Hello Rob,

On Wed, 15 Dec 2021 15:58:40 -0600
Rob Herring [off-list ref] wrote:

On Wed, Dec 15, 2021 at 12:34:32AM +0100, Marek Behún wrote:

quoted

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.

quoted

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

Yes, it is peak to peak.

quoted

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)?

For such a provider I think the best way would be to have the different
PHYs each have a subnode:
  phy-provider {
    phy@0 {
      tx-p2p-microvolt = ...;
    };
  }

quoted

Signed-off-by: Marek Behún <kabel@kernel.org>
---

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.

Thx.

Marek