Re: [PATCH] gatt-server: Implement NegotiatedMTU property on Device1

It can be useful for programs using GATT to be aware of the maximum
transmittable packet size, as negotiated by bluetoothd.  This change
exposes the negotiated MTU over D-Bus, if such a negotiation has
occurred.
---
 client/main.c            |  1 +
 doc/device-api.txt       |  5 +++++
 src/device.c             | 31 +++++++++++++++++++++++++++++++
 src/shared/gatt-server.c | 15 +++++++++++++++
 src/shared/gatt-server.h |  3 +++
 5 files changed, 55 insertions(+)

diff --git a/client/main.c b/client/main.c
index 255cbd5..6435674 100644
--- a/client/main.c
+++ b/client/main.c

@@ -1372,6 +1372,7 @@ static void cmd_info(const char *arg)
 	print_property(proxy, "ServiceData");
 	print_property(proxy, "RSSI");
 	print_property(proxy, "TxPower");
+	print_property(proxy, "NegotiatedMTU");
 }

 static void pair_reply(DBusMessage *message, void *user_data)

diff --git a/doc/device-api.txt b/doc/device-api.txt
index 13b2881..908616b 100644
--- a/doc/device-api.txt
+++ b/doc/device-api.txt

@@ -234,3 +234,8 @@ Properties	string Address [readonly]
 		array{byte} AdvertisingFlags [readonly, experimental]

 			The Advertising Data Flags of the remote device.
+
+		int16 NegotiatedMTU [readonly, optional, experimental]
+
+			The ATT MTU negotiated with the connected host.
+			Available only once MTU negotiation is complete.

diff --git a/src/device.c b/src/device.c
index 8693eb8..1b171ee 100644
--- a/src/device.c
+++ b/src/device.c

@@ -930,6 +930,35 @@ static gboolean

dev_property_exists_tx_power(const GDBusPropertyTable *property,
 	return TRUE;
 }

+static gboolean dev_property_get_negotiated_mtu(
+					const GDBusPropertyTable * property,
+					DBusMessageIter *iter, void *data)
+{
+	const struct btd_device *dev = data;
+	dbus_int16_t val = 0;
+	int err;
+
+	err = bt_gatt_server_get_negotiated_mtu(dev->server, &val);
+	if (err != 0)
+		return FALSE;
+
+	dbus_message_iter_append_basic(iter, DBUS_TYPE_INT16, &val);
+
+	return TRUE;
+}
+
+static gboolean dev_property_exists_negotiated_mtu(
+					const GDBusPropertyTable * property,
+					void *data)
+{
+	const struct btd_device *dev = data;
+	int err;
+
+	err = bt_gatt_server_get_negotiated_mtu(dev->server, NULL);
+
+	return err == 0;
+}
+
 static gboolean
 dev_property_get_svc_resolved(const GDBusPropertyTable *property,
 					DBusMessageIter *iter, void *data)

@@ -2561,6 +2590,8 @@ static const GDBusPropertyTable 

device_properties[] = {
 				NULL, dev_property_service_data_exist },
 	{ "TxPower", "n", dev_property_get_tx_power, NULL,
 					dev_property_exists_tx_power },
+	{ "NegotiatedMTU", "n", dev_property_get_negotiated_mtu, NULL,
+					dev_property_exists_negotiated_mtu },
 	{ "ServicesResolved", "b", dev_property_get_svc_resolved, NULL, NULL 
},
 	{ "AdvertisingFlags", "ay", dev_property_get_flags, NULL,
 					dev_property_flags_exist,

diff --git a/src/shared/gatt-server.c b/src/shared/gatt-server.c
index 79e01c8..2727f09 100644
--- a/src/shared/gatt-server.c
+++ b/src/shared/gatt-server.c

@@ -89,6 +89,7 @@ struct bt_gatt_server {
 	struct bt_att *att;
 	int ref_count;
 	uint16_t mtu;
+	bool mtu_negotiated;

 	unsigned int mtu_id;
 	unsigned int read_by_grp_type_id;

@@ -1378,12 +1379,25 @@ static void exchange_mtu_cb(uint8_t opcode,

const void *pdu,

 	/* Set MTU to be the minimum */
 	server->mtu = final_mtu;
+	server->mtu_negotiated = true;
 	bt_att_set_mtu(server->att, final_mtu);

 	util_debug(server->debug_callback, server->debug_data,
 			"MTU exchange complete, with MTU: %u", final_mtu);
 }

+int bt_gatt_server_get_negotiated_mtu(const struct bt_gatt_server 
*server,
+				int16_t *mtu)
+{
+	if (!server || !server->mtu_negotiated)
+		return -1;
+
+	if (mtu)
+		*mtu = server->mtu;
+
+	return 0;
+}
+
 static bool gatt_server_register_att_handlers(struct bt_gatt_server 
*server)
 {
 	/* Exchange MTU */

@@ -1493,6 +1507,7 @@ struct bt_gatt_server *bt_gatt_server_new(struct

gatt_db *db,
 	server->db = gatt_db_ref(db);
 	server->att = bt_att_ref(att);
 	server->mtu = MAX(mtu, BT_ATT_DEFAULT_LE_MTU);
+	server->mtu_negotiated = false;
 	server->max_prep_queue_len = DEFAULT_MAX_PREP_QUEUE_LEN;
 	server->prep_queue = queue_new();

diff --git a/src/shared/gatt-server.h b/src/shared/gatt-server.h
index 0e480e1..0b99da5 100644
--- a/src/shared/gatt-server.h
+++ b/src/shared/gatt-server.h

@@ -31,6 +31,9 @@ struct bt_gatt_server *bt_gatt_server_new(struct 

gatt_db *db,
 struct bt_gatt_server *bt_gatt_server_ref(struct bt_gatt_server 
*server);
 void bt_gatt_server_unref(struct bt_gatt_server *server);

+int bt_gatt_server_get_negotiated_mtu(const struct bt_gatt_server 
*server,
+					int16_t *mtu);
+
 typedef void (*bt_gatt_server_destroy_func_t)(void *user_data);
 typedef void (*bt_gatt_server_debug_func_t)(const char *str, void 
*user_data);
 typedef void (*bt_gatt_server_conf_func_t)(void *user_data);