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

[Michal Simek]

On 11/20/23 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: -

device-tree specification v0.4. Chapter 2.2.1/Table 2.1 is describing much more
valid characters for node names.
It means above description is not accurate or DT spec should be updated.


> +
> +2. Labels are allowed to use only:
> +
> +   * lowercase characters: [a-z]
> +   * digits: [0-9]
> +   * underscore: _

based on dt spec uppercase is also valid char in label.


> +
> +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>;

Is 0x0 recommended or 0 is enough?

> +	}
> +
> +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

Same comment about /* */ as was mentioned in another thread.

> +
> +	/ {
> +		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

Isn't the order already defined in DT spec in 2.3 in chapters?
compatible
model
status
#address/size cells
reg
virtual-reg
ranges
dma-ranges
dma-coherent
dma-non-coherent

Again I am fine with whatever order but I think we should reflect it in the spec 
too. Especially status property is for my taste too low simply because you start 
to read and then you will find that IP is disabled.

And are you describing all properties starting with # as standard properties?


> +
> +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).

DTS/DTSI - SOMs can actually run as they are that's why it is fair to say that
there doesn't need to be DTS representing the board.

Thanks,
Michal