usb: Propagate the result of submit_xfer.

[hyx0329]

The result is used in the hid keyboard module but was not actually propagated. With this result, the report double buffering will work correctly.

[Josverl]

Could you add a test to verify this in CI?

Or is this already covered by another test?

[dpgeorge]

There is a comment in Interface.submit_xfer():

# If the function returns, the transfer is queued.

I guess that's either incorrect and should be changed, or is correct and then this PR is not needed.

@projectgus maybe we need your input here.

[projectgus]

Hmm, yeah - the underlying machine.USBDevice can return False if the transfer fails to queue due to a DCD error. This is documented on Interface.submit_xfer() as being the third case where it can raise an exception, but the result isn't checked so it won't raise an exception (currently).

In general, submit_xfer() should only fail under "exceptional" circumstances (i.e. broken hardware or broken driver), so it probably is better to change hid.send_report() to ignore the submit_xfer() result and return True. Then also update Interface.submit_xfer() so it raises an exception if the lower layer returns False (i.e. make the behaviour the same as what is documented).

@hyx0329 Thanks for pointing this out and submitting the fix. Are you up for updating your PR along these lines?

[hyx0329]

I've updated the PR with the proposed changes. It should work now.

As to the test, I have no idea how to do it with software only. Core depends on machine.USBDevice.

[projectgus]

Thanks @hyx0329, this fix looks good to me.

We don't currently have any way to test the micropython-lib usb classes in CI. Running the examples manually is the best we can do. 🫤