[PATCH 0/2] i2c: mux: reg: allow fixing the base bus number via fwnode

[PATCH 0/2] i2c: mux: reg: allow fixing the base bus number via fwnode

[Abdurrahman Hussain]

Add an "i2c-mux-base-bus-num" device property to i2c-mux-reg so
DT/ACPI/swnode instances can request a fixed base bus number; the
legacy platdata path already supports this via data.base_nr.

Patch 1 converts the legacy Documentation/devicetree/bindings/i2c/
i2c-mux-reg.txt binding to the YAML schema and documents the new
property. Patch 2 reads the property in the driver.

The series applies on top of the device-property accessor conversion
queued in Wolfram's wsa.git i2c/for-andi:

  c191ec71250b ("i2c: mux: reg: use device property accessors")
  https://lore.kernel.org/all/20260519-i2c-mux-reg-v6-1-a27ff50dee67@nexthop.ai/

Without that conversion there is no probe-time hook on the OF path
for the new property the driver reads.

Signed-off-by: Abdurrahman Hussain <abdurrahman@nexthop.ai>
---
Abdurrahman Hussain (2):
      dt-bindings: i2c: convert i2c-mux-reg to DT schema
      i2c: mux: reg: allow fixing the base bus number via fwnode

 .../devicetree/bindings/i2c/i2c-mux-reg.txt        |  74 --------------
 .../devicetree/bindings/i2c/i2c-mux-reg.yaml       | 110 +++++++++++++++++++++
 drivers/i2c/muxes/i2c-mux-reg.c                    |   4 +-
 3 files changed, 113 insertions(+), 75 deletions(-)
---
base-commit: aa61612ab641d7d62b0b6889f2c7c9251489f6e3
change-id: 20260607-i2c-mux-reg-base-bus-num-738e5db7e99c
prerequisite-patch-id: 3ad9f5acb365e6eb2cd3b0790e983b9049371155

Best regards,
--  
Abdurrahman Hussain <abdurrahman@nexthop.ai>

[PATCH 1/2] dt-bindings: i2c: convert i2c-mux-reg to DT schema

[Abdurrahman Hussain]

Convert Documentation/devicetree/bindings/i2c/i2c-mux-reg.txt to
the YAML schema so the i2c-mux-reg binding is validated by
dt_binding_check.

Also document a new optional "base-bus-num" property: when
present, child adapter N is registered as Linux i2c bus number
(base-bus-num + reg), so static references such as sensors.conf
stanzas remain stable across boots. The matching driver support is
added in the following patch.

Signed-off-by: Abdurrahman Hussain <abdurrahman@nexthop.ai>
---
 .../devicetree/bindings/i2c/i2c-mux-reg.txt        |  74 --------------
 .../devicetree/bindings/i2c/i2c-mux-reg.yaml       | 110 +++++++++++++++++++++
 2 files changed, 110 insertions(+), 74 deletions(-)

diff --git a/Documentation/devicetree/bindings/i2c/i2c-mux-reg.txt b/Documentation/devicetree/bindings/i2c/i2c-mux-reg.txt
deleted file mode 100644
index b9d9755e4172..000000000000
--- a/Documentation/devicetree/bindings/i2c/i2c-mux-reg.txt
+++ /dev/null
@@ -1,74 +0,0 @@
-Register-based I2C Bus Mux
-
-This binding describes an I2C bus multiplexer that uses a single register
-to route the I2C signals.
-
-Required properties:
-- compatible: i2c-mux-reg
-- i2c-parent: The phandle of the I2C bus that this multiplexer's master-side
-  port is connected to.
-* Standard I2C mux properties. See i2c-mux.yaml in this directory.
-* I2C child bus nodes. See i2c-mux.yaml in this directory.
-
-Optional properties:
-- reg: this pair of <offset size> specifies the register to control the mux.
-  The <offset size> depends on its parent node. It can be any memory-mapped
-  address. The size must be either 1, 2, or 4 bytes. If reg is omitted, the
-  resource of this device will be used.
-- little-endian: The existence indicates the register is in little endian.
-- big-endian: The existence indicates the register is in big endian.
-  If both little-endian and big-endian are omitted, the endianness of the
-  CPU will be used.
-- write-only: The existence indicates the register is write-only.
-- idle-state: value to set the muxer to when idle. When no value is
-  given, it defaults to the last value used.
-
-Whenever an access is made to a device on a child bus, the value set
-in the relevant node's reg property will be output to the register.
-
-If an idle state is defined, using the idle-state (optional) property,
-whenever an access is not being made to a device on a child bus, the
-register will be set according to the idle value.
-
-If an idle state is not defined, the most recently used value will be
-left programmed into the register.
-
-Example of a mux on PCIe card, the host is a powerpc SoC (big endian):
-
-	i2c-mux {
-		/* the <offset size> depends on the address translation
-		 * of the parent device. If omitted, device resource
-		 * will be used instead. The size is to determine
-		 * whether iowrite32, iowrite16, or iowrite8 will be used.
-		 */
-		reg = <0x6028 0x4>;
-		little-endian;		/* little endian register on PCIe */
-		compatible = "i2c-mux-reg";
-		#address-cells = <1>;
-		#size-cells = <0>;
-		i2c-parent = <&i2c1>;
-		i2c@0 {
-			reg = <0>;
-			#address-cells = <1>;
-			#size-cells = <0>;
-
-			si5338: clock-generator@70 {
-				compatible = "silabs,si5338";
-				reg = <0x70>;
-				/* other stuff */
-			};
-		};
-
-		i2c@1 {
-			/* data is written using iowrite32 */
-			reg = <1>;
-			#address-cells = <1>;
-			#size-cells = <0>;
-
-			si5338: clock-generator@70 {
-				compatible = "silabs,si5338";
-				reg = <0x70>;
-				/* other stuff */
-			};
-		};
-	};
diff --git a/Documentation/devicetree/bindings/i2c/i2c-mux-reg.yaml b/Documentation/devicetree/bindings/i2c/i2c-mux-reg.yaml
new file mode 100644
index 000000000000..7fadedabde70
--- /dev/null
+++ b/Documentation/devicetree/bindings/i2c/i2c-mux-reg.yaml
@@ -0,0 +1,110 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/i2c/i2c-mux-reg.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Register-based I2C Bus Mux
+
+maintainers:
+  - Peter Rosin <peda@axentia.se>
+
+description: |
+  This binding describes an I2C bus multiplexer that uses a single
+  memory-mapped register to route the I2C signals.
+
+  Whenever an access is made to a device on a child bus, the value
+  set in the relevant node's reg property is output to the register.
+
+  If an idle state is defined via the idle-state property, the
+  register is set to that value whenever no access is being made.
+  Otherwise the most recently used value is left programmed.
+
+allOf:
+  - $ref: /schemas/i2c/i2c-mux.yaml#
+
+properties:
+  compatible:
+    const: i2c-mux-reg
+
+  reg:
+    maxItems: 1
+    description: |
+      Offset and size of the register that selects the active child
+      bus, relative to the parent node's address space. The size
+      determines the access width and must be 1, 2, or 4 bytes. If
+      omitted, the platform device's own memory resource is used
+      instead.
+
+  i2c-parent:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      Phandle of the I2C bus that this multiplexer's master-side port
+      is connected to.
+
+  little-endian:
+    type: boolean
+    description: Register is accessed in little-endian byte order.
+
+  big-endian:
+    type: boolean
+    description: Register is accessed in big-endian byte order.
+
+  write-only:
+    type: boolean
+    description:
+      Register is write-only; the driver must not read back the
+      current selection.
+
+  idle-state:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      Value to write to the register when no child bus is selected.
+
+  base-bus-num:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description: |
+      Base Linux i2c-adapter number for the child buses. Child N is
+      registered as bus (base-bus-num + reg). When unset, child
+      adapters are allocated dynamically by the i2c core.
+
+required:
+  - compatible
+  - i2c-parent
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    i2c-mux@6028 {
+        compatible = "i2c-mux-reg";
+        reg = <0x6028 0x4>;
+        little-endian;
+        #address-cells = <1>;
+        #size-cells = <0>;
+        i2c-parent = <&i2c1>;
+        base-bus-num = <16>;
+
+        i2c@0 {
+            reg = <0>;
+            #address-cells = <1>;
+            #size-cells = <0>;
+
+            clock-generator@70 {
+                compatible = "silabs,si5338";
+                reg = <0x70>;
+            };
+        };
+
+        i2c@1 {
+            reg = <1>;
+            #address-cells = <1>;
+            #size-cells = <0>;
+
+            clock-generator@70 {
+                compatible = "silabs,si5338";
+                reg = <0x70>;
+            };
+        };
+    };
+...

