[PATCH 0/2] Indicate PAST RX ready to userspace

[Pauli Virtanen <pav@iki.fi>]

Currently there is the following race in BASS PAST:

1. Assistant: TX Add Source
2. Delegator user: RX Add Source, listen(), TX Bcast Rcv State Nfy
3a. Assistant: RX Bcast Rcv State Nfy, Send PAST
3b. Delegator kernel (hci_sync)
     - HCI_OP_LE_PAST_PARAMS queued -> Command Complete
     - start waiting for HCI_EV_LE_PAST_RECEIVED

Steps 3a and 3b are not ordered, so LE PAST may arrive when kernel or
controller is not ready to receive it.

This is why test_bass_past_transport_acquire functional test is flaky.

Solve this by new userspace API: listen() socket signals POLLOUT when
kernel is ready for PAST.

Userspace shall notify PAST ready only after that, fixed in separate
BlueZ patch series.  Also add related iso-tester tests + uAPI
documentation there.

This needed an extension to the HCI_REQ handling, to set POLLOUT only
after hci_sync has started waiting for the PA events.  Didn't see easy
ways to simplify this, without completely changing the
hci_connect_pa_sync() -> create_pa_complete() flow.

***

An alternative uAPI here could be to have listen() block until PAST
listener is ready. The time it blocks is however not well bounded if
hci_sync queue is stuck doing something else first. We probably want to
avoid stalls in bluetoothd, so went with async notification here.

***

Testing: test_bass_past_transport_acquire: 24/24 passed at -n 6
(pre-patch baseline: 6/24 failed). In --btvirt-debug delegator 0x205c
(LE Set PAST Parameters) is before assistant 0x205b (transfer) in every
run. Pre-patch failures show the inverse (0x205b first, or in same
btvirt instant).

Not tested on real hardware, only on the emulator.

LLM was used for triage + patch review + test writing.  Design and
implementation is mine.

Assisted-by: opencode:qwen3.8-flash-next opencode:gpt-6-sol

Pauli Virtanen (2):
  Bluetooth: hci_sync: add request wait hook for __hci_cmd_sync_sk
  Bluetooth: ISO: Signal BIS PAST ready to userspace via POLLOUT

 include/net/bluetooth/bluetooth.h |   2 +
 include/net/bluetooth/hci_sync.h  |  10 ++-
 net/bluetooth/hci_core.c          |   9 ++-
 net/bluetooth/hci_sync.c          | 109 ++++++++++++++++++++++++------
 net/bluetooth/iso.c               |  45 +++++++++++-
 5 files changed, 150 insertions(+), 25 deletions(-)

-- 
2.56.0