Use kernel-provided packet reception timestamps to get less jitter in
packet timings. Mostly matters for ISO/SCO which have regular schedule.
A2DP (L2CAP) doesn't currently do RX timestamps in kernel, but we can as
well use the same mechanism for it.
Kernel-provided MTU does not work for USB controllers and the correct
packet size to send can be known currently only from RX. So we are
waiting for RX to get it.
The known problem is USB-specific, we shouldn't need the workaround for
other transport types.
Don't wait for POLLIN for non-USB controllers on connect, but ready
things on POLLOUT as usual.
For non-USB controllers, pick some sensible packet sizes to use
initially, before we switch to same size as for RX.
The kernel-provided SCO write MTU is currently never the correct packet
size for writing, so don't try to use it. Some adapter firmware (eg.
BCM20702A0 0b05:17cb) appears in practice sensitive to the alignment of
the msbc frames, and writes with wrong packet size break things but only
on certain headsets. For other adapters, this doesn't appear to matter.
This is a bitfield, but it's unclear what it achieves since this is the
only member of a bitfield, so it may be more efficient to just make it a
bool.
Fixes a LGTM warning:
Bit field started of type int should have explicitly unsigned integral, explicitly signed integral, or enumeration type.
Move SCO polling to a single place, and abstract mtu handling.
Autodetect suitable tx packet size based on rx, instead of relying on
the kernel providing correct values.
