James Bottomley authored Feb 17, 2021 and PulseAudio Marge Bot committed Apr 05, 2021

Adding processing support for the mSBC codec is somewhat problematic,
because, although it is a SBC codec, the a2dp handling can't simply be
reused because the codec is used on an eSCO link with transparent
data, meaning the transmission unit has to be 48 bytes (fragmenting
the codec packets) and reassembly and boundary detection is required
to be done by the implementation.  Therefore we have to implement
separate render and push routines for msbc that do this fragmentation.

Fragmentation is done by emulating circular buffers.  The receive
(push) buffer is easy, since the mSBC packet size is 60, simply have a
buffer of this size in the sbc_info area where the fragments are
reassembled.  Once we have a full 60 bytes, decode and restart from
zero.  The send (render) buffer is more problematic, since the
transmit must be done from contiguous memory.  This means that the
buffer must be the lowest common multiple of the transmission unit and
the packet size.  This value is 240 since 240/48 == 5 and 240/60 == 4.
So the buffer pointers are reset at 240 which is a whole number of
both rendered packets and eSCO transmission units.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Part-of: <!507>