Hi,
for testing purposes, I tried to lower GATT's ExchangeMTU in main.conf:
> # Exchange MTU size.
> # Possible values: 23-517
> # Defaults to 517
> ExchangeMTU = 23
Unfortunately, bluetoothd doesn't show my adapter anymore after this change:
> daemon.err bluetoothd[450]: /usr/src/debug/bluez5/5.82/src/gatt-database.c:btd_gatt_database_new() Failed to start listening: setsockopt(L2CAP_OPTIONS): Invalid argument (22)
> daemon.err bluetoothd[450]: Failed to create GATT database for adapter
> daemon.err bluetoothd[450]: Unable to register new adapter
The first error message seems to come from btd_gatt_database_new() [1].
...
bredr:
/* BR/EDR socket */
database->bredr_io = bt_io_listen(connect_cb, NULL, NULL, NULL, &gerr,
BT_IO_OPT_SOURCE_BDADDR, addr,
BT_IO_OPT_PSM, BT_ATT_PSM,
BT_IO_OPT_SEC_LEVEL, BT_IO_SEC_MEDIUM,
BT_IO_OPT_MTU, btd_opts.gatt_mtu,
BT_IO_OPT_INVALID);
if (database->bredr_io == NULL) {
error("Failed to start listening: %s", gerr->message);
g_error_free(gerr);
goto fail;
}
...
Interestingly, the ExchangeMTU value from main.conf seems not to be used when
creating the 'le_io' socket (isn't this the most important one?), so I guess
that the error message is raised when creating the 'bredr_io' socket. The
kernel has different allowed ranges for the MTU, depending on the L2CAP CID
value:
static bool l2cap_valid_mtu(struct l2cap_chan *chan, u16 mtu)
{
switch (chan->scid) {
case L2CAP_CID_ATT:
if (mtu && mtu < L2CAP_LE_MIN_MTU)
return false;
break;
default:
if (mtu && mtu < L2CAP_DEFAULT_MIN_MTU)
return false;
}
return true;
}
As the 'bredr_io' (and the 'eatt_io') sockets are not created with the
L2CAP_CID_ATT cid, the default branch is taken here which imposes a minimum
MTU value of 48.
Questions:
1. Is it correct that the value of ExchangeMTU isn't used for the LE socket?
2. Should the kernel allow MTU values such as 23 for all (GATT) sockets?
3. Where/how should this be fixed (kernel / bluetoothd / main.conf)?
regards,
Christian
[1] https://git.kernel.org/pub/scm/bluetooth/bluez.git/tree/src/gatt-database.c#n4103
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/bluetooth/l2cap_sock.c#n709
