MIME-Version: 1.0
From:   =?UTF-8?Q?Sergio_Conde_G=C3=B3mez?= <skgsergio@gmail.com>
Date:   Mon, 10 Jan 2022 09:29:59 +0100
Message-ID: <CAGjK+HS2o8P4wPb869vaJoP6Fm-EMHmguhVKEyBX=nGoBchDKw@mail.gmail.com>
Subject: Unable to connect to Bluetooth LE devices with long advertising times
To:     linux-bluetooth@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

Hello,

I'm on kernel version 5.15.13 (shipped by Arch Linux) with bluez 5.63
and I'm unable to connect to Bluetooth LE devices with 5s, 7,5s, 10s
advertising times. As for hardware, I have a PCIe card with an Intel
AC9260 (WiFi + BT combo)[1].

I've been doing some research together with an open-source firmware
developer and some other users for one of the devices, and we found
references of Linux's HCI_LE_AUTOCONN_TIMEOUT
(include/net/bluetooth/hci.h) being too low[2].

Apparently, the Bluetooth Core Specification allows the advertising to
be from 20ms to 10,485s in multiples of 0.625ms (Vol. 6, Part B,
4.4.2.2.1, page 2749 of the Core Specification 5.3), and Linux's
HCI_LE_AUTOCONN_TIMEOUT is set to 4s.

I've recompiled the kernel package on my machine raising it to 20s (no
scientific reason for this number other than being
HCI_LE_CONN_TIMEOUT) and I could connect to them (with the device with
10s advertising time was not successful every time but at least I
could connect). I retested changing the value again to 12s (to cover
the 10,485s, plus some extra with no scientific reasons) and I got
more or less the same results as with the 20s (but was quick testing
this morning, not as in deep as with the 20s).

The connection procedure was just running "bluetoothctl", "scan on",
and when the device is first seen then "connect DEVICE_MAC".

Now, I'm no expert in Bluetooth, BLE, or Linux Kernel, so I might be
doing it wrong or misunderstanding something, but changing the value
made it work. If this timeout is what is making us unable to connect
to these devices, what's the reason for this low timeout outside the
spec? Would be possible to fix it by not changing the devices to a
shorter advertisement time (which impacts battery life)?

Regards,
Sergio Conde.

[1]: https://www.intel.com/content/www/us/en/products/sku/99445/intel-wirelessac-9260/specifications.html
[2]: Full discussion https://github.com/pvvx/ATC_MiThermometer/issues/172