linux-kernel - [PATCH 2/2] mfd: syscon: Don't auto create "no-auto-mmio" syscons

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Message-ID: <5d94d26b71dc74c6891a2c2b22fcee879e3737b1.1761753288.git.dan.carpenter@linaro.org>
Date: Wed, 29 Oct 2025 20:27:10 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Lee Jones <lee@...nel.org>
Cc: Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] mfd: syscon: Don't auto create "no-auto-mmio" syscons

Most syscons are created automaticall and accessed via MMIO. However,
some syscons can only be accessed via a custom regmap because they are
only accessible via the secure partition or via SCMI etc.  We register
these custom syscons by calling the of_syscon_register_regmap()
function.

However, the problem is that syscons are generally used by looking up
the phandle and if the syscon does not exist it is automatically
created as an MMIO syscon.  This creates an ordering problem where we
need to register the syscon before any other driver looks up a
phandle.

The fix is to add a "no-auto-mmio" property to the device tree.  That
will prevent the syscon from being created as an MMIO syscon.  When
the driver looks up the phandle it will get -EPROBE_DEFER if the
syscon has not been registered yet.

Signed-off-by: Dan Carpenter <dan.carpenter@...aro.org>
---
 drivers/mfd/syscon.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/mfd/syscon.c b/drivers/mfd/syscon.c
index ae71a2710bed..063526f039d0 100644
--- a/drivers/mfd/syscon.c
+++ b/drivers/mfd/syscon.c
@@ -183,7 +183,7 @@ static struct regmap *device_node_get_regmap(struct device_node *np,
 		if (create_regmap)
 			syscon = of_syscon_register(np, check_res);
 		else
-			syscon = ERR_PTR(-EINVAL);
+			syscon = ERR_PTR(-EPROBE_DEFER);
 	}
 	mutex_unlock(&syscon_list_lock);

@@ -271,7 +271,13 @@ EXPORT_SYMBOL_GPL(device_node_to_regmap);
  */
 struct regmap *syscon_node_to_regmap(struct device_node *np)
 {
-	return device_node_get_regmap(np, of_device_is_compatible(np, "syscon"), true);
+	bool create = false;
+
+	if (of_device_is_compatible(np, "syscon") &&
+	    !of_get_property(np, "no-auto-mmio", NULL))
+		create = true;
+
+	return device_node_get_regmap(np, create, true);
 }
 EXPORT_SYMBOL_GPL(syscon_node_to_regmap);

-- 
2.51.0