It can be useful for programs using GATT to be aware of the maximum
transmittable attribute size, as negotiated by bluetoothd. This change
exposes the negotiated size over D-Bus, if such a negotiation has
occurred.
---
attrib/gattrib.c | 6 +++++-
doc/device-api.txt | 11 +++++++++++
src/device.c | 41 +++++++++++++++++++++++++++++++++++++++++
src/shared/att.c | 15 +++++++++++++++
src/shared/gatt-server.c | 1 +
src/shared/gatt-server.h | 3 +++
6 files changed, 76 insertions(+), 1 deletion(-)
diff --git a/attrib/gattrib.c b/attrib/gattrib.c
index 2e1e39a..f080b6b 100644
--- a/attrib/gattrib.c
+++ b/attrib/gattrib.c
@@ -468,7 +468,11 @@ gboolean g_attrib_set_mtu(GAttrib *attrib, int mtu)
attrib->buflen = mtu;
- return bt_att_set_mtu(attrib->att, mtu);
+ if (bt_att_set_mtu(attrib->att, mtu)) {
+ bt_att_set_mtu_negotiated(attrib->att, true);
+ return true;
+ } else
+ return false;
}
gboolean g_attrib_unregister(GAttrib *attrib, guint id)
diff --git a/doc/device-api.txt b/doc/device-api.txt
index 13b2881..fc04011 100644
--- a/doc/device-api.txt
+++ b/doc/device-api.txt
@@ -234,3 +234,14 @@ Properties string Address [readonly]
array{byte} AdvertisingFlags [readonly, experimental]
The Advertising Data Flags of the remote device.
+
+ int16 NegotiatedMaxAttributeSize [readonly,
+ optional,
+ experimental]
+
+ The maximum supported byte size of attributes, as
+ negotiated between this host and the remote device.
+ This attribute is only available once the negotiation
+ is complete. If the attribute is not available,
+ clients should assume the default maximum size for
+ the underlying technology, e.g. 20 bytes for BTLE.
diff --git a/src/device.c b/src/device.c
index 8693eb8..cd3983b 100644
--- a/src/device.c
+++ b/src/device.c
@@ -92,6 +92,8 @@
#define GATT_INCLUDE_UUID_STR "2802"
#define GATT_CHARAC_UUID_STR "2803"
+#define ATT_PACKET_HEADER_SIZE 3
+
static DBusConnection *dbus_conn = NULL;
static unsigned service_state_cb_id;
@@ -930,6 +932,41 @@ static gboolean dev_property_exists_tx_power(const GDBusPropertyTable *property,
return TRUE;
}
+static gboolean dev_property_get_negotiated_max_att_payload(
+ const GDBusPropertyTable * property,
+ DBusMessageIter *iter, void *data)
+{
+ const struct btd_device *dev = data;
+ dbus_int16_t val = 0;
+ int err;
+
+ /* The spec for this API requires a failure if
+ * MTU negotiation is incomplete.
+ */
+ if (!bt_att_is_mtu_negotiated(dev->att))
+ return FALSE;
+
+ val = bt_att_get_mtu(dev->att);
+ // subtract off the ATT header length to get the max payload
+ val -= ATT_PACKET_HEADER_SIZE;
+
+ dbus_message_iter_append_basic(iter, DBUS_TYPE_INT16, &val);
+
+ return TRUE;
+}
+
+static gboolean dev_property_exists_negotiated_max_att_payload(
+ const GDBusPropertyTable * property,
+ void *data)
+{
+ const struct btd_device *dev = data;
+
+ /* The spec for this API requires the property
+ * be absent until MTU negotiation is incomplete.
+ */
+ return bt_att_is_mtu_negotiated(dev->att);
+}
+
static gboolean
dev_property_get_svc_resolved(const GDBusPropertyTable *property,
DBusMessageIter *iter, void *data)
@@ -2561,6 +2598,10 @@ static const GDBusPropertyTable device_properties[] = {
NULL, dev_property_service_data_exist },
{ "TxPower", "n", dev_property_get_tx_power, NULL,
dev_property_exists_tx_power },
+ { "NegotiatedMaxAttributeSize", "n",
+ dev_property_get_negotiated_max_att_payload,
+ NULL, dev_property_exists_negotiated_max_att_payload,
+ G_DBUS_PROPERTY_FLAG_EXPERIMENTAL },
{ "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/att.c b/src/shared/att.c
index ca2d051..bf20a01 100644
--- a/src/shared/att.c
+++ b/src/shared/att.c
@@ -70,6 +70,7 @@ struct bt_att {
uint8_t *buf;
uint16_t mtu;
+ bool mtu_negotiated;
unsigned int next_send_id; /* IDs for "send" ops */
unsigned int next_reg_id; /* IDs for registered callbacks */
@@ -1100,6 +1101,20 @@ bool bt_att_set_debug(struct bt_att *att, bt_att_debug_func_t callback,
return true;
}
+bool bt_att_is_mtu_negotiated(struct bt_att *att)
+{
+ if (!att)
+ return false;
+
+ return att->mtu_negotiated;
+}
+
+void bt_att_set_mtu_negotiated(struct bt_att *att, bool negotiated)
+{
+ if (att && !att->mtu_negotiated)
+ att->mtu_negotiated = negotiated;
+}
+
uint16_t bt_att_get_mtu(struct bt_att *att)
{
if (!att)
diff --git a/src/shared/gatt-server.c b/src/shared/gatt-server.c
index 79e01c8..c76ca23 100644
--- a/src/shared/gatt-server.c
+++ b/src/shared/gatt-server.c
@@ -1379,6 +1379,7 @@ static void exchange_mtu_cb(uint8_t opcode, const void *pdu,
/* Set MTU to be the minimum */
server->mtu = final_mtu;
bt_att_set_mtu(server->att, final_mtu);
+ bt_att_set_mtu_negotiated(server->att, true);
util_debug(server->debug_callback, server->debug_data,
"MTU exchange complete, with MTU: %u", final_mtu);
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);
--
2.10.0
--
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