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

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Message-ID: <CAL_JsqKkOjBHrJ0WELq3JnJDqgtA=mdF+EtAxHSCGqZMQ9tuSQ@mail.gmail.com>
Date:   Wed, 22 Nov 2023 07:34:59 -0700
From:   Rob Herring <robh+dt@...nel.org>
To:     Michal Simek <michal.simek@....com>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Dragan Simic <dsimic@...jaro.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        wens@...nel.org,
        Rafał Miłecki <zajec5@...il.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, Andrew Davis <afd@...com>,
        Arnd Bergmann <arnd@...db.de>,
        Bjorn Andersson <andersson@...nel.org>,
        Heiko Stuebner <heiko@...ech.de>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Nishanth Menon <nm@...com>, Olof Johansson <olof@...om.net>,
        linux-rockchip@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org,
        linux-amlogic@...ts.infradead.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH v2] docs: dt-bindings: add DTS Coding Style document

On Wed, Nov 22, 2023 at 1:57 AM Michal Simek <michal.simek@....com> wrote:
>
> Hi Geert,
>
> On 11/22/23 09:53, Geert Uytterhoeven wrote:
> > Hi Michal,
> >
> > On Wed, Nov 22, 2023 at 9:50 AM Michal Simek <michal.simek@....com> wrote:
> >> On 11/22/23 09:29, Dragan Simic wrote:
> >>> On 2023-11-22 09:21, Krzysztof Kozlowski wrote:
> >>>> On 22/11/2023 09:09, Chen-Yu Tsai wrote:
> >>>>> On Wed, Nov 22, 2023 at 4:05 PM Krzysztof Kozlowski
> >>>>> <krzysztof.kozlowski@...aro.org> wrote:
> >>>>>>
> >>>>>> On 21/11/2023 14:50, Rafał Miłecki wrote:
> >>>>>>>> +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.
> >>>>>>>
> >>>>>>> I think it would really help to include position of #address-cells and
> >>>>>>> #size-cells here. In some files I saw them above "compatible" that seems
> >>>>>>> unintuitive. Some prefer putting them at end which I think makes sense
> >>>>>>> as they affect children nodes.
> >>>>>>>
> >>>>>>> Whatever you choose it'd be just nice to have things consistent.
> >>>>>>
> >>>>>> This is a standard/common property, thus it goes to (4) above.
> >>>>>
> >>>>> It's probably a mix, but AFAIK a lot of the device trees in tree have
> >>>>> #*-cells after "status". In some cases they are added in the board
> >>>>> .dts files, not the chip/module .dtsi files.
> >>>>
> >>>> Existing DTS is not a good example :)
> >>>>
> >>>>>
> >>>>> +1 that it makes sense at the end as they affect child nodes.
> >>>>
> >>>> I still insist that status must be the last, because:
> >>>> 1. Many SoC nodes have address/size cells but do not have any children
> >>>> (I2C, SPI), so we put useless information at the end.
> >>>> 2. Status should be the final information to say whether the node is
> >>>> ready or is not. I read the node, check properties and then look at the end:
> >>>> a. Lack of status means it is ready.
> >>>> b. status=disabled means device still needs board resources/customization
> >>>
> >>> I agree with the "status" belonging to the very end, because it's both logical
> >>> and much more readable.  Also, "status" is expected to be modified in the
> >>> dependent DT files, which makes it kind of volatile and even more deserving to
> >>> be placed last.
> >>
> >> I am just curious if having status property at the end won't affect
> >> execution/boot up time. Not sure how it is done in Linux but in U-Boot at least
> >> (we want to have DTs in sync between Linux and U-Boot) of_find_property is
> >> pretty much big loop over all properties. And status property defined at the end
> >> means going over all of them to find it out to if device is present.
> >> Not sure if Linux works in the same way but at least of_get_property is done in
> >> the same way.
> >
> > As the default is "okay", you have to loop over all properties anyway.
>
> No doubt if you don't define status property that you need to loop over all of
> them. We normally describe the whole SOC with pretty much all IPs status =
> disabled and then in board file we are changing it to okay based on what it is
> actually wired out.
> It means on our systems all nodes have status properties. If you have it at
> first you don't need to go over all.

Order in the source and order in the OS are independent. If checking
status needs to be optimized, then we could just put it first in the
property list or make the state a field in struct device_node. But
provide some data that it matters first.

I've had this idea to randomize the order nodes are processed so
there's no reliance on the DT order. Maybe I need the same on
properties...

Rob