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

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Date: Tue, 27 Feb 2024 08:07:02 +0000
From: Conor Dooley <conor.dooley@...rochip.com>
To: Vignesh Raghavendra <vigneshr@...com>
CC: Théo Lebrun <theo.lebrun@...tlin.com>, 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>, 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

On Tue, Feb 27, 2024 at 09:54:30AM +0530, Vignesh Raghavendra wrote:
> On 26/02/24 20:05, Théo Lebrun wrote:
> > 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".

This A and B stuff confused me, I should just have used the actual
compatibles. I meant
| What should have been "B+A" is actually documented as "A+B", 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.
> > 
> 
> IIRC, idea behind adding new compatible for AM64 even though register
> map is very much compatible is just being future proof as AM64 and J721e
> belong to different product groups and thus have differences wrt SoC
> level integration etc which may need SoC specific handling later on.

That is fine, I don't think anyone here is disputing a soc-specific
compatible existing for this device.

> Also, note that AM64 SoC support was added long after J721e. So ideally
> should be B+A if at all we need a fallback compatible.

Correct, I accidentally wrote "A+B", but you can see that that
conflicted with the actual example I had given above.

> I don't see any DT (now or in the past) using
> 
> compatible = B,A or compatible = A,B
> 
> So do we really need A+B to be supported by binding?

Given the mistake, I am going to take this as meaning should the
fallback be supported. My take is that if we are going to remove
something, it should be "ti,am64-usb" isolation that should go.
The devicetrees can be update without concerns about compatibility.

Cheers,
Conor.



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