tty: n_gsm: fix restart handling via CLD command
n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010. See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516 The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to the newer 27.010 here. Chapter 5.8.2 states that both sides will revert to the non-multiplexed mode via a close-down message (CLD). The usual program flow is as following: - start multiplex mode by sending AT+CMUX to the mobile - establish the control channel (DLCI 0) - establish user channels (DLCI >0) - terminate user channels - send close-down message (CLD) - revert to AT protocol (i.e. leave multiplexed mode) The AT protocol is out of scope of the n_gsm driver. However, gsm_disconnect() sends CLD if gsm_config() detects that the requested parameters require the mux protocol to restart. The next immediate action is to start the mux protocol by opening DLCI 0 again. Any responder side which handles CLD commands correctly forces us to fail at this point because AT+CMUX needs to be sent to the mobile to start the mux again. Therefore, remove the CLD command in this phase and keep both sides in multiplexed mode. Remove the gsm_disconnect() function as it become unnecessary and merge the remaining parts into gsm_cleanup_mux() to handle the termination order and locking correctly. Fixes: 71e07791 ("tty: n_gsm: do not send/receive in ldisc close path") Cc: stable@vger.kernel.org Signed-off-by:Daniel Starke <daniel.starke@siemens.com> Link: https://lore.kernel.org/r/20220414094225.4527-2-daniel.starke@siemens.com Signed-off-by: