Hi, all
I'm a beginner with QEMU. I'm trying to write a CAN device and also a backend.
However while I try to write a backend receiving routine, some thing confuse me. I hope I can get some help here.
Actually, I try to figure out how backend udp works at first, also a serial pci acts as a frontend. The functions qemu_set_fd_handler2() and qemu_chr_add_handlers() are used for backend udp and frontend serial pci seperatly to register the necessary routines. In the function main_loop_wait() in main-loop.c will check whether we need to receive some data, if needed, data will be read from backend.
Following is the question. I change serial_can_receive1()-hw/serial.c which is registered to test if we need data to return 1 always. So that means we always need data. In main_loop_wait() which actually call main_loop_wait()->os_host_main_loop_wait()->select(), I have checked that before select() called the udp file descriptor has set in rfds. So, if we send some data to the udp backend, the bit in rfds should be set and finally the receive routine we register though qemu_chr_add_handlers() should be called. However, the receiving routine has never been called. I do not know why?
And then I try to start QEMU with the following commands,
-serial none -chardev udp,id=UDPserial,port=3334 -device pci-serial,chardev=UDPserial
and connect it with "nc -u -l -p 3334". Before I insert the linux driver, I try to send some data to qemu, the receiving routine still never be called. After the driver has been inserted, input command "echo test>/dev/ttyS0" then the receiving routine is called. Why? When input "cat /dev/ttyS0" and input some characters in host, the receiving routine is also called. I think this is correct.