-- 
2.54.0

[PATCH 2/2] i2c: mux: reg: allow fixing the base bus number via fwnode

[Abdurrahman Hussain]

i2c_mux_reg supports per-channel force_nr through mux->data.base_nr
(when set, the channel-N adapter is registered as
i2c_add_numbered_adapter(base_nr + N)), but the only way to populate
base_nr was the legacy i2c_mux_reg_platform_data path. DT/ACPI/swnode
instances have always defaulted to dynamic allocation, which makes
sensors.conf bus stanzas and other static references break across
boots whenever the i2c-core pool shifts.

Read base_nr from the new "base-bus-num" device property in
i2c_mux_reg_probe_fw(). When the property is absent, base_nr stays
zero and the existing dynamic-allocation behaviour is preserved.

Anchor the per-channel bus number to the channel index (values[i]
== the child node's reg) rather than the iteration counter:

  nr = base_nr ? base_nr + values[i] : 0

values[i] is the channel id encoded in the child node's reg property
(0..n-1), which is also what i2c_mux_add_adapter() receives as
chan_id. Using values[i] makes the mapping deterministic across
fwnode-iteration orderings (some OF kernels walk children in reverse
source order) and across DTS gaps (e.g. a mux that wires channels
0, 1, 3 still gets sane, consecutive bus numbers).

Signed-off-by: Abdurrahman Hussain <abdurrahman@nexthop.ai>
---
 drivers/i2c/muxes/i2c-mux-reg.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/i2c/muxes/i2c-mux-reg.c b/drivers/i2c/muxes/i2c-mux-reg.c
index 13da757100fe..5ea59ecb4ae5 100644
--- a/drivers/i2c/muxes/i2c-mux-reg.c
+++ b/drivers/i2c/muxes/i2c-mux-reg.c
@@ -141,6 +141,8 @@ static int i2c_mux_reg_probe_fw(struct regmux *mux, struct device *dev)
 	if (!device_property_read_u32(dev, "idle-state", &mux->data.idle))
 		mux->data.idle_in_use = true;
 
+	device_property_read_u32(dev, "base-bus-num", &mux->data.base_nr);
+
 	return 0;
 }
 
@@ -197,7 +199,7 @@ static int i2c_mux_reg_probe(struct platform_device *pdev)
 		muxc->deselect = i2c_mux_reg_deselect;
 
 	for (i = 0; i < mux->data.n_values; i++) {
-		nr = mux->data.base_nr ? (mux->data.base_nr + i) : 0;
+		nr = mux->data.base_nr ? (mux->data.base_nr + mux->data.values[i]) : 0;
 
 		ret = i2c_mux_add_adapter(muxc, nr, mux->data.values[i]);
 		if (ret)

-- 
2.54.0

Re: [PATCH 1/2] dt-bindings: i2c: convert i2c-mux-reg to DT schema

[Conor Dooley]

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On Sun, Jun 07, 2026 at 01:20:55AM -0700, Abdurrahman Hussain wrote:
> Convert Documentation/devicetree/bindings/i2c/i2c-mux-reg.txt to
> the YAML schema so the i2c-mux-reg binding is validated by
> dt_binding_check.
> 
> Also document a new optional "base-bus-num" property: when
> present, child adapter N is registered as Linux i2c bus number
> (base-bus-num + reg), so static references such as sensors.conf
> stanzas remain stable across boots. The matching driver support is
> added in the following patch.

