Claude review: drm/atomic: Return user readable error in atomic_ioctl

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

Patch Review

**Issues:**

```c
+	if (!arg->reserved)
+		drm_dbg_atomic(dev,
+			       "memory not allocated for drm_atomic error reporting\n");
+	else
+		/* Update the error code if any error to allow user handling it */
+		error_code_ptr = (struct drm_mode_atomic_err_code __user *)
+				 (unsigned long)arg->reserved;
```
- **No Validation**: Userspace pointer is blindly trusted. No `access_ok()` check.
- **Comment After else**: Comment should explain what happens, not repeat what code does
- **Double Cast**: The `(unsigned long)` cast is unnecessary noise

```c
+	memset(&state->error_code, 0, sizeof(*error_code_ptr));
```
- **Size Mismatch**: Using `sizeof(*error_code_ptr)` (userspace size) to clear kernel's `state->error_code`. Should be `sizeof(state->error_code)`. These happen to be the same type, but the pattern is wrong.

```c
	if (!file_priv->atomic) {
+		drm_mode_atomic_add_error_msg(&state->error_code,
+					      DRM_MODE_ATOMIC_INVALID_API_USAGE,
+					      "drm atomic capability not enabled");
		drm_dbg_atomic(dev,
			       "commit failed: atomic cap not enabled\n");
```
- **Duplication**: Error message is duplicated between `add_error_msg` and `drm_dbg_atomic`. The `drm_dbg_atomic` call inside `add_error_msg` already logs.

```c
	if (arg->flags & ~DRM_MODE_ATOMIC_FLAGS) {
-		drm_dbg_atomic(dev, "commit failed: invalid flag\n");
-		ret = -EINVAL;
-		goto out;
-	}
-
-	if (arg->reserved) {
-		drm_dbg_atomic(dev, "commit failed: reserved field set\n");
```
- **Logic Error**: The removal of `arg->reserved` check means old code that validated reserved==0 is gone. This is an ABI break.

```c
+out:
+	/* Update the error code if any error to allow user handling it */
+	if (ret < 0 && arg->reserved) {
+		if (copy_to_user(error_code_ptr, &state->error_code,
+				 sizeof(state->error_code)))
+			return -EFAULT;
+	}
```
- **Cleanup Leak**: Returning -EFAULT directly bypasses remaining cleanup code (modeset locks, state free). Should set `ret = -EFAULT;` and fall through.
- **Error Path**: If `copy_to_user` fails, we're already in an error path and now leaking more resources.

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Generated by Claude Code Patch Reviewer