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From:   David Lechner <david@lechnology.com>
To:     linux-bluetooth@vger.kernel.org
Cc:     David Lechner <david@lechnology.com>
Subject: [PATCH BlueZ] device: add MTU D-Bus property
Date:   Thu, 26 Aug 2021 11:42:11 -0500
Message-Id: <20210826164211.2936133-1-david@lechnology.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

When using GATT write without response, it is useful to know how much
data can be sent in a single write. This value is the negotiated MTU
minus 3 bytes.

The D-bus method org.bluez.GattCharacteristic1.AcquireWrite returns the
MTU exactly for this reason. However, when using the alternate API
org.bluez.GattCharacteristic1.WriteValue with the options dictionary
{ "type": "command" }, there is no current way to get the MTU value.
If the value is too large, then the method returns "Failed to initiate
write" [org.bluez.Error.Failed].

This adds an "MTU" property to the org.bluez.Device1 interface that
is emitted in gatt_client_ready_cb() which is after the MTU exchange
has taken place.

Signed-off-by: David Lechner <david@lechnology.com>
---
 client/main.c      |  1 +
 doc/device-api.txt |  4 ++++
 src/device.c       | 24 ++++++++++++++++++++++++
 3 files changed, 29 insertions(+)

diff --git a/client/main.c b/client/main.c
index 506602bbd..b12a7da3e 100644
--- a/client/main.c
+++ b/client/main.c
@@ -1754,6 +1754,7 @@ static void cmd_info(int argc, char *argv[])
 	print_property(proxy, "TxPower");
 	print_property(proxy, "AdvertisingFlags");
 	print_property(proxy, "AdvertisingData");
+	print_property(proxy, "MTU");
 
 	battery_proxy = find_proxies_by_path(battery_proxies,
 					g_dbus_proxy_get_path(proxy));
diff --git a/doc/device-api.txt b/doc/device-api.txt
index 4e824d2de..030873821 100644
--- a/doc/device-api.txt
+++ b/doc/device-api.txt
@@ -272,3 +272,7 @@ Properties	string Address [readonly]
 			Example:
 				<Transport Discovery> <Organization Flags...>
 				0x26                   0x01         0x01...
+
+		uint16 MTU [readonly, optional]
+
+			The exchanged MTU (GATT client only).
diff --git a/src/device.c b/src/device.c
index 26a01612a..898f98da7 100644
--- a/src/device.c
+++ b/src/device.c
@@ -1471,6 +1471,26 @@ static gboolean dev_property_wake_allowed_exist(
 	return device_get_wake_support(device);
 }
 
+static gboolean
+dev_property_get_mtu(const GDBusPropertyTable *property,
+		     DBusMessageIter *iter, void *data)
+{
+	struct btd_device *device = data;
+
+	dbus_uint16_t mtu = bt_gatt_client_get_mtu(device->client);
+	dbus_message_iter_append_basic(iter, DBUS_TYPE_UINT16, &mtu);
+
+	return TRUE;
+}
+
+static gboolean
+dev_property_mtu_exist(const GDBusPropertyTable *property, void *data)
+{
+	struct btd_device *device = data;
+
+	return bt_gatt_client_get_mtu(device->client) != 0;
+}
+
 static bool disconnect_all(gpointer user_data)
 {
 	struct btd_device *device = user_data;
@@ -3014,6 +3034,7 @@ static const GDBusPropertyTable device_properties[] = {
 	{ "WakeAllowed", "b", dev_property_get_wake_allowed,
 				dev_property_set_wake_allowed,
 				dev_property_wake_allowed_exist },
+	{ "MTU", "q", dev_property_get_mtu, NULL, dev_property_mtu_exist },
 	{ }
 };
 
@@ -5245,6 +5266,9 @@ static void gatt_client_ready_cb(bool success, uint8_t att_ecode,
 		return;
 	}
 
+	g_dbus_emit_property_changed(dbus_conn, device->path,
+					DEVICE_INTERFACE, "MTU");
+
 	register_gatt_services(device);
 
 	btd_gatt_client_ready(device->client_dbus);
-- 
2.25.1