This should not be "hidden" inside a conversion. It must be a different
patch.
I think it is fairly likely that this property won't be acceptable.
The conversion itself looks okay to me.

pw-bot: changes-requested

Cheers,
Conor.

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Re: [PATCH 1/2] dt-bindings: i2c: convert i2c-mux-reg to DT schema

[Rob Herring (Arm)]

On Sun, 07 Jun 2026 01:20:55 -0700, Abdurrahman Hussain wrote:
> Convert Documentation/devicetree/bindings/i2c/i2c-mux-reg.txt to
> the YAML schema so the i2c-mux-reg binding is validated by
> dt_binding_check.
> 
> Also document a new optional "base-bus-num" property: when
> present, child adapter N is registered as Linux i2c bus number
> (base-bus-num + reg), so static references such as sensors.conf
> stanzas remain stable across boots. The matching driver support is
> added in the following patch.
> 
> Signed-off-by: Abdurrahman Hussain <abdurrahman@nexthop.ai>
> ---
>  .../devicetree/bindings/i2c/i2c-mux-reg.txt        |  74 --------------
>  .../devicetree/bindings/i2c/i2c-mux-reg.yaml       | 110 +++++++++++++++++++++
>  2 files changed, 110 insertions(+), 74 deletions(-)
> 

My bot found errors running 'make dt_binding_check' on your patch:

yamllint warnings/errors:

dtschema/dtc warnings/errors:
Documentation/devicetree/bindings/i2c/i2c-mux-reg.example.dtb: /example-0/i2c-mux@6028/i2c@0/clock-generator@70: failed to match any schema with compatible: ['silabs,si5338']
Documentation/devicetree/bindings/i2c/i2c-mux-reg.example.dtb: /example-0/i2c-mux@6028/i2c@1/clock-generator@70: failed to match any schema with compatible: ['silabs,si5338']

doc reference errors (make refcheckdocs):

See https://patchwork.kernel.org/project/devicetree/patch/20260607-i2c-mux-reg-base-bus-num-v1-1-f193b5a8fedc@nexthop.ai

The base for the series is generally the latest rc1. A different dependency
should be noted in *this* patch.

If you already ran 'make dt_binding_check' and didn't see the above
error(s), then make sure 'yamllint' is installed and dt-schema is up to
date:

pip3 install dtschema --upgrade

Please check and re-submit after running the above command yourself. Note
that DT_SCHEMA_FILES can be set to your schema file to speed up checking
your schema. However, it must be unset to test all examples with your schema.

Re: [PATCH 1/2] dt-bindings: i2c: convert i2c-mux-reg to DT schema

[Abdurrahman Hussain]

On Sun, 7 Jun 2026 15:06:43 +0100, Conor Dooley wrote:
> I think it is fairly likely that this property won't be acceptable.
> The conversion itself looks okay to me.

Thanks for the hint -- I dug into the i2c core and realised the case
I was trying to solve already works on OF without any new binding.
i2c_mux_add_adapter() sets the child adapter's of_node to the
matching i2c@N node (drivers/i2c/i2c-mux.c:330..367), and
i2c_add_adapter() then honours of_alias_get_id(dev->of_node, "i2c")
(drivers/i2c/i2c-core-base.c:1657).  So in DT, the existing aliases
mechanism already pins mux child buses:

  aliases {
      i2c16 = &my_mux_bus0;
      i2c17 = &my_mux_bus1;
  };

  i2c-mux@... {
      compatible = "i2c-mux-reg";
      ...
      my_mux_bus0: i2c@0 { reg = <0>; ... };
      my_mux_bus1: i2c@1 { reg = <1>; ... };
  };

That's clearly the right answer; sorry for not finding it before
sending.  The only wrinkle is that of_alias_get_id() is OF-only, so
this doesn't help the ACPI/swnode platform that originally motivated
me -- but that's a Linux-side problem (udev rules, stable adapter
names) and not something a new DT property should paper over.

I'll respin v2 as just the .txt -> YAML conversion (faithful port,
no new property), and fix the silabs,si5338 example that Rob's bot
flagged.

Thanks,
Abdurrahman.