Subject: Re: [PATCH devicetree 2/2] dt-bindings: phy: Add `tx-amplitude-microvolt` property binding

From: Rob Herring <[email-masked]>
Date: Wed, 15 Dec 2021 15:58:40 -0600

On Wed, Dec 15, 2021 at 12:34:32AM +0100, Marek Behún wrote:
> Common PHYs often have the possibility to specify peak-to-peak voltage
> on the differential pair - the default voltage sometimes needs to be
> changed for a particular board.

I can envision needing this, but I can't say that I've seen custom 
properties being proposed for this purpose.

> 
> Add properties `tx-amplitude-microvolt` and
> `tx-amplitude-microvolt-names` for this purpose. The second property is
> needed to specify

Is the amplitude peak to peak? You just said it was, but perhaps make 
the property name more clearly defined: tx-p2p-microvolt

> 
> Example usage with only one voltage (it will be used for all supported
> PHY modes, the `tx-amplitude-microvolt-names` property is not needed in
> this case):
> 
>   tx-amplitude-microvolt = <915000>;
> 
> Example usage with voltages for multiple modes:
> 
>   tx-amplitude-microvolt = <915000>, <1100000>, <1200000>;
>   tx-amplitude-microvolt-names = "2500base-x", "usb", "pcie";

I'm not wild about the -names, but I think outside of ethernet most 
cases will only be 1 entry.

For a phy provider with multiple phys, what if each one needs a 
different voltage (for the same mode)?

> 
> Signed-off-by: Marek Behún <[email-masked]>
> ---
> 
> I wanted to constrain the values allowed in the
> `tx-amplitude-microvolt-names` property to:
> - ethernet SerDes modes (sgmii, qsgmii, 10gbase-r, 2500base-x, ...)
> - PCIe modes (pattern: ^pcie[1-6]?$)
> - USB modes (pattern: ^usb((-host|-device|-otg)?-(ls|fs|hs|ss|ss\+|4))?$)
> - DisplayPort modes (pattern: ^dp(-rbr|-hbr[23]?|-uhbr-(10|13.5|20))?$)
> - Camera modes (mipi-dphy, mipi-dphy-univ, mipi-dphy-v2.5-univ)
> - Storage modes (sata, ufs-hs, ufs-hs-a, ufs-hs-b)
> 
> But was unable to. The '-names' suffix implies string-array type, and
> string-array type does not allow to specify a type for all items in a
> simple way, i.e.:
>   items:
>     enum:
>       - sgmii
>       - sata
>       - usb
>       ...

Works here: Documentation/devicetree/bindings/arm/samsung/pmu.yaml:56

The requirement is you need to constrain the size with maxItems. It can 
be a 'should be enough for anyone' value.

> Such constraint does work when changing ethernet controller's
> `phy-connection-type` to array, see
> https://lore.kernel.org/netdev/[email-masked]/
> 
> But it seems that string-array type prohibits this.
> 
> ---
>  .../devicetree/bindings/phy/phy.yaml          | 22 +++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/phy/phy.yaml b/Documentation/devicetree/bindings/phy/phy.yaml
> index a5b6b04cff5b..8915065cf6c2 100644
> --- a/Documentation/devicetree/bindings/phy/phy.yaml
> +++ b/Documentation/devicetree/bindings/phy/phy.yaml
> @@ -31,9 +31,29 @@ properties:
>        Phandle to a regulator that provides power to the PHY. This regulator
>        will be managed during the PHY power on/off sequence.
>  
> +  tx-amplitude-microvolt:
> +    description:
> +      Transmit amplitude voltages in microvolts, peak-to-peak. If this property
> +      contains multiple values for various PHY modes, the
> +      'tx-amplitude-microvolt-names' property must be provided and contain
> +      corresponding mode names.
> +
> +  tx-amplitude-microvolt-names:
> +    description: |
> +      Names of the modes corresponding to voltages in the
> +      'tx-amplitude-microvolt' property. Required only if multiple voltages
> +      are provided.
> +
> +      If a value of 'default' is provided, the system should use it for any PHY
> +      mode that is otherwise not defined here. If 'default' is not provided, the
> +      system should use manufacturer default value.
> +
>  required:
>    - '#phy-cells'
>  
> +dependencies:
> +  tx-amplitude-microvolt-names: [ tx-amplitude-microvolt ]
> +
>  additionalProperties: true
>  
>  examples:
> @@ -46,6 +66,8 @@ examples:
>      phy: phy {
>        #phy-cells = <1>;
>        phy-supply = <&phy_regulator>;
> +      tx-amplitude-microvolt = <915000>, <1100000>, <1200000>;
> +      tx-amplitude-microvolt-names = "2500base-x", "usb-hs", "usb-ss";
>      };
>  
>      ethernet-controller {
> -- 
> 2.32.0
> 
>