Claude review: dma: coherent: store reserved memory coherent regions

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

Patch Review

**Issue: Static array with hardcoded limit**

```c
#define MAX_COHERENT_REGIONS 64

static struct reserved_mem *rmem_coherent_areas[MAX_COHERENT_REGIONS];
static unsigned int rmem_coherent_areas_num;
```

64 regions is probably sufficient for practical purposes, but using a static array feels fragile. Since `rmem_dma_setup()` is called early (from `__reservedmem_of_table`), dynamic allocation may not be available, so this is acceptable. However, the error return value should be `-ENOSPC` rather than `-EINVAL`:

```c
	if (rmem_coherent_areas_num >= MAX_COHERENT_REGIONS) {
		...
		return -EINVAL;
	}
```

`-ENOSPC` or `-ENOMEM` would be more descriptive.

**Issue: `#include <linux/dma-heap.h>` added to coherent.c but not used**

```c
+#include <linux/dma-heap.h>
```

This header is added but nothing from it is used in this file. The `IS_ENABLED(CONFIG_DMABUF_HEAPS_COHERENT)` check doesn't require it. This include should be dropped.

**Design concern: `IS_ENABLED()` check couples DMA core to heap config**

```c
	if (IS_ENABLED(CONFIG_DMABUF_HEAPS_COHERENT)) {
		int ret = rmem_coherent_insert_area(rmem);
```

This is noted in the commit message. The `IS_ENABLED()` approach means the storage array and insertion code are always compiled in but guarded at runtime. Since this is `__init` code, it's not a runtime concern, but it does add code to `kernel/dma/coherent.c` that only matters when the heap module is loaded. An alternative would be to have the heap module iterate reserved memory regions itself, avoiding coupling. However, since `rmem_dma_setup()` is an `__init` function and the rmem structs are available post-init, the module could potentially walk the reserved_mem array directly. That said, this approach works.

**Thread safety**: The array is populated at `__init` time (single-threaded) and read later by the module init. No races possible. Fine.

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