Bluetooth HCI redesign

[jhedberg]

Bluetooth HCI redesign ideas discussed at Bluetooth WG F2F meeting.

• Use H:4 encoding instead of bt_buf_{set,get}_type(), or rather make bt_buf_set_type() insert the value into the payload rather than user data, so we're API backwards compatible. Bluetooth: HCI: Use H:4 encoding for buffers #88710
• Do all buffer allocation management on the driver API user side. Possible options:
  • vtable struct to open() instead of just a recv callback
  • proactively have the API user "push" available buffers to the driver
• Thread separation, two options:
  • k_fifo (i.e. net_buf)
  • Flat buffers where sending side indicates availability of data and receiving side requests it. Likely requires an additional copy.
• Move setup() callback away from drivers and into an optional extension of the host (how does the user select which extension to use with the configured HCI instance?)