nimble/l2cap: Add TX_UNSTALLED event to L2CAP CoC

With this patch if L2CAP CoC has been stalled due to lack of credits
BLE_HS_ESTALLED code will be returned.

When NimBLE gets back credits and finish sending SDU,
BLE_L2CAP_EVENT_COC_TX_UNSTALLED will be sent to the application as the
indication that L2CAP CoC is unstalled and ready for sending new data.

nimble/host/include/host/ble_hs.h

@@ -94,6 +94,7 @@ extern "C" {
 #define BLE_HS_ESTORE_FAIL          28
 #define BLE_HS_EPREEMPTED           29
 #define BLE_HS_EDISABLED            30
+#define BLE_HS_ESTALLED             31
 
 /** Error base for ATT errors */
 #define BLE_HS_ERR_ATT_BASE         0x100

nimble/host/include/host/ble_l2cap.h

@@ -75,6 +75,7 @@ struct ble_hs_conn;
 #define BLE_L2CAP_EVENT_COC_DISCONNECTED              1
 #define BLE_L2CAP_EVENT_COC_ACCEPT                    2
 #define BLE_L2CAP_EVENT_COC_DATA_RECEIVED             3
+#define BLE_L2CAP_EVENT_COC_TX_UNSTALLED              4
 
 typedef void ble_l2cap_sig_update_fn(uint16_t conn_handle, int status,
                                      void *arg);
@@ -173,6 +174,28 @@ struct ble_l2cap_event {
             /** The mbuf with received SDU. */
             struct os_mbuf *sdu_rx;
         } receive;
+
+        /**
+         * Represents tx_unstalled data. Valid for the following event
+         * types:
+         *     o BLE_L2CAP_EVENT_COC_TX_UNSTALLED
+         */
+        struct {
+            /** Connection handle of the relevant connection */
+            uint16_t conn_handle;
+
+            /** The L2CAP channel of the relevant L2CAP connection. */
+            struct ble_l2cap_chan *chan;
+
+            /**
+             * The status of the send attempt which was stalled due to
+             * lack of credits; This can be non zero only if there
+             * is an issue with memory allocation for following SDU fragments.
+             * In such a case last SDU has been partially sent to peer device
+             * and it is up to application to decide how to handle it.
+             */
+            int status;
+        } tx_unstalled;
     };
 };
 

nimble/host/src/ble_l2cap_coc.c

@@ -316,6 +316,23 @@ ble_l2cap_coc_cleanup_chan(struct ble_l2cap_chan *chan)
     os_mbuf_free_chain(chan->coc_tx.sdu);
 }
 
+static void
+ble_l2cap_event_coc_unstalled(struct ble_l2cap_chan *chan, int status)
+{
+    struct ble_l2cap_event event = { };
+
+    if (!chan->cb) {
+        return;
+    }
+
+    event.type = BLE_L2CAP_EVENT_COC_TX_UNSTALLED;
+    event.tx_unstalled.conn_handle = chan->conn_handle;
+    event.tx_unstalled.chan = chan;
+    event.tx_unstalled.status = status;
+
+    chan->cb(&event, chan->cb_arg);
+}
+
 static int
 ble_l2cap_coc_continue_tx(struct ble_l2cap_chan *chan)
 {
@@ -402,16 +419,30 @@ ble_l2cap_coc_continue_tx(struct ble_l2cap_chan *chan)
                 os_mbuf_free_chain(tx->sdu);
                 tx->sdu = 0;
                 tx->data_offset = 0;
+                if (tx->flags & BLE_L2CAP_COC_FLAG_STALLED) {
+                    ble_l2cap_event_coc_unstalled(chan, 0);
+                    tx->flags &= ~BLE_L2CAP_COC_FLAG_STALLED;
+                }
                 break;
         }
     }
 
+    if (tx->sdu) {
+        /* Not complete SDU sent, wait for credits */
+        tx->flags |= BLE_L2CAP_COC_FLAG_STALLED;
+        return BLE_HS_ESTALLED;
+    }
+
     return 0;
 
 failed:
     os_mbuf_free_chain(tx->sdu);
     tx->sdu = NULL;
     os_mbuf_free_chain(txom);
+    if (tx->flags & BLE_L2CAP_COC_FLAG_STALLED) {
+        ble_l2cap_event_coc_unstalled(chan, rc);
+        tx->flags &= ~BLE_L2CAP_COC_FLAG_STALLED;
+    }
 
     return rc;
 }

nimble/host/src/ble_l2cap_coc_priv.h

@@ -35,11 +35,14 @@ extern "C" {
 
 struct ble_l2cap_chan;
 
+#define BLE_L2CAP_COC_FLAG_STALLED              0x01
+
 struct ble_l2cap_coc_endpoint {
+    struct os_mbuf *sdu;
     uint16_t mtu;
     uint16_t credits;
     uint16_t data_offset;
-    struct os_mbuf *sdu;
+    uint8_t flags;
 };
 
 struct ble_l2cap_coc_srv {

nimble/host/test/src/ble_l2cap_test.c

@@ -736,6 +736,9 @@ ble_l2cap_test_event(struct ble_l2cap_event *event, void *arg)
                                     OS_MBUF_PKTLEN(event->receive.sdu_rx));
         TEST_ASSERT(memcmp(sdu_rx->om_data, ev->data, ev->data_len) == 0);
         return 0;
+    case BLE_L2CAP_EVENT_COC_TX_UNSTALLED:
+        /* TODO Add tests for this */
+        return 0;
     default:
         return 0;
     }