How to use USBTMC notification support

[hinxx]

USBTMC change in #2494 adds notification support. I'm trying to figure out how to use this feature on the RP2040 port.

How is bool tud_usbtmc_transmit_notification_data(const void * data, size_t len) supposed to be used?
What should be done in bool tud_usbtmc_notification_complete_cb(void)?
Is host supposed to "poll" for the SRQ using pyvisa instrument.read_stb()?

Can anyone shed some light onto this subject?

[hinxx]

I think I figured it out, might be mistaken, though..

One needs to add the tud_usbtmc_notification_complete_cb() to the application which in turn would call tud_usbtmc_transmit_notification_data() if there is a SRQ condition to report, otherwise do nothing(?).

Then the host needs to issue IN token(s) on the interrupt EP effectively polling for device data. Host either sees a timeout (device had no data to reported) or 2 bytes of data (0x81 <status_byte>).

Unfortunately, there seems to be a bug (See #2735) in the tud_usbtmc_transmit_notification_data at the moment preventing any device data to reach the host. Once resolved the notification reporting mechanism works as I would expect.