On Thu, Jan 19, 2017 at 10:57:58AM -0800, Christopher Collins wrote:
> On Thu, Jan 19, 2017 at 03:46:49AM -0800, Simon Ratner wrote:
> > A related question: how does this map to large ATT_MTU and fragmented
> > packets at the L2CAP level (assuming no data length extension)? Does each
> > fragment get its own mbuf, which are then chained together, or does the
> > entire packet get reassembled into a single mbuf if there is room?
>
> If the host needs to send a large packet, it packs the payload into an
> mbuf chain. By "packs," I mean each buffer holds as much data as
> possible with no regard to the maximum L2CAP fragment size.
>
> When the host sends an L2CAP fragment, it splits the fragment payload
> off from the front of the mbuf chain, constructs an ACL data packet, and
> sends it to the controller. If a buffer at the front of mbuf can be
> freed, now that data has been removed, the host frees it.
>
> If you are interested, the function which handles fragmentation and
> freeing is mem_split_frag() (util/mem/src/mem.c).
I rushed this response a bit, and there are some important details I
neglected.
* For the final L2CAP fragment in a packet, the host doesn't
do an allocating or copying. Instead, it just prepends an ACL data
header to the mbuf chain and sends it to the controller.
* For all L2CAP fragments *other than the last*, the host allocates an
additional mbuf chain to hold the ACL data packet. The host then copies
the fragment data into this new chain, sends it, and frees buffers from
the front of the original chain if possible. The number of buffers that
get allocated for the fragment depends on how the maximum L2CAP fragment
size compares to the msys mbuf size. If an msys mbuf buffer has
sufficient capacity for a maximum size L2CAP fragment, then only one
buffer will get allocated. If the mbuf capacity is less, the chain that
gets allocated will consist of multiple buffers.
* An L2CAP fragment mbuf chain contains the following:
* mbuf pkthdr (8 bytes)
* HCI ACL data header (4 bytes)
* Basic L2CAP header (4 bytes)
* Payload (varies)
* For incoming data, the host does not do any packing. Each L2CAP
fragment is simply chained together.
