linux-kernel - Re: [PATCH] scripts/dtc: Allow ports to have a single port@0 child

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Message-ID: <5d251b55-7050-c212-c262-49127f86f32c@ti.com>
Date:   Tue, 31 Oct 2023 22:08:04 +0530
From:   Aradhya Bhatia <a-bhatia1@...com>
To:     Rob Herring <robh@...nel.org>
CC:     Frank Rowand <frowand.list@...il.com>,
        Devicetree List <devicetree@...r.kernel.org>,
        Linux Kernel List <linux-kernel@...r.kernel.org>,
        Andrew Davis <afd@...com>, Nishanth Menon <nm@...com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Devarsh Thakkar <devarsht@...com>,
        Jai Luthra <j-luthra@...com>,
        Jayesh Choudhary <j-choudhary@...com>
Subject: Re: [PATCH] scripts/dtc: Allow ports to have a single port@0 child



On 26-Oct-23 01:01, Rob Herring wrote:
> On Fri, Oct 13, 2023 at 02:24:24PM +0530, Aradhya Bhatia wrote:
>> Exempt 'ports' from the rule which asserts that nodes with single child
>> node having reg = 0, should not have the '#size-cells' and
>> '#address-cells' properties.
>>
>> Ports of certain hardware do need to be described as only having a
>> single child node 'port@0', especially when hardware has multiple ports,
>> and the other ports 'port@x' are planned to be added subsequently. In
>> such cases, just using 'port', would be an inaccurate hardware
>> description.
>>
>> For example, Texas Instruments' DSS (display-subsystem), which has 2 or
>> 4 video ports depending on the SoC. Describing the first video port with
>> just 'port' under ports would be inaccurate and even slightly
>> misleading. Simply using port@0 (when other ports are not added)
>> produces the following warning, while making dtbs with W=1 flag set[0].
> 
> There's a reason this is behind W=1.
> 
> In general, if you only have a single 'port' it should be just 'port' 
> which is equivalent to port 0. There's exceptions to that, so the 
> warning is off by default.

Thank you for reviewing the patch, Rob!

I had a discussion offline, and I agree that the patch may not be needed
after all.

Moreover, upon looking at the tests provided in upstream dtc tree, I
also realized that the check is exclusively limited to port@0, and does
not include any random 'child@0'. This makes the patch make a lot less
sense too.

Regards
Aradhya

> 
>> code-block ::
>>
>> 	Warning (graph_child_address): /bus@...000/dss@...0000/ports:
>> 	graph node has single child node 'port@0',
>> 	#address-cells/#size-cells are not necessary
>>
>> Signed-off-by: Aradhya Bhatia <a-bhatia1@...com>
>>
>> [0]: https://lore.kernel.org/all/570903b6-8239-d44a-5fac-71700804cb5d@ti.com/
>> ---
>>  scripts/dtc/checks.c | 11 ++++++++++-
>>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> This is a copy of upstream dtc. We don't take patches for it (except in 
> emergency). Look at the commit history.
> 
> Rob