linux-kernel - Re: [PATCH v2 phy 12/16] dt-bindings: phy: lynx-28g: add compatible strings per SerDes and instantiation

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Message-ID: <20250925130510.GA451343-robh@kernel.org>
Date: Thu, 25 Sep 2025 08:05:10 -0500
From: Rob Herring <robh@...nel.org>
To: Vladimir Oltean <vladimir.oltean@....com>
Cc: linux-phy@...ts.infradead.org, Ioana Ciornei <ioana.ciornei@....com>,
	Vinod Koul <vkoul@...nel.org>,
	Kishon Vijay Abraham I <kishon@...nel.org>,
	Josua Mayer <josua@...id-run.com>, linux-kernel@...r.kernel.org,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, devicetree@...r.kernel.org
Subject: Re: [PATCH v2 phy 12/16] dt-bindings: phy: lynx-28g: add compatible
 strings per SerDes and instantiation

On Wed, Sep 24, 2025 at 06:45:34PM +0300, Vladimir Oltean wrote:
> Hi Rob,
> 
> On Wed, Sep 24, 2025 at 08:54:29AM -0500, Rob Herring wrote:
> > > +description: |
> > 
> > Don't need '|' if no formatting to preserve.
> 
> Thanks, will drop.
> 
> > > +  "#address-cells":
> > > +    const: 1
> > > +    description: "Address cells for child lane nodes"
> > 
> > You don't need generic descriptions of common properties.
> 
> Ok, I'll also drop the description from #size-cells but keep it in
> #phy-cells (less obvious).
> 
> > > +
> > > +  "#size-cells":
> > > +    const: 0
> > > +    description: "Size cells for child lane nodes"
> > > +
> > >    "#phy-cells":
> > > +    description: "Number of cells in PHY specifier (legacy binding only)"
> > >      const: 1
> > >  
> > > @@ -32,9 +124,51 @@ examples:
> > >      soc {
> > >        #address-cells = <2>;
> > >        #size-cells = <2>;
> > > -      serdes_1: phy@...0000 {
> > > -        compatible = "fsl,lynx-28g";
> > > +
> > > +      serdes_1: serdes@...0000 {
> > > +        compatible = "fsl,lx2160a-serdes1";
> > >          reg = <0x0 0x1ea0000 0x0 0x1e30>;
> > > -        #phy-cells = <1>;
> > > +        #address-cells = <1>;
> > > +        #size-cells = <0>;
> > > +
> > > +        phy@0 {
> > > +          reg = <0>;
> > > +          #phy-cells = <0>;
> > > +        };
> > 
> > There's really no difference between having child nodes 0-7 and 8 phy 
> > providers vs. putting 0-7 into a phy cell arg and 1 phy provider. 
> > 
> > The only difference I see is it is more straight-forward to determine 
> > what lanes are present in the phy driver if the driver needs to know 
> > that. But you can also just read all 'phys' properties in the DT with a 
> > &serdes_1 phandle and determine that. Is that efficient? No, but you 
> > have to do that exactly once and probably has no measurable impact.
> > 
> > With that, then can't you simply just add a more specific compatible:
> > 
> > compatible = "fsl,lx2160a-serdes1", "fsl,lynx-28g";
> > 
> > Then you maintain some compatibility.
> > 
> > Rob
> 
> With the patches that have been presented to you thus far -- yes, this
> is the correct conclusion, there is not much of a difference. But this
> is not all.

That's all I can base my conclusion on if you don't tell me more...

> If I want in the future to apply the properties from
> Documentation/devicetree/bindings/phy/transmit-amplitude.yaml to just
> one of the lanes, how would I do that with just 1 phy provider? It's not
> so clear. Compared to 8 phy providers, each with its OF node => much
> easier to structure and to understand.

That's unfortunate that binding wasn't designed to support more than 
1 instance. You could do:

lane@0 {
  reg = <0>;
  tx-p2p-microvolt = <123>;
};

lane@1 {
  reg = <1>;
  tx-p2p-microvolt = <123>;
};

Yeah, that's about what you had, but it avoids changing the cell size. 
That should be a bit simpler to implement in the driver and to add to 
existing DTs as a fixup (because you don't have to change 'phys' entries 
everywhere).

Another option is go to cell size of 2 and stick the voltage in a cell. 
That approach doesn't work well if you have a 3rd, 4th, etc. cell to add 
later for more properties.

Your overlaying the old and new bindings approach works too. That 
approach is fine with me.

Rob