Clean up MRAM no-latency unit test after race mitigation redesign

[Copilot]

The unit test was updated to validate a redesigned MRAM no-latency management that handles multiple radio_reset()/radio_stop() calls before async callback completion. This PR cleans up the implementation.

Changes

• Remove dead code: Deleted #if 0 block containing superseded callback implementation
• Add missing include: <zephyr/sys/atomic.h> for atomic operations (atomic_inc, atomic_dec, atomic_get)
• Document race mitigation design: Comprehensive overview of the asynchronous state management strategy

Design Summary

The implementation prevents race conditions through:

1. Atomic reference counting (mram_refcnt): Only first reset requests MRAM no-latency, only last stop releases
2. Request/release tracking (start_req/stop_req): Volatile counters accumulate operations between callbacks
3. Callback batching: Processes accumulated requests/releases atomically, handling scenarios like:
   • reset → stop(pending) → callback → release
   • reset → stop → reset → callback → state_retained

// Callback counts operations since last acknowledgment
uint8_t req = mram_no_latency_start_req - mram_no_latency_start_ack;
uint8_t rel = mram_no_latency_stop_req - mram_no_latency_stop_ack;

// Executes cancel/release if releases exceed requests
if (rel > req) {
    mram_no_latency_cancel_or_release();
}

This mirrors the actual hal_radio_reset()/hal_radio_stop() implementation for nRF54H20 cpurad Bluetooth Controller support.

Original prompt

Context

PR zephyrproject-rtos#95417 adds nRF54H20 cpurad support for the Bluetooth Controller (bt_ll_sw_split). The nRF54H20's hal_radio_reset() and hal_radio_stop() in radio_nrf54hx.h manage hardware resources (currently nrf_lrcconf clock). MRAM no-latency management (mram_no_latency_request() / mram_no_latency_cancel_or_release()) needs to be integrated and its reference-counting behavior validated.

Task

Add a unit test file at tests/bluetooth/controller/radio_nrf54hx_mram/src/main.c (with appropriate testcase.yaml and CMakeLists.txt and prj.conf) that tests the MRAM no-latency state management with stubs for mram_no_latency_request() and mram_no_latency_cancel_or_release().

The test should exercise the state by calling radio_reset() (which calls mram_no_latency_request()) and radio_stop() (which calls mram_no_latency_cancel_or_release()), and then invoking mram_no_latency_callback() to resolve the async state.

Test invariants to verify:

1. n calls to mram_no_latency_request() and n calls to mram_no_latency_cancel_or_release(), then when mram_no_latency_callback() is called → mram_no_latency is OFF.

2. (n+1) calls to mram_no_latency_request() and n calls to mram_no_latency_cancel_or_release(), then when mram_no_latency_callback() is called → mram_no_latency is ON.

Implementation details:

• Stubs: Create stub implementations for mram_no_latency_request() and mram_no_latency_cancel_or_release() that track a pending_requests counter (incremented on request, decremented on release).

• mram_no_latency_callback(): A test function that resolves state: if pending_requests > 0 → mram_no_latency is ON; if pending_requests == 0 → mram_no_latency is OFF.

• radio_reset() / radio_stop(): Simplified functions that call the stubs, mirroring what hal_radio_reset() / hal_radio_stop() in radio_nrf54hx.h would do.

Test cases to include:

• test_n_requests_n_releases_then_callback_off: For n = 1, 2, 3, 5, 10 — verify mram_no_latency is OFF after equal requests and releases + callback.
• test_n_plus_1_requests_n_releases_then_callback_on: For n = 0, 1, 2, 3, 5, 10 — verify mram_no_latency is ON after (n+1) requests and n releases + callback.
• test_interleaved_reset_stop: Interleave radio_reset/radio_stop calls, verify state after each callback.
• test_single_radio_event_cycle: Single reset → stop → callback → OFF.
• test_single_reset_no_stop: Single reset → callback → ON.
• test_zero_requests_callback_off: No operations → callback → OFF.
• test_multiple_callbacks: Multiple callbacks between operations maintain correct state.

Test file structure:

tests/bluetooth/controller/radio_nrf54hx_mram/
├── CMakeLists.txt
├── prj.conf
├── testcase.yaml
└── src/
    └── main.c

Reference files:

• radio_nrf54hx.h (new in PR): subsys/bluetooth/controller/ll_sw/nordic/hal/nrf5/radio/radio_nrf54hx.h — contains hal_radio_reset() and hal_radio_stop() with nrf_lrcconf clock management at lines 398-412.
• mram_latency.h: soc/nordic/common/mram_latency.h — declares mram_no_latency_request(), mram_no_latency_cancel_or_release() using struct onoff_client.
• mram_latency.c: soc/nordic/common/mram_latency.c — real implementation using onoff_manager.
• Existing MRAM latency tests: tests/boards/nrf/mram_latency/src/main.c — for reference on test patterns.

Use the Zephyr ztest framework. The test should be a native/unit test that can run without hardware (using native_posix or similar).

This pull request was created from Copilot chat.

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