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The USB interface between the host and the bluetooth adapter used for SCO packets uses an USB isochronous endpoint with a fragmentation scheme that does not tolerate errors. Except USB isochronous transfers do not provide a reliable stream with guaranteed delivery. (There is no retry on error, see USB spec v2.0 5.6 and 8.5.5.) To fragment a packet, the bluetooth HCI simply splits it in parts and transfer them as-is. The receiver is expected to reconstruct the packet by assuming the first fragment contains the header and parsing its size field. There is no error detection either. If a fragment is lost, the end result is that the kernel is no longer synchronized and will pass malformed data to the upper layers, since it has no way to tell if the first fragment is an actual first fragment or a continuation fragment. Resynchronization can only happen by luck and requires an unbounded amount of time. The typical symptom for a HSP/HFP bluetooth headset is that the microphone stops working and dmesg contains piles of rate-limited "Bluetooth: hci0: SCO packet for unknown connection handle XXXX" errors for an indeterminate amount of time, until the kernel accidentally resynchronize. A workaround is to ask the upper layer to prevalidate the first fragment header. This is not possible with user channels so this workaround is disabled in this case. This problem is the most severe when using an ath3k adapter on an i.MX 6 board, where packet loss occur regularly, possibly because it is an USB1 device connected on an USB2 hub and this is a special case requiring split transactions. Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr> Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com> |
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| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.