huawei: handle empty response to AT^SYSCFG=?

Some Huawei modems (e.g. E220) may give an empty response for AT^SYSCFG=?, even
if they do support the command. Handle this case by prividing a default fallback
format string when this happens.

plugins/huawei/mm-broadband-modem-huawei.c

@@ -912,8 +912,19 @@ syscfg_test_ready (MMBroadbandModemHuawei *self,
     GError *error = NULL;
 
     response = mm_base_modem_at_command_finish (MM_BASE_MODEM (self), res, &error);
-    if (response)
+    if (response) {
-        self->priv->syscfg_supported_modes = mm_huawei_parse_syscfg_test (response, &error);
+        /* There are 2G+3G Huawei modems out there which support mode switching with
+         * AT^SYSCFG, but fail to provide a valid response for AT^SYSCFG=? (they just
+         * return an empty string). So handle that case by providing a default response
+         * string to get parsed. Ugly, ugly, blame Huawei.
+         */
+        if (response[0])
+            self->priv->syscfg_supported_modes = mm_huawei_parse_syscfg_test (response, &error);
+        else {
+            self->priv->syscfg_supported_modes = mm_huawei_parse_syscfg_test (MM_HUAWEI_DEFAULT_SYSCFG_FMT, NULL);
+            g_assert (self->priv->syscfg_supported_modes != NULL);
+        }
+    }
 
     if (self->priv->syscfg_supported_modes) {
         MMModemModeCombination mode;

plugins/huawei/mm-modem-helpers-huawei.h

@@ -73,6 +73,10 @@ const MMHuaweiPrefmodeCombination *mm_huawei_parse_prefmode_response (const gcha
 /*****************************************************************************/
 /* ^SYSCFG test parser */
 
+/* This is the default string we use as fallback when the modem gives
+ * an empty response to AT^SYSCFG=? */
+#define MM_HUAWEI_DEFAULT_SYSCFG_FMT "^SYSCFG:(2,13,14,16),(0-3),,,"
+
 typedef struct {
     guint mode;
     guint acqorder;

plugins/huawei/tests/test-modem-helpers-huawei.c

@@ -439,6 +439,41 @@ typedef struct {
 } SyscfgTest;
 
 static const SyscfgTest syscfg_tests[] = {
+    {
+        MM_HUAWEI_DEFAULT_SYSCFG_FMT,
+        {
+            {
+                .mode = 2,
+                .acqorder = 0,
+                .allowed = (MM_MODEM_MODE_3G | MM_MODEM_MODE_2G),
+                .preferred = MM_MODEM_MODE_NONE
+            },
+            {
+                .mode = 2,
+                .acqorder = 1,
+                .allowed = (MM_MODEM_MODE_3G | MM_MODEM_MODE_2G),
+                .preferred = MM_MODEM_MODE_2G
+            },
+            {
+                .mode = 2,
+                .acqorder = 2,
+                .allowed = (MM_MODEM_MODE_3G | MM_MODEM_MODE_2G),
+                .preferred = MM_MODEM_MODE_3G
+            },
+            {
+                .mode = 13,
+                .acqorder = 0,
+                .allowed = MM_MODEM_MODE_2G,
+                .preferred = MM_MODEM_MODE_NONE
+            },
+            {
+                .mode = 14,
+                .acqorder = 0,
+                .allowed = MM_MODEM_MODE_3G,
+                .preferred = MM_MODEM_MODE_NONE
+            }
+        }
+    },
     {
         "^SYSCFG:(2,13,14,16),(0-3),((400000,\"WCDMA2100\")),(0-2),(0-4)\r\n",
         {