modem: cmux: Decouple modem cmux and cellular driver

The modem modules cmux module is currently directly coupled to the
presence of specific modems, rather than being selected by drivers
for whatever hardware wants to request the default MTU of 127 bytes.

This commit the makes the device drivers (for now, modem_cellular)
select the symbol, thus decoupling the modem modules from the
presence of any specific device.

Signed-off-by: Bjarki Arge Andreasen <[email protected]>

Author: bjarki-andreasen

drivers/modem/Kconfig.cellular

@@ -21,6 +21,13 @@ config MODEM_CELLULAR
 		    DT_HAS_QUECTEL_EG25_G_ENABLED || DT_HAS_NORDIC_NRF91_SLM_ENABLED || \
 		    DT_HAS_SQN_GM02S_ENABLED || DT_HAS_U_BLOX_LARA_R6_ENABLED || \
 		    DT_HAS_QUECTEL_EG800Q_ENABLED)
+	select MODEM_CMUX_DEFAULT_MTU_127 if \
+		(DT_HAS_QUECTEL_BG95_ENABLED || DT_HAS_QUECTEL_EG25_G_ENABLED || \
+		 DT_HAS_SIMCOM_SIM7080_ENABLED || DT_HAS_U_BLOX_SARA_R4_ENABLED || \
+		 DT_HAS_U_BLOX_SARA_R5_ENABLED || DT_HAS_SWIR_HL7800_ENABLED || \
+		 DT_HAS_TELIT_ME910G1_ENABLED || DT_HAS_TELIT_ME310G1_ENABLED || \
+		 DT_HAS_SQN_GM02S_ENABLED || DT_HAS_QUECTEL_EG800Q_ENABLED || \
+		 DT_HAS_SIMCOM_A76XX_ENABLED)
 	help
 	  This driver uses the generic 3gpp AT commands, along
 	  with the standard protocols CMUX and PPP, to configure

subsys/modem/Kconfig

@@ -31,15 +31,8 @@ if MODEM_CMUX
 
 config MODEM_CMUX_DEFAULT_MTU_127
 	bool
-	default y if (DT_HAS_QUECTEL_BG95_ENABLED || DT_HAS_QUECTEL_EG25_G_ENABLED || \
-		      DT_HAS_SIMCOM_SIM7080_ENABLED || DT_HAS_U_BLOX_SARA_R4_ENABLED || \
-		      DT_HAS_U_BLOX_SARA_R5_ENABLED || DT_HAS_SWIR_HL7800_ENABLED || \
-		      DT_HAS_TELIT_ME910G1_ENABLED || DT_HAS_TELIT_ME310G1_ENABLED || \
-		      DT_HAS_SQN_GM02S_ENABLED || DT_HAS_QUECTEL_EG800Q_ENABLED || \
-		      DT_HAS_SIMCOM_A76XX_ENABLED)
 	help
-	  Use the default MTU size of 127 bytes for the CMUX module on certain modems.
-	  This must match the AT+CMUX commands in the modem_cellular driver.
+	  Hint that the default MODEM_CMUX_MTU size should be 127 bytes.
 
 config MODEM_CMUX_MTU
 	int "CMUX MTU size in bytes"