Subject: Re: [RFC PATCH net-next 1/2] net: dcb: add new pcp selector to app object

From: Petr Machata <[email-masked]>
Date: Mon, 12 Sep 2022 18:15:01 +0200

Daniel Machon <[email-masked]> writes:

> Add new PCP selector for the 8021Qaz APP managed object.
>
> The purpose of adding the PCP selector, is to be able to offload
> PCP-based queue classification to the 8021Q Priority Code Point table,
> see 6.9.3 of IEEE Std 802.1Q-2018.
>
> PCP and DEI is encoded in the protocol field as 8*dei+pcp, so that a
> mapping of PCP 2 and DEI 1 to priority 3 is encoded as {255, 10, 3}.
>
> While PCP is not a standard 8021Qaz selector, it seems very convenient
> to add it to the APP object, as this is where similar priority mapping
> is handled, and it perfectly fits the {selector, protocol, priority}
> triplet.
>
> Signed-off-by: Daniel Machon <[email-masked]>
> ---
>  include/uapi/linux/dcbnl.h | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/include/uapi/linux/dcbnl.h b/include/uapi/linux/dcbnl.h
> index a791a94013a6..8eab16e5bc13 100644
> --- a/include/uapi/linux/dcbnl.h
> +++ b/include/uapi/linux/dcbnl.h
> @@ -217,6 +217,7 @@ struct cee_pfc {
>  #define IEEE_8021QAZ_APP_SEL_DGRAM	3
>  #define IEEE_8021QAZ_APP_SEL_ANY	4
>  #define IEEE_8021QAZ_APP_SEL_DSCP       5
> +#define IEEE_8021QAZ_APP_SEL_PCP	255
>  
>  /* This structure contains the IEEE 802.1Qaz APP managed object. This
>   * object is also used for the CEE std as well.

I'm thinking how to further isolate this from the IEEE standard values.
I think it would be better to pass the non-standard APP contributions in
a different attribute. IIUIC, this is how the APP table is passed:

DCB_ATTR_IEEE_APP_TABLE {
    DCB_ATTR_IEEE_APP {
        struct dcb_app { ... };
    }
    DCB_ATTR_IEEE_APP {
        struct dcb_app { ... };
    }
}

Well, instead, the non-standard stuff could be passed in a different
attribute:

DCB_ATTR_IEEE_APP_TABLE {
    DCB_ATTR_IEEE_APP {
        struct dcb_app { ... }; // standard contribution to APP table
    }
    DCB_ATTR_DCB_APP {
        struct dcb_app { ... }; // non-standard contribution
    }
}

The new selector could still stay as 255. This will allow us to keep the
internal bookkeeping simple for the likely case that 255 never becomes a
valid IEEE selector. But if it ever does, the UAPI can stay the same,
just the internals will need to be updated.