[PATCH v2] docs: dt-bindings: add DTS Coding Style document

[Neil Armstrong]

On 20/11/2023 09:40, Krzysztof Kozlowski wrote:
> Document preferred coding style for Devicetree sources (DTS and DTSI),
> to bring consistency among all (sub)architectures and ease in reviews.
> 
> Cc: Andrew Davis <afd at ti.com>
> Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno at collabora.com>
> Cc: Arnd Bergmann <arnd at arndb.de>
> Cc: Bjorn Andersson <andersson at kernel.org>
> Cc: Geert Uytterhoeven <geert+renesas at glider.be>
> Cc: Heiko Stuebner <heiko at sntech.de>
> Cc: Konrad Dybcio <konrad.dybcio at linaro.org>
> Cc: Matthias Brugger <matthias.bgg at gmail.com>
> Cc: Michal Simek <michal.simek at amd.com>
> Cc: Neil Armstrong <neil.armstrong at linaro.org>
> Cc: Nishanth Menon <nm at ti.com>
> Cc: Olof Johansson <olof at lixom.net>
> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski at linaro.org>
> 
> ---
> 
> Merging idea: Rob/DT bindings
> 
> Changes in v2
> =============
> 1. Hopefully incorporate entire feedback from comments:
> a. Fix \ { => / { (Rob)
> b. Name: dts-coding-style (Rob)
> c. Exceptions for ordering nodes by name for Renesas and pinctrl (Geert,
>     Konrad)
> d. Ordering properties by common/vendor (Rob)
> e. Array entries in <> (Rob)
> 
> 2. New chapter: Organizing DTSI and DTS
> 
> 3. Several grammar fixes (missing articles)
> 
> Cc: linux-rockchip at lists.infradead.org
> Cc: linux-mediatek at lists.infradead.org
> Cc: linux-samsung-soc at vger.kernel.org
> Cc: linux-amlogic at lists.infradead.org
> Cc: linux-arm-kernel at lists.infradead.org
> Cc: linux-arm-msm at vger.kernel.org
> ---
>   .../devicetree/bindings/dts-coding-style.rst  | 163 ++++++++++++++++++
>   Documentation/devicetree/bindings/index.rst   |   1 +
>   2 files changed, 164 insertions(+)
>   create mode 100644 Documentation/devicetree/bindings/dts-coding-style.rst
> 
> diff --git a/Documentation/devicetree/bindings/dts-coding-style.rst b/Documentation/devicetree/bindings/dts-coding-style.rst
> new file mode 100644
> index 000000000000..cc7e3b4d1b92
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/dts-coding-style.rst
> @@ -0,0 +1,163 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +.. _dtscodingstyle:
> +
> +=====================================
> +Devicetree Sources (DTS) Coding Style
> +=====================================
> +
> +When writing Devicetree Sources (DTS) please observe below guidelines.  They
> +should be considered complementary to any rules expressed already in Devicetree
> +Specification and dtc compiler (including W=1 and W=2 builds).
> +
> +Individual architectures and sub-architectures can add additional rules, making
> +the style stricter.
> +
> +Naming and Valid Characters
> +---------------------------
> +
> +1. Node and property names are allowed to use only:
> +
> +   * lowercase characters: [a-z]
> +   * digits: [0-9]
> +   * dash: -
> +
> +2. Labels are allowed to use only:
> +
> +   * lowercase characters: [a-z]
> +   * digits: [0-9]
> +   * underscore: _
> +
> +3. Unit addresses should use lowercase hex, without leading zeros (padding).
> +
> +4. Hex values in properties, e.g. "reg", should use lowercase hex.  The address
> +   part can be padded with leading zeros.
> +
> +Example::
> +
> +	gpi_dma2: dma-controller at 800000 {
> +		compatible = "qcom,sm8550-gpi-dma", "qcom,sm6350-gpi-dma";
> +		reg = <0x0 0x00800000 0x0 0x60000>;
> +	}
> +
> +Order of Nodes
> +--------------
> +
> +1. Nodes within any bus, thus using unit addresses for children, shall be
> +   ordered incrementally by unit address.
> +   Alternatively for some sub-architectures, nodes of the same type can be
