linux-kernel - Re: [PATCH v3 1/8] dt-bindings: usb: ti,j721e-usb: drop useless compatible list

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Message-Id: <CZF33W51MC4M.3GUBZFQXT39DB@bootlin.com>
Date: Mon, 26 Feb 2024 15:35:59 +0100
From: Théo Lebrun <theo.lebrun@...tlin.com>
To: "Conor Dooley" <conor.dooley@...rochip.com>
Cc: "Conor Dooley" <conor@...nel.org>, "Greg Kroah-Hartman"
 <gregkh@...uxfoundation.org>, "Rob Herring" <robh+dt@...nel.org>,
 "Krzysztof Kozlowski" <krzysztof.kozlowski+dt@...aro.org>, "Conor Dooley"
 <conor+dt@...nel.org>, "Roger Quadros" <rogerq@...nel.org>, "Peter Chen"
 <peter.chen@...nel.org>, "Pawel Laszczak" <pawell@...ence.com>, "Nishanth
 Menon" <nm@...com>, "Vignesh Raghavendra" <vigneshr@...com>, "Tero Kristo"
 <kristo@...nel.org>, "Thomas Petazzoni" <thomas.petazzoni@...tlin.com>,
 Grégory Clement <gregory.clement@...tlin.com>, "Kevin
 Hilman" <khilman@...nel.org>, "Alan Stern" <stern@...land.harvard.edu>,
 <linux-usb@...r.kernel.org>, <devicetree@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 1/8] dt-bindings: usb: ti,j721e-usb: drop useless
 compatible list

Hello,

On Mon Feb 26, 2024 at 12:56 PM CET, Conor Dooley wrote:
> On Mon, Feb 26, 2024 at 11:33:06AM +0100, Théo Lebrun wrote:
> > Hello Conor,
> > 
> > On Fri Feb 23, 2024 at 7:12 PM CET, Conor Dooley wrote:
> > > On Fri, Feb 23, 2024 at 05:05:25PM +0100, Théo Lebrun wrote:
> > > > Compatible can be A or B, not A or B or A+B. Remove last option.
> > > > A=ti,j721e-usb and B=ti,am64-usb.
> > > > 
> > > > Signed-off-by: Théo Lebrun <theo.lebrun@...tlin.com>
> > > > ---
> > > >  Documentation/devicetree/bindings/usb/ti,j721e-usb.yaml | 9 +++------
> > > >  1 file changed, 3 insertions(+), 6 deletions(-)
> > > > 
> > > > diff --git a/Documentation/devicetree/bindings/usb/ti,j721e-usb.yaml b/Documentation/devicetree/bindings/usb/ti,j721e-usb.yaml
> > > > index 95ff9791baea..949f45eb45c2 100644
> > > > --- a/Documentation/devicetree/bindings/usb/ti,j721e-usb.yaml
> > > > +++ b/Documentation/devicetree/bindings/usb/ti,j721e-usb.yaml
> > > > @@ -11,12 +11,9 @@ maintainers:
> > > >  
> > > >  properties:
> > > >    compatible:
> > > > -    oneOf:
> > > > -      - const: ti,j721e-usb
> > > > -      - const: ti,am64-usb
> > > > -      - items:
> > > > -          - const: ti,j721e-usb
> > > > -          - const: ti,am64-usb
> > >
> > > Correct, this makes no sense. The devices seem to be compatible though,
> > > so I would expect this to actually be:
> > > oneOf:
> > >   - const: ti,j721e-usb
> > >   - items:
> > >       - const: ti,am64-usb
> > >       - const: ti,j721e-usb
> > 
> > I need your help to grasp what that change is supposed to express? Would
> > you mind turning it into english sentences?
> > A=ti,j721e-usb and B=ti,am64-usb. My understanding of your proposal is
> > that a device can either be compat with A or B. But B is compatible
> > with A so you express it as a list of items. If B is compat with A then
> > A is compat with B. Does the order of items matter?
>
> The two devices are compatible with each other, based on an inspection of
> the driver and the existing "A+B" setup. If this was a newly submitted
> binding, "B" would not get approved because "A+B" allows support without
> software changes and all that jazz.
>
> Your patch says that allowing "A", "B" and "A+B" makes no sense and you
> suggest removing "A+B". I am agreeing that it makes no sense to allow
> all 3 of these situations.
>
> What I also noticed is other problems with the binding. What should have
> been "A+B" is actually documented as "B+A", but that doesn't make sense
> when the originally supported device is "A".
>
> Therefore my suggestion was to only allow "A" and "A+B", which is what
> we would (hopefully) tell you to do were you submitting the am64 support
> as a new patch today.

Thank you for the in-depth explanation! It makes much more sense now,
especially the handling of historic stuff that ideally wouldn't have
been done this way but that won't be changed from now on.

> > I've not applied your proposal to check for dtbs_check but I'd guess it
> > would throw warnings for the single existing upstream DTSI (as of
> > v6.8-rc6) that uses "ti,am64-usb"? See:
> > arch/arm64/boot/dts/ti/k3-am64-main.dtsi.
>
> Yeah, it would but it's not as if that cannot be changed. There's no
> concerns here about backwards compatibility here, right?

I'm not involved in the maintenance of this platform so I do not believe
I should be answering this question. I asked the question because I
taught there always were concerns of backwards-compat when it comes to
DT and dt-bindings (in the best of all possible worlds).

K3 maintainers are already in cc.

Thanks,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com