Claude review: accel/ivpu: Document why full JSM message size is always used

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

Patch Review

**Assessment: No issues found.**

The existing code at `ivpu_ipc.c:100` has this TODO:
```c
/* TODO: Set data_size to actual JSM message size, not union of all messages */
```

This implies `sizeof(*req)` (where `req` is `struct vpu_jsm_msg *`) is a bug or technical debt to be fixed. The patch replaces it with:

```c
/*
 * Firmware expects full JSM message size regardless of the payload size.
 * Unused fields must be zeroed to allow future extensions of existing
 * commands without breaking compatibility.
 */
```

The replacement comment is accurate and well-justified:

1. **Zeroing is handled**: `memset(tx_buf, 0, sizeof(*tx_buf))` at line 98 zeroes the entire buffer including the JSM portion, which supports the "unused fields must be zeroed" claim.

2. **`vpu_jsm_msg` is indeed a union-based struct**: Looking at the callers in `ivpu_jsm_msg.c`, every message is declared as `struct vpu_jsm_msg req = { .type = ... }` and only populates the relevant payload union member, confirming that the full struct size is always sent regardless of which command is used.

3. **The comment follows kernel multi-line comment style** (`/* ... */` block) correctly.

**Minor observation**: The commit message is well-written and self-contained — it explains both the firmware requirement and the forward-compatibility rationale without being verbose.

No concerns with this patch. It can be applied as-is.

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