[PATCH v3] docs: dt-bindings: add DTS Coding Style document

[Krzysztof Kozlowski]

On 25/11/2023 20:37, Laurent Pinchart wrote:
> Hi Krzysztof,
> 
> Thank you for the patch.
> 
> On Sat, Nov 25, 2023 at 07:44:22PM +0100, 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: Andrew Lunn <andrew at lunn.ch>
>> Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno at collabora.com>
>> Cc: Arnd Bergmann <arnd at arndb.de>
>> Cc: Bjorn Andersson <andersson at kernel.org>
>> Cc: Chen-Yu Tsai <wens at kernel.org>
>> Cc: Dmitry Baryshkov <dmitry.baryshkov at linaro.org>
>> Cc: Geert Uytterhoeven <geert+renesas at glider.be>
>> Cc: Heiko Stuebner <heiko at sntech.de>
>> Cc: Jonathan Corbet <corbet at lwn.net>
>> 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>
>> Cc: Rafał Miłecki <zajec5 at gmail.com>
>> Acked-by: Neil Armstrong <neil.armstrong at linaro.org>
>> Acked-by: Heiko Stuebner <heiko at sntech.de>
>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski at linaro.org>
>>
>> ---
>>
>> Merging idea: Rob/DT bindings
>>
>> Changes in v3
>> =============
>> 1. should->shall (Angelo)
>> 2. Comments // -> /* (Angelo, Michal)
>> 3. Use imaginary example in "Order of Properties in Device Node"
>>    (Angelo)
>> 4. Added paragraphs for three sections with justifications of chosen
>>    style.
>> 5. Allow two style of ordering overrides in board DTS: alphabetically or
>>    by order of DTSI (Rob).
>> 6. I did not incorporate feedback about, due to lack of consensus and my
>>    disagreement:
>>    a. SoM being DTS without DTSI in "Organizing DTSI and DTS"
>>
>> 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
>> Cc: workflows at vger.kernel.org
>> Cc: linux-doc at vger.kernel.org
>> ---
>>  .../devicetree/bindings/dts-coding-style.rst  | 194 ++++++++++++++++++
>>  Documentation/devicetree/bindings/index.rst   |   1 +
>>  2 files changed, 195 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..e374bec0f555
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/dts-coding-style.rst
>> @@ -0,0 +1,194 @@
>> +.. 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
>> +---------------------------
>> +
>> +Devicetree specification allows broader range of characters in node and
> 
> s/Devicetree specification/The Devicetree specification/
> s/broader range/a broad range/

Ack, but I really expect people finish with grammar and style fixes at
v3. Please point the language things now or just let it go.

> 
>> +property names, but for code readability the choice shall be narrowed.
>> +
>> +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 shall use lowercase hex, without leading zeros (padding).
> 
> I'm curious, what's the reason for this ? I think it makes the sources
> less readable. If the rule is "just" because that's how DT sources are
> written today and it would be too complicated to change that, that's
> fine with me.

Warning (unit_address_format): /cpus/cpu at 0100: unit name should not have
leading 0s

And we fixed all or most of DTS some time ago.

> 
>> +
>> +4. Hex values in properties, e.g. "reg", shall 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 shall 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 shall be
>> +   ordered either alpha-numerically or by keeping the order from DTSI (choice
>> +   depending on sub-architecture).
>> +
>> +Above ordering rules are easy to enforce during review, reduce chances of
>> +conflicts for simultaneous additions (new nodes) to a file and help in
>> +navigating through the DTS source.
>> +
>> +Example::
>> +
>> +	/* SoC DTSI */
>> +
>> +	/ {
>> +		cpus {
>> +			/* ... */
>> +		};
>> +
>> +		psci {
>> +			/* ... */
>> +		};
>> +
>> +		soc@ {
>> +			dma: dma-controller at 10000 {
>> +				/* ... */
>> +			};
>> +
>> +			clk: clock-controller at 80000 {
>> +				/* ... */
>> +			};
>> +		};
>> +	};
>> +
>> +	/* Board DTS - alphabetical order */
>> +
>> +	&clk {
>> +		/* ... */
>> +	};
>> +
>> +	&dma {
>> +		/* ... */
>> +	};
>> +
>> +	/* Board DTS - alternative order, keep as DTSI */
>> +
>> +	&dma {
>> +		/* ... */
>> +	};
>> +
>> +	&clk {
>> +		/* ... */
>> +	};
>> +
>> +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.
>> +
>> +Above order follows approach:
>> +
>> +1. Most important properties start the node: compatible then bus addressing to
>> +   match unit address.
>> +2. Each node will have common properties in similar place.
>> +3. Status is the last information to annotate that device node is or is not
>> +   finished (board resources are needed).
>> +
>> +Example::
>> +
>> +	/* SoC DTSI */
>> +
>> +	device_node: device-class at 6789abc {
>> +		compatible = "vendor,device";
>> +		reg = <0x0 0x06789abc 0x0 0xa123>;
>> +		ranges = <0x0 0x0 0x06789abc 0x1000>;
>> +		#dma-cells = <1>;
>> +		clocks = <&clock_controller 0>, <&clock_controller 1>;
>> +		clock-names = "bus", "host";
>> +		vendor,custom-property = <2>;
>> +		status = "disabled";
>> +
>> +		child_node: child-class at 100 {
>> +			reg = <0x100 0x200>;
>> +			/* ... */
>> +		};
>> +	};
>> +
>> +	/* Board DTS */
>> +
>> +	&device_node {
>> +		vdd-supply = <&board_vreg1>;
>> +		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)
>> +   shall 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 shall 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 shall 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.
> 
> I'm looking forward to discussing how to organize overlays. That
> discussion should be separate though, or this patch will never get
> merged :-)
> 
> Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>

Thanks

Best regards,
Krzysztof