Remove unnecessary ISO Rx/Tx ack serialization

[Copilot]

ISO TX acknowledgments were being serialized with RX processing in rx_demux(), similar to ACL connections. This serialization is unnecessary for ISO—TX acks can be processed independently.

Changes

Removed ISO from rx_demux (ull.c)

• Eliminated ISO ack peeking and processing from main RX demux loop
• Removed rx_demux_iso_tx_ack() function

Added independent ISO TX ack handler (ull_iso.c)

• New iso_tx_ack_demux() processes ISO TX acks in separate mayfly context
• ull_iso_lll_ack_enqueue() now schedules independent processing after MFIFO enqueue
• Helper functions made static (internal-only)

Cleaned up exports (ull_iso_internal.h)

• Removed 4 functions no longer needed externally

Architecture

Before:

LLL → iso_ack MFIFO → rx_demux() (with RX serialization)

After:

LLL → iso_ack MFIFO → iso_tx_ack_demux() (independent)

ISO TX acks now follow the same independent processing pattern as ISO RX (iso_rx_demux()), eliminating unnecessary coupling between ISO RX and TX paths.

Net: -48 lines

Original prompt

Add iso_ack MFIFO for Context-Safe ISO TX Acknowledgment

Problem Statement

The current implementation in subsys/bluetooth/controller/ll_sw/ull_iso.c has architectural violations where LLL (Link Layer Low) context directly calls ULL (Upper Link Layer) functions:

1. FIXME at lines 1487-1493: "ll_tx_ack_put is not LLL callable" - for CIS without vendor data path
2. FIXME at lines 1499-1505: "ll_tx_ack_put is not LLL callable" - for BIS/ADV_ISO
3. TODO at lines 1496-1497: "Can be unified with CIS and use ISOAL"

Current Unsafe Pattern

void ull_iso_lll_ack_enqueue(uint16_t handle, struct node_tx_iso *node_tx)
{
    // Called from LLL context
    ll_tx_ack_put(handle, (void *)node_tx);  // ❌ ULL function!
    ll_rx_sched();  // ❌ Context violation
}

Design Requirement

Implement an iso_ack MFIFO similar to the existing conn_ack MFIFO pattern used for ACL connections to provide context-safe handoff between LLL and ULL.

Reference: ACL TX Ack Design Pattern

File: subsys/bluetooth/controller/ll_sw/ull_conn.c

// MFIFO definition (lines 125-127)
static MFIFO_DEFINE(conn_ack, sizeof(struct lll_tx),
                    (CONFIG_BT_BUF_ACL_TX_COUNT + LLCP_TX_CTRL_BUF_COUNT));

// LLL enqueue (lines 1531-1541)
void ull_conn_lll_ack_enqueue(uint16_t handle, struct node_tx *tx)
{
    struct lll_tx *lll_tx;
    uint8_t idx;

    idx = MFIFO_ENQUEUE_GET(conn_ack, (void **)&lll_tx);
    LL_ASSERT_ERR(lll_tx);

    lll_tx->handle = handle;
    lll_tx->node = tx;

    MFIFO_ENQUEUE(conn_ack, idx);
}

// ULL dequeue (lines 1493-1507)
memq_link_t *ull_conn_ack_peek(uint8_t *ack_last, uint16_t *handle,
                                struct node_tx **tx)
{
    struct lll_tx *lll_tx;

    lll_tx = MFIFO_DEQUEUE_GET(conn_ack);
    if (!lll_tx) {
        return NULL;
    }

    *ack_last = mfifo_fifo_conn_ack.l;
    *handle = lll_tx->handle;
    *tx = lll_tx->node;

    return (*tx)->link;
}

Processing in ULL context (ull.c lines 2700-2760):

• rx_demux() calls rx_demux_conn_tx_ack() in ULL context
• Serializes TX acks with RX processing
• Eventually calls ll_tx_ack_put() safely in ULL context

Implementation Design

Step 1: Define iso_ack MFIFO

File: subsys/bluetooth/controller/ll_sw/ull_iso.c

Add after existing includes and before function definitions:

