lib: devres: add a comment about the devm_of_iomap() function

We recently introduced a bug when we tried to convert of_iomap() to
devm_of_iomap().  The problem was that there were two drivers mapping
the same io region.  The first driver was using of_iomap() and the
second driver was using devm_of_iomap() and the kernel booted fine.
When we converted the first drive to use devm_of_iomap() then the second
driver failed with -EBUSY and the kernel couldn't boot.

Let's add a comment to prevent this sort of mistake in the future.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200609104642.GA43074@mwanda
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

lib/devres.c

@@ -213,6 +213,12 @@ void __iomem *devm_ioremap_resource_wc(struct device *dev,
  *	base = devm_of_iomap(&pdev->dev, node, 0, NULL);
  *	if (IS_ERR(base))
  *		return PTR_ERR(base);
+ *
+ * Please Note: This is not a one-to-one replacement for of_iomap() because the
+ * of_iomap() function does not track whether the region is already mapped.  If
+ * two drivers try to map the same memory, the of_iomap() function will succeed
+ * but the the devm_of_iomap() function will return -EBUSY.
+ *
  */
 void __iomem *devm_of_iomap(struct device *dev, struct device_node *node, int index,
 			    resource_size_t *size)