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

[Laurent Pinchart]

Hi Krzysztof,

On Sun, Nov 26, 2023 at 11:32:17AM +0100, Krzysztof Kozlowski wrote:
> On 25/11/2023 20:37, Laurent Pinchart wrote:
> > 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.

v3 is the first version that ended up in my inbox. I haven't noticed any
other spelling or grammar error in this patch, but I can't guarantee I
won't see any in new text that gets introduced in a future version :-)

> >> +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.

It's the current practice in DT sources and I understand it won't get
changed, but I was more curious about the rationale behind that.

> >> +
> >> +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>

-- 
Regards,

Laurent Pinchart