[PATCH|RFC] of: let of_match_device() always return best match

[Marc Kleine-Budde <mkl@pengutronix.de>]

The function of_match_device() should tell if a struct device matches an
of_device_id list and return the specific entry of that table matches the
device best.

The underlying __of_match_node() implements the wrong search algorithm. It
iterates over the list of of_device_ids, comparing the first compatible with
_all_ compatibles of the struct device, then the second compatible of
of_device_id and so on.

This leads to a problem, if the device has more than one compatible that match
the of_device_id list. The implemented search algorithm may find not the "best"
match. As the compatible list in the device is sorted from most to least
specific.

For example:

The imx28.dtsi gives this compatible string for its CAN core:

> 	compatible = "fsl,imx28-flexcan", "fsl,p1010-flexcan";

The flexcan driver defines:

> static const struct of_device_id flexcan_of_match[] = {
> 	{ .compatible = "fsl,p1010-flexcan", .data = &fsl_p1010_devtype_data, },
> 	{ .compatible = "fsl,imx28-flexcan", .data = &fsl_imx28_devtype_data, },
> 	{ .compatible = "fsl,imx6q-flexcan", .data = &fsl_imx6q_devtype_data, },
> 	{ /* sentinel */ },
> };

The "p1010" was the first Freescale SoC with the flexcan core. But this SoC has
a bug, so a workaround has to be enabled in the driver. The mx28 has this bug
fixed, so we don't need this quite costly workaround.

The __of_match_node() will compare:

    from  of_device_id		from device
    fsl,p1010-flexcan		fsl,imx28-flexcan
    fsl,p1010-flexcan		fsl,p1010-flexcan	-> MATCH

The of_match_device() function as it currently is implemented will always
return p1010 not the mx28.

This patch fixes the problem by exchanging outer and inner loop. The first
compatible of a device is compared against all compatible from the of_device_id
list, then the second device compatible and so on.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
---
 drivers/of/base.c | 40 +++++++++++++++++++++++++++++++++++-----
 1 file changed, 35 insertions(+), 5 deletions(-)

diff --git a/drivers/of/base.c b/drivers/of/base.c
index 865d3f6..5bff2fe 100644
--- a/drivers/of/base.c
+++ b/drivers/of/base.c
@@ -349,6 +349,35 @@ static int __of_device_is_compatible(const struct device_node *device,
 	return 0;
 }
 
+/** Returns the specific of_device_id, if the it matches one of the
+ * strings in the device's "compatible" property
+ */
+static
+const struct of_device_id *__of_device_matches_compatible(const struct device_node *device,
+							  const struct of_device_id *matches)
+{
+	const struct of_device_id *m;
+	const char* cp;
+	int cplen, l;
+
+	cp = __of_get_property(device, "compatible", &cplen);
+	if (cp == NULL)
+		return NULL;
+	while (cplen > 0) {
+		m = matches;
+		while (m->compatible[0]) {
+			if (of_compat_cmp(cp, m->compatible, strlen(m->compatible)) == 0)
+				return m;
+			m++;
+		}
+		l = strlen(cp) + 1;
+		cp += l;
+		cplen -= l;
+	}
+
+	return NULL;
+}
+
 /** Checks if the given "compat" string matches one of the strings in
  * the device's "compatible" property
  */
@@ -717,22 +746,23 @@ const struct of_device_id *__of_match_node(const struct of_device_id *matches,
 {
 	if (!matches)
 		return NULL;
-
-	while (matches->name[0] || matches->type[0] || matches->compatible[0]) {
+	while (matches->name[0] || matches->type[0]) {
 		int match = 1;
+
 		if (matches->name[0])
 			match &= node->name
 				&& !strcmp(matches->name, node->name);
 		if (matches->type[0])
 			match &= node->type
 				&& !strcmp(matches->type, node->type);
-		if (matches->compatible[0])
-			match &= __of_device_is_compatible(node,
-							   matches->compatible);
 		if (match)
 			return matches;
 		matches++;
 	}
+
+	if (matches->compatible[0])
+		return __of_device_matches_compatible(node, matches);
+
 	return NULL;
 }
 
-- 
1.8.4.rc3