> +   grouped together (e.g. all I2C controllers one after another even if this
> +   breaks unit address ordering).
> +
> +2. Nodes without unit addresses should be ordered alpha-numerically by the node
> +   name.  For a few types of nodes, they can be ordered by the main property
> +   (e.g. pin configuration states ordered by value of "pins" property).
> +
> +3. When extending nodes in the board DTS via &label, the entries should be
> +   ordered alpha-numerically.
> +
> +Example::
> +
> +	// SoC DTSI
> +
> +	/ {
> +		cpus {
> +			// ...
> +		};
> +
> +		psci {
> +			// ...
> +		};
> +
> +		soc@ {
> +			dma: dma-controller at 10000 {
> +				// ...
> +			};
> +
> +			clk: clock-controller at 80000 {
> +				// ...
> +			};
> +		};
> +	};
> +
> +	// Board DTS
> +
> +	&clk {
> +		// ...
> +	};
> +
> +	&dma {
> +		// ...
> +	};
> +
> +
> +Order of Properties in Device Node
> +----------------------------------
> +
> +Following order of properties in device nodes is preferred:
> +
> +1. compatible
> +2. reg
> +3. ranges
> +4. Standard/common properties (defined by common bindings, e.g. without
> +   vendor-prefixes)
> +5. Vendor-specific properties
> +6. status (if applicable)
> +7. Child nodes, where each node is preceded with a blank line
> +
> +The "status" property is by default "okay", thus it can be omitted.
> +
> +Example::
> +
> +	// SoC DTSI
> +
> +	usb_1_hsphy: phy at 88e3000 {
> +		compatible = "qcom,sm8550-snps-eusb2-phy";
> +		reg = <0x0 0x088e3000 0x0 0x154>;
> +		#phy-cells = <0>;
> +		resets = <&gcc GCC_QUSB2PHY_PRIM_BCR>;
> +		status = "disabled";
> +	};
> +
> +	// Board DTS
> +
> +	&usb_1_hsphy {
> +		clocks = <&tcsr TCSR_USB2_CLKREF_EN>;
> +		clock-names = "ref";
> +		status = "okay";
> +	};
> +
> +
> +Indentation
> +-----------
> +
> +1. Use indentation according to :ref:`codingstyle`.
> +2. For arrays spanning across lines, it is preferred to align the continued
> +   entries with opening < from the first line.
> +3. Each entry in arrays with multiple cells (e.g. "reg" with two IO addresses)
> +   should be enclosed in <>.
> +
> +Example::
> +
> +	thermal-sensor at c271000 {
> +		compatible = "qcom,sm8550-tsens", "qcom,tsens-v2";
> +		reg = <0x0 0x0c271000 0x0 0x1000>,
> +		      <0x0 0x0c222000 0x0 0x1000>;
> +	};
> +
> +Organizing DTSI and DTS
> +-----------------------
> +
> +The DTSI and DTS files should be organized in a way representing the common
> +(and re-usable) parts of the hardware.  Typically this means organizing DTSI
> +and DTS files into several files:
> +
> +1. DTSI with contents of the entire SoC (without nodes for hardware not present
> +   on the SoC).
> +2. If applicable: DTSI with common or re-usable parts of the hardware (e.g.
> +   entire System-on-Module).
> +3. DTS representing the board.
> +
> +Hardware components which are present on the board should be placed in the
> +board DTS, not in the SoC or SoM DTSI.  A partial exception is a common
> +external reference SoC-input clock, which could be coded as a fixed-clock in
> +the SoC DTSI with its frequency provided by each board DTS.
> diff --git a/Documentation/devicetree/bindings/index.rst b/Documentation/devicetree/bindings/index.rst
> index d9002a3a0abb..cc1fbdc05657 100644
> --- a/Documentation/devicetree/bindings/index.rst
> +++ b/Documentation/devicetree/bindings/index.rst
> @@ -4,6 +4,7 @@
>      :maxdepth: 1
>   
>      ABI
> +   dts-coding-style
>      writing-bindings
>      writing-schema
>      submitting-patches

Please add my:
Acked-by: Neil Armstrong <neil.armstrong at linaro.org>

I agree with everything in this document and it's a huge step in the right direction.

Thanks,
Neil