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

Krzysztof Kozlowski
2023-11-26T10:32:34Z

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@ti.com>
>> cc: Andrew Lunn <andrew@lunn.ch>
>> Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
>> Cc: Arnd Bergmann <arnd@arndb.de>
>> Cc: Bjorn Andersson <andersson@kernel.org>
>> Cc: Chen-Yu Tsai <wens@kernel.org>
>> Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
>> Cc: Geert Uytterhoeven <geert+renesas@glider.be>
>> Cc: Heiko Stuebner <heiko@sntech.de>
>> Cc: Jonathan Corbet <corbet@lwn.net>
>> Cc: Konrad Dybcio <konrad.dybcio@linaro.org>
>> Cc: Matthias Brugger <matthias.bgg@gmail.com>
>> Cc: Michal Simek <michal.simek@amd.com>
>> Cc: Neil Armstrong <neil.armstrong@linaro.org>
>> Cc: Nishanth Menon <nm@ti.com>
>> Cc: Olof Johansson <olof@lixom.net>
>> Cc: Rafał Miłecki <zajec5@gmail.com>
>> Acked-by: Neil Armstrong <neil.armstrong@linaro.org>
>> Acked-by: Heiko Stuebner <heiko@sntech.de>
>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@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@lists.infradead.org
>> Cc: linux-mediatek@lists.infradead.org
>> Cc: linux-samsung-soc@vger.kernel.org
>> Cc: linux-amlogic@lists.infradead.org
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-arm-msm@vger.kernel.org
>> Cc: workflows@vger.kernel.org
>> Cc: linux-doc@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@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@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@10000 {
>> + /* ... */
>> + };
>> +
>> + clk: clock-controller@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@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@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@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@ideasonboard.com>

Thanks

Best regards,
Krzysztof