Subject: Re: [net-next PATCH v4 10/14] dt-bindings: net: dsa: qca8k: add LEDs definition example

From: Marek Behún <[email-masked]>
Date: Fri, 17 Mar 2023 09:14:10 +0100

Hello Christian, also Rob Herring, Andrew Lunn and Pavel Machek,

On Fri, 17 Mar 2023 03:31:21 +0100
Christian Marangi <[email-masked]> wrote:

> Add LEDs definition example for qca8k Switch Family to describe how they
> should be defined for a correct usage.
> 
> Signed-off-by: Christian Marangi <[email-masked]>
> ---
>  .../devicetree/bindings/net/dsa/qca8k.yaml    | 24 +++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/net/dsa/qca8k.yaml b/Documentation/devicetree/bindings/net/dsa/qca8k.yaml
> index 389892592aac..2e9c14af0223 100644
> --- a/Documentation/devicetree/bindings/net/dsa/qca8k.yaml
> +++ b/Documentation/devicetree/bindings/net/dsa/qca8k.yaml
> @@ -18,6 +18,8 @@ description:
>    PHY it is connected to. In this config, an internal mdio-bus is registered and
>    the MDIO master is used for communication. Mixed external and internal
>    mdio-bus configurations are not supported by the hardware.
> +  Each phy has at least 3 LEDs connected and can be declared
> +  using the standard LEDs structure.
>  
>  properties:
>    compatible:
> @@ -117,6 +119,7 @@ unevaluatedProperties: false
>  examples:
>    - |
>      #include <dt-bindings/gpio/gpio.h>
> +    #include <dt-bindings/leds/common.h>
>  
>      mdio {
>          #address-cells = <1>;
> @@ -226,6 +229,27 @@ examples:
>                      label = "lan1";
>                      phy-mode = "internal";
>                      phy-handle = <&internal_phy_port1>;
> +
> +                    leds {
> +                        #address-cells = <1>;
> +                        #size-cells = <0>;
> +
> +                        led@0 {
> +                            reg = <0>;
> +                            color = <LED_COLOR_ID_WHITE>;
> +                            function = LED_FUNCTION_LAN;
> +                            function-enumerator = <1>;
> +                            default-state = "keep";
> +                        };
> +
> +                        led@1 {
> +                            reg = <1>;
> +                            color = <LED_COLOR_ID_AMBER>;
> +                            function = LED_FUNCTION_LAN;
> +                            function-enumerator = <1>;
> +                            default-state = "keep";
> +                        };
> +                    };
>                  };

I have nothing against this, but I would like to point out the
existence of the trigger-sources DT property, and I would like to
discuss how this property should be used by the LED subsystem to choose
default behaviour of a LED.

Consider that we want to specify in device-tree that a PHY LED (or any
other LED) should blink on network activity of the network device
connected to this PHY (let's say the attached network device is eth0).
(Why would we want to specify this in devicetree? Because currently the
 drivers either keep the behaviour from boot or change it to something
 specific that is not configurable.)

We could specify in DT something like:
  eth0: ethernet-controller {
    ...
  }

  ethernet-phy {
    leds {
      led@0 {
        reg = <0>;
        color = <LED_COLOR_ID_GREEN>;
        trigger-sources = <&eth0>;
        function = LED_FUNCTION_ ?????? ;
      }
    }
  }

The above example specifies that the LED has a trigger source (eth0),
but we still need to specify the trigger itself (for example that
the LED should blink on activity, or the different kinds of link). In my
opinion, this should be specified by the function property, but this
property is currently used in other way: it is filled in with something
like "wan" or "lan" or "wlan", an information which, IMO,
should instead come from the devicename part of the LED, not the
function part.

Recall that the LED names are of the form
  devicename:color:function
where the devicename part is supposed to be something like mmc0 or
sda1. With LEDs that are associated with network devices I think the
corresponding name should be the name of the network device (like eth0),
but there is the problem of network namespaces and also that network
devices can be renamed :(.

So one option how to specify the behaviour of the LED to blink on
activity would be to set
  function = LED_FUNCTION_ACTIVITY;
but this would conflict with how currently some devicetrees use "lan",
"wlan" or "wan" as the function (which is IMO incorrect, as I said
above).

Another option would be to ignore the function and instead use
additional argument in the trigger-source property, something like
  trigger-sources = <&eth0 TRIGGER_SOURCE_ACTIVITY>;

I would like to start a discussion on this and hear about your opinions,
because I think that the trigger-sources and function properties were
proposed in good faith, but currently the implementation and usage is a
mess.

Marek