I am a beginner to Linux drivers and I started with writing an application for a cdc-acm based USB device in linux. Therefore, I have used the cdc_acm driver. The USB device that I am using has 2 Bulk endpoints (read and write) and one interrupts endpoint.
Now, my question is whether all these endpoints operate on the same /dev/ttyACM0 file, and how does the write call on this tty file get convert into acm_write_bulk fops call?
If I write a data to trigger a USB functionality into the ttyACM0 file, will the data get sent through bulk out endpoint? Or how should I send the data to the bulk endpoint directly from user space. Should I write any supporting driver in kernel space? Similarly, how do I read the data from interrupt endpoint in the userspace?
I appreciate your help in advance.
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Answer
There is no need to write a kernel space driver. You can open /dev/ttyACM0 with the open system call, set parameters for it using termios (optional), and then use the read and write system calls to read and write bulk data from your device. These system calls are easiest to access from C and C++, but most languages have a library you can use to access serial ports.
I don’t know of a great way to read data from the interrupt endpoint in Linux but you can at least look into the TIOCMGET, TIOCGICOUNT, and TIOCMIWAIT ioctls in you really need to do that.
The Linux serial port interface abstracts away all details about USB, endpoints, and bulk transfers, so you can use a simpler, more abstract API to communicate with the serial port. In fact, you can use the same code on any type of serial port, regardless of which kernel driver implements the serial port. It might help you to search the Internet for things like “linux serial port programming” or “posix serial port programming” to understand more about how to do this.
If you really are curious about how the Linux CDC ACM driver works, and how it converts a write system call into the corresponding USB transfer, you can read the source of the cdc-acm driver. However, that is way beyond what you need to do to simply use a serial port.