wwan: T9326: fix dual-stack connect on single-PDN-context networks

Some networks only ever admit a single combined IPv4+IPv6 PDN context per APN
and reject a standalone IPv6 "Start Network" request outright (QMI
CallEndReason: ip-version-mismatch) once an IPv4 session already exists for
that APN.

This means "interfaces wwan <name>" with both
* "address dhcp"
* "address dhcpv6" or "ipv6 address autoconf"

never gets an IPv6 address on such networks, even though the modem and network
both genuinely support dual-stack.

Authored-by: Adam Smith <adamsmith@yzguy.io>
Co-Authored-by: Christian Breunig <christian@breunig.cc>

1 files changed, 17 insertions, 0 deletions

diff --git a/src/conf_mode/interfaces_wwan.py b/src/conf_mode/interfaces_wwan.py
index 59303196d..87ced1c6f 100755
--- a/src/conf_mode/interfaces_wwan.py
+++ b/src/conf_mode/interfaces_wwan.py
@@ -195,6 +195,23 @@ def apply(wwan):
         if 'authentication' in wwan:
             options += ',user={username},password={password}'.format(**wwan['authentication'])
 
+        # Some networks only ever admit a single combined IPv4+IPv6 PDN context
+        # per APN and reject a standalone IPv6 "Start Network" request outright
+        # (QMI CallEndReason ip-version-mismatch) once an IPv4 session already
+        # exists for that APN. ModemManager's --simple-connect with
+        # ip-type=ipv4v6 opens two separate WDS sessions (one per family) rather
+        # than negotiating both together, which is what a normal handset attach
+        # does and why this is invisible on most other devices.
+        #
+        # Pre-negotiating the attach-time PDN type upfront avoids the rejection;
+        # the resulting bearer only sets up what the *next* --simple-connect is
+        # allowed to request and can't be connected directly, so a failure here
+        # isn't fatal - unsupported modems simply proceed to --simple-connect
+        # exactly as before.
+        if ip_type in ('ipv4v6', 'ipv6'):
+            call(f'{base_cmd} --3gpp-set-initial-eps-bearer-settings="{options}"',
+                stdout=DEVNULL)
+
         command = f'{base_cmd} --simple-connect="{options}"'
         call(command, stdout=DEVNULL)