ChibiOS Free Embedded RTOS

What I don't understand is : What if nothing is happening and you want to send data out ? To do this, you need to wake up the thread. In order to do wake it up, how then, can you make the transition from none empty to empty happen from another thread ?
Do you have to wait for flag==IO_OUTPUT_EMPTY to output ?

You can simply start writing from any thread, you don't need to wait an event in order to write.

Giovanni