Re: [PATCH net-next 1/3] net: dsa: mv88e6xxx: Allow dynamic reconfiguration of tag protocol

[Andrew Lunn <andrew@lunn.ch>]

On Fri, Mar 26, 2021 at 11:56:46AM +0100, Tobias Waldekranz wrote:
> All devices are capable of using regular DSA tags. Support for
> Ethertyped DSA tags sort into three categories:
> 
> 1. No support. Older chips fall into this category.
> 
> 2. Full support. Datasheet explicitly supports configuring the CPU
>    port to receive FORWARDs with a DSA tag.
> 
> 3. Undocumented support. Datasheet lists the configuration from
>    category 2 as "reserved for future use", but does empirically
>    behave like a category 2 device.

> +static int mv88e6xxx_change_tag_protocol(struct dsa_switch *ds, int port,
> +					 enum dsa_tag_protocol proto)
> +{
> +	struct mv88e6xxx_chip *chip = ds->priv;
> +	enum dsa_tag_protocol old_protocol;
> +	int err;
> +
> +	switch (proto) {
> +	case DSA_TAG_PROTO_EDSA:
> +		if (chip->info->tag_protocol != DSA_TAG_PROTO_EDSA)
> +			dev_warn(chip->dev, "Relying on undocumented EDSA tagging behavior\n");
> +
> +		break;
> +	case DSA_TAG_PROTO_DSA:
> +		break;
> +	default:
> +		return -EPROTONOSUPPORT;
> +	}

You are handling cases 2 and 3 here, but not 1. Which makes it a bit
of a foot cannon for older devices.

Now that we have chip->tag_protocol, maybe we should change
chip->info->tag_protocol to mean supported protocols?

BIT(0) DSA
BIT(1) EDSA
BIT(2) Undocumented EDSA

Andrew