During modem initialization we detected the flow control settings
supported by the modem, and selected the best one to use from them,
notifying it to the device via AT+IFC. The device was therefore
instructed to use that flow control setting for data transmission in
the TTY (i.e. not during AT control commands).

The missing thing was to also configure ourselves our end of the
serial port with the same flow control settings when getting into data
mode. By doing it ourselves, we avoid requiring any explicit setting
in pppd for flow control; pppd can assume the flow control settings
are already the expected ones.

Worth noting that all this setup is completely ignored for TTYs
exposed directly via USB.

https://bugs.freedesktop.org/show_bug.cgi?id=100394
---
 src/mm-broadband-bearer.c | 32 ++++++++++++++++++++++++++++++++
 src/mm-broadband-modem.c  | 36 +++++++++++++++++++++++++-----------
 src/mm-broadband-modem.h  |  4 ++++
 3 files changed, 61 insertions(+), 11 deletions(-)

diff --git a/src/mm-broadband-bearer.c b/src/mm-broadband-bearer.c
index 9491f5a8..bccee72b 100644
--- a/src/mm-broadband-bearer.c
+++ b/src/mm-broadband-bearer.c
@@ -259,6 +259,14 @@ dial_cdma_ready (MMBaseModem *modem,
      * connect_succeeded(), we do it right away so that we stop our polling. */
     mm_port_set_connected (ctx->data, TRUE);
 
+    /* Configure flow control to use while connected */
+    if (!mm_port_serial_set_flow_control (MM_PORT_SERIAL (ctx->data),
+                                          
mm_broadband_modem_get_connected_flow_control (MM_BROADBAND_MODEM (modem)),
+                                          &error)) {
+        mm_warn ("Couldn't set flow control settings: %s", error->message);
+        g_clear_error (&error);
+    }
+
     /* Keep port open during connection */
     ctx->close_data_on_exit = FALSE;
 
@@ -557,6 +565,8 @@ atd_ready (MMBaseModem *modem,
            GAsyncResult *res,
            Dial3gppContext *ctx)
 {
+    GError *error = NULL;
+
     /* DO NOT check for cancellable here. If we got here without errors, the
      * bearer is really connected and therefore we need to reflect that in
      * the state machine. */
@@ -576,6 +586,14 @@ atd_ready (MMBaseModem *modem,
         return;
     }
 
+    /* Configure flow control to use while connected */
+    if (!mm_port_serial_set_flow_control (MM_PORT_SERIAL (ctx->dial_port),
+                                          
mm_broadband_modem_get_connected_flow_control (MM_BROADBAND_MODEM (modem)),
+                                          &error)) {
+        mm_warn ("Couldn't set flow control settings: %s", error->message);
+        g_clear_error (&error);
+    }
+
     /* The ATD command has succeeded, and therefore the TTY is in data mode 
now.
      * Instead of waiting for setting the port as connected later in
      * connect_succeeded(), we do it right away so that we stop our polling. */
@@ -1452,9 +1470,16 @@ data_flash_cdma_ready (MMPortSerial *data,
                        DetailedDisconnectContext *ctx)
 {
     GError *error = NULL;
+    GError *flow_control_error = NULL;
 
     mm_port_serial_flash_finish (data, res, &error);
 
+    /* Cleanup flow control */
+    if (!mm_port_serial_set_flow_control (MM_PORT_SERIAL (data), 
MM_FLOW_CONTROL_NONE, &flow_control_error)) {
+        mm_dbg ("Couldn't reset flow control settings: %s", 
flow_control_error->message);
+        g_clear_error (&flow_control_error);
+    }
+
     /* We kept the serial port open during connection, now we close that open
      * count */
     mm_port_serial_close (data);
@@ -1569,9 +1594,16 @@ data_flash_3gpp_ready (MMPortSerial *data,
                        DetailedDisconnectContext *ctx)
 {
     GError *error = NULL;
+    GError *flow_control_error = NULL;
 
     mm_port_serial_flash_finish (data, res, &error);
 
+    /* Cleanup flow control */
+    if (!mm_port_serial_set_flow_control (MM_PORT_SERIAL (data), 
MM_FLOW_CONTROL_NONE, &flow_control_error)) {
+        mm_dbg ("Couldn't reset flow control settings: %s", 
flow_control_error->message);
+        g_clear_error (&flow_control_error);
+    }
+
     /* We kept the serial port open during connection, now we close that open
      * count */
     mm_port_serial_close (data);
diff --git a/src/mm-broadband-modem.c b/src/mm-broadband-modem.c
index e7376310..83302f63 100644
--- a/src/mm-broadband-modem.c
+++ b/src/mm-broadband-modem.c
@@ -146,6 +146,7 @@ struct _MMBroadbandModemPrivate {
     guint modem_cind_max_signal_quality;
     guint modem_cind_indicator_roaming;
     guint modem_cind_indicator_service;
+    MMFlowControl modem_flow_control;
 
