[PATCH 3/3] mfd: syscon: Allow syscon nodes without a "syscon" compatible

["Rob Herring (Arm)" <robh@kernel.org>]

of_syscon_register_regmap() was added for nodes which need a custom
regmap setup. It's not really correct for those nodes to claim they are
compatible with "syscon" as the default handling likely doesn't work in
those cases. If device_node_get_regmap() happens to be called first,
then of_syscon_register() will be called and an incorrect regmap will be
created (barring some other error). That may lead to unknown results in
the worst case. In the best case, of_syscon_register_regmap() will fail
with -EEXIST. This problem remains unless these cases drop "syscon" (an
ABI issue) or we exclude them using their specific compatible. ATM,
there is only one user: "google,gs101-pmu"

There are also cases of adding "syscon" compatible to existing nodes
after the fact in order to register the syscon. That presents a
potential DT ABI problem. Instead, if there's a kernel change needing a
syscon for a node, then it should be possible to allow the kernel to
register a syscon without a DT change. That's only possible by using
of_syscon_register_regmap() currently, but in the future we may want to
support a match list for cases which don't need a custom regmap.

With this change, the lookup functions will succeed for any node
registered by of_syscon_register_regmap() regardless of whether the node
compatible contains "syscon".

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
---
 drivers/mfd/syscon.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/mfd/syscon.c b/drivers/mfd/syscon.c
index bfb1f69fcff1..e6df2825c14d 100644
--- a/drivers/mfd/syscon.c
+++ b/drivers/mfd/syscon.c
@@ -171,8 +171,10 @@ static struct regmap *device_node_get_regmap(struct device_node *np,
 			break;
 		}
 
-	if (!syscon)
+	if (!syscon && of_device_is_compatible(np, "syscon"))
 		syscon = of_syscon_register(np, check_res);
+	else
+		syscon = ERR_PTR(-EINVAL);
 
 	mutex_unlock(&syscon_list_lock);
 
@@ -238,9 +240,6 @@ EXPORT_SYMBOL_GPL(device_node_to_regmap);
 
 struct regmap *syscon_node_to_regmap(struct device_node *np)
 {
-	if (!of_device_is_compatible(np, "syscon"))
-		return ERR_PTR(-EINVAL);
-
 	return device_node_get_regmap(np, true);
 }
 EXPORT_SYMBOL_GPL(syscon_node_to_regmap);

-- 
2.45.2