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From:   Petr Machata <petrm@nvidia.com>
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Subject: Re: [PATCH net-next 0/6] Introduce new DCB rewrite table
Date:   Fri, 13 Jan 2023 17:11:50 +0100
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Daniel Machon <daniel.machon@microchip.com> writes:

> There is currently no support for per-port egress mapping of priority to PCP and
> priority to DSCP. Some support for expressing egress mapping of PCP is supported
> through ip link, with the 'egress-qos-map', however this command only maps
> priority to PCP, and for vlan interfaces only. DCB APP already has support for
> per-port ingress mapping of PCP/DEI, DSCP and a bunch of other stuff. So why not
> take advantage of this fact, and add a new table that does the reverse.
>
> This patch series introduces the new DCB rewrite table. Whereas the DCB
> APP table deals with ingress mapping of PID (protocol identifier) to priority,
> the rewrite table deals with egress mapping of priority to PID.
>
> It is indeed possible to integrate rewrite in the existing APP table, by
> introducing new dedicated rewrite selectors, and altering existing functions
> to treat rewrite entries specially. However, I feel like this is not a good
> solution, and will pollute the APP namespace. APP is well-defined in IEEE, and
> some userspace relies of advertised entries - for this fact, separating APP and
> rewrite into to completely separate objects, seems to me the best solution.
>
> The new table shares much functionality with the APP table, and as such, much
> existing code is reused, or slightly modified, to work for both.
>
> ================================================================================
> DCB rewrite table in a nutshell
> ================================================================================
> The table is implemented as a simple linked list, and uses the same lock as the
> APP table. New functions for getting, setting and deleting entries have been
> added, and these are exported, so they can be used by the stack or drivers.
> Additionnaly, new dcbnl_setrewr and dcnl_delrewr hooks has been added, to
> support hardware offload of the entries.

Looks good to me overall.

I just want to add that to configure rewrite, mlxsw currently reverses
the APP prioritization table. That's not ideal, and is lossy as
well--certain configurations simply can't be expressed however you set
up in-driver heuristics. The proposed interfaces would make
configuration of the rewrite functionality very straightforward.