     /*<--- Modem 3GPP interface --->*/
     /* Properties */
@@ -3101,17 +3102,20 @@ modem_setup_flow_control_finish (MMIfaceModem  *self,
 }
 
 static void
-ifc_test_ready (MMBaseModem  *self,
+ifc_test_ready (MMBaseModem  *_self,
                 GAsyncResult *res,
                 GTask        *task)
 {
-    GError        *error = NULL;
-    const gchar   *response;
-    MMFlowControl  mask;
-    const gchar   *cmd;
+    MMBroadbandModem *self;
+    GError           *error = NULL;
+    const gchar      *response;
+    MMFlowControl     mask;
+    const gchar      *cmd;
+
+    self = MM_BROADBAND_MODEM (_self);
 
     /* Completely ignore errors in AT+IFC=? */
-    response = mm_base_modem_at_command_finish (self, res, &error);
+    response = mm_base_modem_at_command_finish (_self, res, &error);
     if (!response)
         goto out;
 
@@ -3125,17 +3129,20 @@ ifc_test_ready (MMBaseModem  *self,
      *  XON/XOFF
      *  None.
      */
-    if (mask & MM_FLOW_CONTROL_RTS_CTS)
+    if (mask & MM_FLOW_CONTROL_RTS_CTS) {
+        self->priv->modem_flow_control = MM_FLOW_CONTROL_RTS_CTS;
         cmd = "+IFC=2,2";
-    else if (mask & MM_FLOW_CONTROL_XON_XOFF)
+    } else if (mask & MM_FLOW_CONTROL_XON_XOFF) {
+        self->priv->modem_flow_control = MM_FLOW_CONTROL_XON_XOFF;
         cmd = "+IFC=1,1";
-    else if (mask & MM_FLOW_CONTROL_NONE)
+    } else if (mask & MM_FLOW_CONTROL_NONE) {
+        self->priv->modem_flow_control = MM_FLOW_CONTROL_NONE;
         cmd = "+IFC=0,0";
-    else
+    } else
         g_assert_not_reached ();
 
     /* Set flow control settings and ignore result */
-    mm_base_modem_at_command (self, cmd, 3, FALSE, NULL, NULL);
+    mm_base_modem_at_command (_self, cmd, 3, FALSE, NULL, NULL);
 
 out:
     /* Ignore errors */
@@ -10315,6 +10322,12 @@ mm_broadband_modem_get_current_charset 
(MMBroadbandModem *self)
     return self->priv->modem_current_charset;
 }
 
+MMFlowControl
+mm_broadband_modem_get_connected_flow_control (MMBroadbandModem *self)
+{
+    return self->priv->modem_flow_control;
+}
+
 gchar *
 mm_broadband_modem_create_device_identifier (MMBroadbandModem *self,
                                              const gchar *ati,
@@ -10635,6 +10648,7 @@ mm_broadband_modem_init (MMBroadbandModem *self)
     self->priv->current_sms_mem1_storage = MM_SMS_STORAGE_UNKNOWN;
     self->priv->current_sms_mem2_storage = MM_SMS_STORAGE_UNKNOWN;
     self->priv->sim_hot_swap_supported = FALSE;
+    self->priv->modem_flow_control = MM_FLOW_CONTROL_NONE;
 }
 
 static void
diff --git a/src/mm-broadband-modem.h b/src/mm-broadband-modem.h
index d6b55d9d..9ac4eabd 100644
--- a/src/mm-broadband-modem.h
+++ b/src/mm-broadband-modem.h
@@ -24,6 +24,7 @@
 
 #include <ModemManager.h>
 
+#include "mm-modem-helpers.h"
 #include "mm-charsets.h"
 #include "mm-base-modem.h"
 
@@ -109,6 +110,9 @@ gchar 
*mm_broadband_modem_take_and_convert_to_current_charset (MMBroadbandModem
 
 MMModemCharset mm_broadband_modem_get_current_charset (MMBroadbandModem *self);
 
+/* Get the flow control setting to apply when connected, if any */
+MMFlowControl mm_broadband_modem_get_connected_flow_control (MMBroadbandModem 
*self);
+
 /* Create a unique device identifier string using the ATI and ATI1 replies and 
some
  * additional internal info */
 gchar *mm_broadband_modem_create_device_identifier (MMBroadbandModem *self,
-- 
2.12.0

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