#if defined(CONFIG_BT_CTLR_ADV_ISO) || defined(CONFIG_BT_CTLR_CONN_ISO)
// MFIFO for ISO TX acknowledgments from LLL to ULL
static MFIFO_DEFINE(iso_ack, sizeof(struct lll_tx), 
                    BT_CTLR_ISO_TX_PDU_BUFFERS);
#endif

Step 2: Modify LLL Enqueue Function

Replace ull_iso_lll_ack_enqueue() (lines 1469-1508) to use MFIFO:

#if defined(CONFIG_BT_CTLR_ADV_ISO) || defined(CONFIG_BT_CTLR_CONN_ISO)
void ull_iso_lll_ack_enqueue(uint16_t handle, struct node_tx_iso *node_tx)
{
    // Check if datapath exists and use ISOAL if available
    if (IS_ENABLED(CONFIG_BT_CTLR_CONN_ISO) && IS_CIS_HANDLE(handle)) {
        struct ll_conn_iso_stream *cis;
        struct ll_iso_datapath *dp;

        cis = ll_conn_iso_stream_get(handle);
        dp  = cis->hdr.datapath_in;

        if (dp) {
            // Use ISOAL for datapath case
            isoal_tx_pdu_release(dp->source_hdl, node_tx);
            return;
        }
#if defined(CONFIG_BT_CTLR_ISO_VENDOR_DATA_PATH)
        ll_data_path_tx_pdu_release(handle, node_tx);
        return;
#endif
    } else if (IS_ENABLED(CONFIG_BT_CTLR_ADV_ISO) && IS_ADV_ISO_HANDLE(handle)) {
        struct lll_adv_iso_stream *stream;
        struct ll_iso_datapath *dp;
        uint16_t stream_handle;

        stream_handle = LL_BIS_ADV_IDX_FROM_HANDLE(handle);
        stream = ull_adv_iso_stream_get(stream_handle);
        dp = stream ? stream->dp : NULL;

        if (dp) {
            // ✓ UNIFIED: Use ISOAL like CIS
            isoal_tx_pdu_release(dp->source_hdl, node_tx);
            return;
        }
#if defined(CONFIG_BT_CTLR_ISO_VENDOR_DATA_PATH)
        ll_data_path_tx_pdu_release(handle, node_tx);
        return;
#endif
    }

    // ✓ Context-safe fallback: Enqueue to iso_ack MFIFO
    struct lll_tx *lll_tx;
    uint8_t idx;

    idx = MFIFO_ENQUEUE_GET(iso_ack, (void **)&lll_tx);
    LL_ASSERT_ERR(lll_tx);

    lll_tx->handle = handle;
    lll_tx->node = (struct node_tx *)node_tx;

    MFIFO_ENQUEUE(iso_ack, idx);
}
#endif /* CONFIG_BT_CTLR_ADV_ISO || CONFIG_BT_CTLR_CONN_ISO */

Step 3: Add ULL Dequeue Functions

Add helper functions for ULL to process iso_ack MFIFO:

#if defined(CONFIG_BT_CTLR_ADV_ISO) || defined(CONFIG_BT_CTLR_CONN_ISO)
/**
 * @brief Get last index of iso_ack MFIFO for serialization
 */
uint8_t ull_iso_ack_last_idx_get(void)
{
    return mfifo_fifo_iso_ack.l;
}

/**
 * @brief Peek at next ISO TX ack without dequeuing
 */
static memq_link_t *ull_iso_ack_peek(uint8_t *ack_last, uint16_t *handle,
                                     struct node_tx_iso **tx)
{
    struct lll_tx *lll_tx;

    lll_tx = MFIFO...

</details>



<!-- START COPILOT CODING AGENT SUFFIX -->

*This pull request was created from Copilot chat.*
>

<!-- START COPILOT CODING AGENT TIPS -->
---

✨ Let Copilot coding agent [set things up for you](https://github.com/cvinayak/zephyr/issues/new?title=✨+Set+up+Copilot+instructions&body=Configure%20instructions%20for%20this%20repository%20as%20documented%20in%20%5BBest%20practices%20for%20Copilot%20coding%20agent%20in%20your%20repository%5D%28https://gh.io/copilot-coding-agent-tips%29%2E%0A%0A%3COnboard%20this%20repo%3E&assignees=copilot) — coding agent works faster and does higher quality work when set up for your repo.