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Message-Id: <20250729-spi-bus-extension-v1-4-b20c73f2161a@beagleboard.org>
Date: Tue, 29 Jul 2025 15:21:03 +0530
From: Ayush Singh <ayush@...gleboard.org>
To: Mark Brown <broonie@...nel.org>, herve.codina@...tlin.com,
luca.ceresoli@...tlin.com, conor+dt@...nel.org,
Jason Kridner <jkridner@...gleboard.org>,
Deepak Khatri <lorforlinux@...gleboard.org>, Dhruva Gole <d-gole@...com>,
Robert Nelson <robertcnelson@...gleboard.org>, Andrew Davis <afd@...com>,
Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>
Cc: linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org,
devicetree@...r.kernel.org, Ayush Singh <ayush@...gleboard.org>
Subject: [PATCH 4/4] devicetree: bindings: spi: Introduce SPI bus
extensions
An SPI bus can be wired to the connector and allows an add-on board to
connect additional SPI devices to this bus.
Those additional SPI devices could be described as sub-nodes of the SPI
bus controller node however for hotplug connectors described via device
tree overlays there is additional level of indirection, which is needed
to decouple the overlay and the base tree:
--- base device tree ---
spi1: spi@...d0000 {
compatible = "xyz,foo";
spi-bus-extension@0 {
spi-bus = <&spi_ctrl>;
};
...
};
spi5: spi@...e0000 {
compatible = "xyz,bar";
spi-bus-extension@0 {
spi-bus = <&spi_sensors>;
};
...
};
connector {
spi_ctrl: spi-ctrl {
spi-parent = <&spi1>;
#address-cells = <1>;
#size-cells = <0>;
};
spi_sensors: spi-sensors {
spi-parent = <&spi5>;
#address-cells = <1>;
#size-cells = <0>;
};
};
--- device tree overlay ---
...
// This node will overlay on the spi-ctrl node of the base tree
spi-ctrl {
eeprom@50 { compatible = "atmel,24c64"; ... };
};
...
--- resulting device tree ---
spi1: spi@...d0000 {
compatible = "xyz,foo";
spi-bus-extension@0 {
spi-bus = <&spi_ctrl>;
};
...
};
spi5: spi@...e0000 {
compatible = "xyz,bar";
spi-bus-extension@0 {
spi-bus = <&spi_sensors>;
};
...
};
connector {
spi_ctrl: spi-ctrl {
spi-parent = <&spi1>;
#address-cells = <1>;
#size-cells = <0>;
device@1 { compatible = "xyz,foo"; ... };
};
spi_sensors: spi-sensors {
spi-parent = <&spi5>;
#address-cells = <1>;
#size-cells = <0>;
};
};
Here spi-ctrl (same goes for spi-sensors) represent the part of SPI bus
that is on the hot-pluggable add-on. On hot-plugging it will physically
connect to the SPI adapter on the base board. Let's call the 'spi-ctrl'
node an "extension node".
In order to decouple the overlay from the base tree, the SPI adapter
(spi@...d0000) and the extension node (spi-ctrl) are separate nodes.
The extension node is linked to the SPI bus controller in two ways. The
first one with the spi-bus-extension available in SPI controller
sub-node and the second one with the spi-parent property available in
the extension node itself.
The purpose of those two links is to provide the link in both direction
from the SPI controller to the SPI extension and from the SPI extension
to the SPI controller.
Signed-off-by: Ayush Singh <ayush@...gleboard.org>
---
.../devicetree/bindings/spi/spi-controller.yaml | 66 +++++++++++++++++++++-
1 file changed, 65 insertions(+), 1 deletion(-)
diff --git a/Documentation/devicetree/bindings/spi/spi-controller.yaml b/Documentation/devicetree/bindings/spi/spi-controller.yaml
index 82d051f7bd6e09dab9809c85ff13475d2b118efd..9b44ce4542f9552c94cb0658ffe3f6d3f29bc434 100644
--- a/Documentation/devicetree/bindings/spi/spi-controller.yaml
+++ b/Documentation/devicetree/bindings/spi/spi-controller.yaml
@@ -25,6 +25,13 @@ properties:
"#size-cells":
const: 0
+ spi-parent:
+ $ref: /schemas/types.yaml#/definitions/phandle
+ description:
+ In case of an SPI bus extension, reference to the SPI bus controller
+ this extension is connected to. In other word, reference the SPI bus
+ controller on the fixed side that drives the bus extension.
+
cs-gpios:
description: |
GPIOs used as chip selects.
@@ -111,7 +118,26 @@ properties:
- compatible
patternProperties:
- "^.*@[0-9a-f]+$":
+ 'spi-bus-extension@[0-9a-f]+$':
+ type: object
+ description:
+ An SPI bus extension connected to an SPI bus. Those extensions allow to
+ decouple SPI busses when they are wired to connectors.
+
+ properties:
+ reg:
+ maxItems: 1
+
+ spi-bus:
+ $ref: /schemas/types.yaml#/definitions/phandle
+ description:
+ Reference to the extension bus.
+
+ required:
+ - reg
+ - spi-bus
+
+ "^(?!spi-bus-extension@).*@[0-9a-f]+$":
type: object
$ref: spi-peripheral-props.yaml
additionalProperties: true
@@ -214,3 +240,41 @@ examples:
spi-cs-high;
};
};
+
+ # SPI bus extension example involving an SPI bus controller and a connector.
+ #
+ # +--------------+ +-------------+ +-------------+
+ # | spi@...d0000 | | Connector | | Addon board |
+ # | (spi1) +-----+ (spi-addon) +-----+ (device@10) |
+ # | | | | | |
+ # +--------------+ +-------------+ +-------------+
+ #
+ # The spi1 SPI bus is wired from a SPI controller to a connector. It is
+ # identified at connector level as spi-addon bus.
+ # An addon board can be connected to this connector and connects a device
+ # (device@10) to this spi-addon extension bus.
+ - |
+ spi1: spi@...d0000 {
+ compatible = "brcm,bcm2835-spi";
+ reg = <0xabcd0000 0x100>;
+ #address-cells = <1>;
+ #size-cells = <0>;
+
+ spi-bus-extension@0 {
+ reg = <0>;
+ spi-bus = <&spi_addon>;
+ };
+ };
+
+ connector {
+ spi_addon: spi-addon {
+ spi-parent = <&spi1>;
+ #address-cells = <1>;
+ #size-cells = <0>;
+
+ device@2 {
+ compatible = "xyz,foo";
+ reg = <0x02>;
+ };
+ };
+ };
--
2.50.1
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