Claude review: gpu: nova-core: fsp: wait to consume message before sending another

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Purpose:** Before sending a new message to FSP via EMEM, poll until the FSP's queue head equals its queue tail, indicating the previous message has been consumed.

The diff adds:

```rust
+        // Wait for FSP to consume any previous message before sending.
+        read_poll_timeout(
+            || {
+                let head = bar.read(regs::NV_PFSP_QUEUE_HEAD).address().get();
+                let tail = bar.read(regs::NV_PFSP_QUEUE_TAIL).address().get();
+                Ok(head == tail)
+            },
+            |&ready| ready,
+            Delta::from_millis(1),
+            Delta::from_secs(2),
+        )?;
```

**Observations:**

- **Correct use of `read_poll_timeout`**: The signature is `(op, cond, sleep_delta, timeout_delta) -> Result<T>`. The `op` closure returns `Ok(bool)`, the `cond` checks `|&ready| ready`, and the `?` propagates the `ETIMEDOUT` error. This matches the established pattern used in `gfw.rs` and elsewhere in nova-core.

- **Timeout value (2 seconds)**: This is reasonable for FSP message consumption. If the FSP is alive and functioning, it should consume messages quickly. 2 seconds is generous enough to handle edge cases without stalling the driver indefinitely.

- **No error context on timeout**: Unlike patch 3 which uses `.inspect_err()` to log a message on timeout, this poll silently returns `ETIMEDOUT`. Consider adding an `.inspect_err()` call to log a message like `"FSP did not consume previous message"` — this would help debug unload/reload failures in the field. Minor nit, not a blocker.

- **Placement before `write_emem`**: This is the correct location — ensuring the queue is drained before writing new data to EMEM and updating the queue tail.

**Verdict:** Looks good. The only suggestion is adding error logging on timeout for consistency with other error paths in the driver.

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Generated by Claude Code Patch Reviewer