mfd: syscon: Allow syscon nodes without a "syscon" compatible

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>
Tested-by: Will McVicker <willmcvicker@google.com>
Reviewed-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20241217-syscon-fixes-v2-3-4f56d750541d@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>

drivers/mfd/syscon.c

@@ -171,9 +171,12 @@ static struct regmap *device_node_get_regmap(struct device_node *np,
 			break;
 		}
 
-	if (!syscon)
-		syscon = of_syscon_register(np, check_res);
-
+	if (!syscon) {
+		if (of_device_is_compatible(np, "syscon"))
+			syscon = of_syscon_register(np, check_res);
+		else
+			syscon = ERR_PTR(-EINVAL);
+	}
 	mutex_unlock(&syscon_list_lock);
 
 	if (IS_ERR(syscon))
@@ -238,9 +241,6 @@ 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);