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

quoted

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 <daniel.machon@microchip.com>
---
 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.

I get your sentiment, but it seems a little far-fetched to me. The 
trade-off will be extra code, in trade for something that IMO very likely 
will not happen. Like you said earlier - how many selectors could one
possibly prioritize on?