linux-kernel - Re: [PATCH 1/2] dt-bindings: phy: imx8mq-usb: add phy tuning properties

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Date:   Wed, 12 Apr 2023 08:39:21 -0500
From:   Rob Herring <robh@...nel.org>
To:     Johannes Zink <j.zink@...gutronix.de>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        vkoul@...nel.org, kishon@...nel.org, shawnguo@...nel.org,
        s.hauer@...gutronix.de, kernel@...gutronix.de, festevam@...il.com,
        linux-imx@....com, krzysztof.kozlowski+dt@...aro.org,
        jun.li@....com, haibo.chen@....com, linux-phy@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH 1/2] dt-bindings: phy: imx8mq-usb: add phy tuning
 properties

On Tue, Apr 11, 2023 at 04:22:37PM +0200, Johannes Zink wrote:
> Hi Krzystof,
> 
> thank you for your explanations. As I'm still quite new to writing
> bindings, I still have some questions:
> 
> On Fri, 2023-04-07 at 11:03 +0200, Krzysztof Kozlowski wrote:
> > On 05/04/2023 14:14, Johannes Zink wrote:
> > > Hi Krysztof,
> > > 
> > > thanks for your review, please find my questions below.
> > > 
> > > On Wed, 2023-04-05 at 13:51 +0200, Krzysztof Kozlowski wrote:
> > > > [snip]
> > > > >        A phandle to the regulator for USB VBUS.
> > > > >  
> > > > > +  fsl,phy-tx-vref-tune:
> > > > > +    description:
> > > > > +      HS DC Voltage level adjustment
> > > > 
> > > > "Level" in what units?
> > > > 
> > > 
> > > The datasheet just shows percent, ranging from -6 to +24%, in 2%
> > > increments. What unit would you suggest?
> > 
> > percent
> > https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/property-units.yaml
> 
> I am still a bit confused how to use this properly. How can I restrict
> the values to multiples of 2 in order to avoid illegal values?
> 
> At the moment the only thing I could come up with is something like
> 
> fsl,phy-tx-vref-tune-percent:                 
>   description: |                              
>     Adjusts the high-speed DC level voltage   
>   $ref: /schemas/types.yaml#/definitions/int32

Note that with standard unit suffixes, you don't need a type.

>   minimum: -6                                 
>   maximum: 24                                 
>   default: 0                                  
> 
> Does something like this work? I am not quite sure if I am on the right
> track here, especially as this requires a signed int, of which I have
> not seen many examples so far.

We'd have to change the type for -percent to signed. That's possible, 
but for vendor specific properties there's not much advantage to use 
standard units instead of just using the register values directly.

Rob