Claude review: drm/sun4i: Use backend/mixer as dedicated DMA device

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

Patch Review

This patch replaces the `of_dma_configure()` hack with proper `drm_dev_set_dma_dev()` calls. The pattern used is:

```c
+	if (drm_dev_dma_dev(drm) == drm->dev)
+		drm_dev_set_dma_dev(drm, dev);
```

This "set only if not already set" logic ensures only the first bound backend/mixer becomes the DMA device, which matches the intent described in the comment. This is correct since `drm_dev_dma_dev()` returns `drm->dev` when no dedicated device has been set (i.e., `dma_dev` is NULL).

A few minor observations:

1. **Comment mismatch in the mixer**: The new comment in `sun8i_mixer.c` says "backends" in two places:

   ```c
   +	 * Always use the first bound backend as the DMA device. While
   +	 * our device trees always have all backends enabled, some in
   ```

   This should say "mixer" instead of "backend" since this is the mixer code. It looks like the comment was copied from `sun4i_backend.c` without adjustment.

2. **Removal of the conditional check**: The old backend code checked `of_property_present(dev->of_node, "interconnects")` and the old mixer code checked `of_property_present(dev->of_node, "iommus")` before calling `of_dma_configure()`. The new code unconditionally sets the DMA device. This is fine — `drm_dev_set_dma_dev()` just takes a device reference and stores it; it doesn't configure anything DMA-related on the device itself. The backend/mixer device already has its DMA configured correctly by the bus code. So this simplification is safe and arguably more correct.

3. **No unbind cleanup**: Neither the backend nor mixer unbind paths clear the DMA device (`drm_dev_set_dma_dev(drm, NULL)`). Looking at the `drm_dev_set_dma_dev` implementation, it does `get_device(dma_dev)` and later `put_device(dev->dma_dev)`, so the DRM device holds a reference to the component device. Since the DRM device is torn down via component framework and `drm_dev_put()` should handle releasing `dma_dev`, this should be fine in practice — but it's worth considering whether there's a lifecycle issue if the backend/mixer device is unbound while the DRM device still holds a reference to it. The `drm_dev_set_dma_dev` function does take a reference via `get_device()`, which would keep the device alive. This matches how MediaTek handles it (no cleanup in deinit either), so it's consistent.

**Summary**: The only concrete issue is the copy-paste comment error in `sun8i_mixer.c` saying "backend" instead of "mixer". Everything else looks correct.

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Generated by Claude Code Patch Reviewer