Dmitry Timoshkov dmitry@baikal.ru wrote:
Wolfgang Walter wine@stwm.de wrote:
I think that happens:
- application writes data to com port.
- all is written, serial buffer is empty
- application calls WaitCommEvent()
- wait_on() is called
- wait_on() calls get_irq_info()
- get_irq_info() sets commio->irq_info->temt = 1
- wait_on() calls check_events() and uses sets commio->irq_info for old an new
- so old->temt == new->temt and EV_TXEMPTY is not set
- if there are no other events (in real world basically EV_RXCHAR):
- wait_for_event() is startet with commio->irq_info->temt set to one
- wait_for_event() calls get_irq_info() with new_irq_info()
- get_irq_info() sets new_irq_info->temt = 1 because the buffer is still empty
- wait_for_event() calls check_events() with new_irq_info and commio->irq_info
- again check_events() will not set EV_TXEMPTY as both have temt == 1
It seems that we do not recover from that hang.
Please correct me if I misread the code and I'm wrong.
I think it's better if WaitCommEvent() returns with EV_TXEMPTY even if there has no new data been sent in between:
That means that WaitCommEvent(EV_TXEMPTY) without any prior WriteFile would always return success and signal the EV_TXEMPTY event. My tests show that WaitCommEvent(EV_TXEMPTY) fails with a timeout in this case. Since the serial device is very slow it should be unlikely that after successful WriteFile() with some data WaitCommEvent() sees an already empty transmitter's output queue.
One of the solutions to this would be that WriteFile sets a flag for a being written handle indicating that there were data pending for this handle, then WaitCommEvent(EV_TXEMPTY) in the case of empty output queue would see that flag, clear it and set EV_TXEMPTY flag in the events mask.
That would also explain why SetCommMask() called after WriteFile() makes WaitCommEvent(EV_TXEMPTY) fail with a timeout in some testbot VMs, in that case SetCommMask() would also clear the data pending write flag.