The host can not get any midi message from device

[Hi-Music]

Thanks everyone, I want that the device send a midi message to host when device receive a midi message from host, so I write the code in the example/device/ midi_test/src/main.c ,
"""
while ( tud_midi_available() ) {
tud_midi_packet_read(packet);
uint8_t note_on[3] = { 0x90 | channel, note_sequence[note_pos], 127 };
tud_midi_stream_write(cable_num, note_on, 3);
}
'""

But the host can not get any midi message from device. Can you help me, thank you very much!