Added idr_replace() function which was apparently added in Linux 2.6.18

Someone should probably double-check my work here since this is the
first time I've touched drm_compat.[ch]

linux-core/drm_compat.c

@@ -678,4 +678,51 @@ void idr_remove_all(struct idr *idp)
        idp->layers = 0;
 }
 EXPORT_SYMBOL(idr_remove_all);
+
+#endif /* DRM_IDR_COMPAT_FN */
+
+
+
+#if (LINUX_VERSION_CODE < KERNEL_VERSION(2,6,18))
+/**
+ * idr_replace - replace pointer for given id
+ * @idp: idr handle
+ * @ptr: pointer you want associated with the id
+ * @id: lookup key
+ *
+ * Replace the pointer registered with an id and return the old value.
+ * A -ENOENT return indicates that @id was not found.
+ * A -EINVAL return indicates that @id was not within valid constraints.
+ *
+ * The caller must serialize vs idr_find(), idr_get_new(), and idr_remove().
+ */
+void *idr_replace(struct idr *idp, void *ptr, int id)
+{
+	int n;
+	struct idr_layer *p, *old_p;
+
+	n = idp->layers * IDR_BITS;
+	p = idp->top;
+
+	id &= MAX_ID_MASK;
+
+	if (id >= (1 << n))
+		return ERR_PTR(-EINVAL);
+
+	n -= IDR_BITS;
+	while ((n > 0) && p) {
+		p = p->ary[(id >> n) & IDR_MASK];
+		n -= IDR_BITS;
+	}
+
+	n = id & IDR_MASK;
+	if (unlikely(p == NULL || !test_bit(n, &p->bitmap)))
+		return ERR_PTR(-ENOENT);
+
+	old_p = p->ary[n];
+	p->ary[n] = ptr;
+
+	return (void *)old_p;
+}
+EXPORT_SYMBOL(idr_replace);
 #endif

linux-core/drm_compat.h

@@ -316,4 +316,9 @@ int idr_for_each(struct idr *idp,
 void idr_remove_all(struct idr *idp);
 #endif
 
+
+#if (LINUX_VERSION_CODE < KERNEL_VERSION(2,6,18))
+void *idr_replace(struct idr *idp, void *ptr, int id);
+#endif
+
 #endif