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

[Dragan Simic]

On 2023-11-22 15:34, Rob Herring wrote:
> On Wed, Nov 22, 2023 at 1:57 AM Michal Simek <michal.simek at amd.com> 
> wrote:
>> On 11/22/23 09:53, Geert Uytterhoeven wrote:
>> > On Wed, Nov 22, 2023 at 9:50 AM Michal Simek <michal.simek at amd.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 at linaro.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.

That's exactly what I plan to do, i.e. to perform some benchmarks before 
and after, to see does it actually matter to the point where introducing 
the changes is worth it.

> 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...