response is truncated at seemingly random points and sometimes characters are missing in the middle as well.
voidsetup() { Serial.begin(115200);}voidloop() {int r = Serial.read();if (r >=0) Serial.write((uint8_t)r);}
Challenge:
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using version 1.3.1 of this core on an official Arduino Nano 33 BLE, Arduino IDE 1.8.13 on Ubuntu 20.045.4.0-65-lowlatency #73-Ubuntu SMP PREEMPT.
I suspect this is some kind of timing issue or a data race in the low-level USB code. I've verified my own code by simulating it in a multi-threaded desktop C++ program and running it using ThreadSanitizer. I'm reasonably confident that it is correct and race-free, and I've tried just wrapping everything in my code that even remotely interacts with variables used in USB callbacks in a critical section to make sure it doesn't race with my code, but the problem persists.
If it's useful, I'll clean up my code and push it to GitHub.
I did some digging into the ArduinoCore-mbed source files and I realize that the root cause probably lies buried somewhere in the Nordic-specific Mbed OS USB code, but since it also affects the Arduino Core, I decided to first open an issue here. I hope someone who's familiar with Arduino Mbed port could maybe point me to the right resources.
I searched for issues in the ARM Mbed OS repository, and found these two:ARMmbed/mbed-os#11912 &ARMmbed/mbed-os#10862, but although they do mention problems with critical sections in the USB code, I don't know how relevant they are to this issue.
I've been stuck on this problem for about a week now, and I don't know how to proceed. At this point, it seems impossible to reliably receive multiple packets worth of data over USB, neither usingSerial, nor using MIDI.