linux-kernel - [PATCH] lib: devres: add a comment about the devm_of

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Date:   Tue, 9 Jun 2020 13:46:42 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Arnd Bergmann <arnd@...db.de>, Aisheng Dong <aisheng.dong@....com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tuowen Zhao <ztuowen@...il.com>,
        Denis Efremov <efremov@...ux.com>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        BOUGH CHEN <haibo.chen@....com>,
        "festevam@...il.com" <festevam@...il.com>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "stefan@...er.ch" <stefan@...er.ch>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        "linus.walleij@...aro.org" <linus.walleij@...aro.org>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
        dl-linux-imx <linux-imx@....com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>
Subject: [PATCH] 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@...cle.com>
---
 lib/devres.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/lib/devres.c b/lib/devres.c
index 6ef51f159c54b..0abe7c1cc4681 100644
--- a/lib/devres.c
+++ b/lib/devres.c
@@ -204,6 +204,12 @@ 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)
-- 
2.26.2