modem: cmux: Handle C/R bit from address field

[SeppoTakalo]

The C/R bit in the address field should be handled as explained in 5.2.1.2 in the spec (3GPP TS 127.010).

To detect if the frame is a command or a response, we need to know who was the initiator of the CMUX channel.

Initiator is the station that take the initiative to initialize
the multiplexer (i.e. sends the SABM command at DLCI 0 )

See the table from given section of the specification.

Also, on UIH frames 5.4.3.1 says

The frames sent by the initiating station have the C/R bit set to 1
and those sent by the responding station have the C/R bit set to 0.

NOTE: This is different than a C/R bit in the Type field.

[bjarki-andreasen]

Could we add a test case for this?

[SeppoTakalo]

@bjarki-andreasen

Could we add a test case for this?

Good idea.
I added

ZTEST(modem_cmux, test_modem_cmux_invalid_cr)
{
	uint32_t events;
	int ret;

	/* We are initiator, so any CMD with CR set, should be dropped */
	modem_backend_mock_put(&bus_mock, cmux_frame_control_cld_cmd,
			       sizeof(cmux_frame_control_cld_cmd));

	modem_backend_mock_put(&bus_mock, cmux_frame_control_sabm_cmd,
			       sizeof(cmux_frame_control_sabm_cmd));


	events = k_event_wait_all(&cmux_event, (MODEM_CMUX_EVENT_CONNECTED | MODEM_CMUX_EVENT_DISCONNECTED),
				  false, K_MSEC(100));

	zassert_false(events, "Wrong CMD should have been ignored");
}

So the mock tries to send Close-Down command and open command, but C/R bit set, so those should be ignored because in the test setup the mock is not an initiator, so C/R should be 0.

[SeppoTakalo]

Fixed the comment like Tomi suggested.

[sonarqubecloud]

Quality Gate passed

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0 New issues
0 Accepted issues

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