mlxsw: spectrum: Track priorities in struct mlxsw_sp_hdroom

The mapping from priorities to buffers determines which buffers should be
configured. Lossiness of these priorities combined with the mapping
determines whether a given buffer should be lossy.

Currently this configuration is stored implicitly in DCB ETS, PFC and
ethtool PAUSE configuration. Keeping it together with the rest of the
headroom configuration and deriving it as needed from PFC / ETS / PAUSE
will make things clearer. To that end, add a field "prios" to struct
mlxsw_sp_hdroom.

Previously, __mlxsw_sp_port_headroom_set() took prio_tc as an argument, and
assumed that the same mapping as we use on the egress should be used on
ingress as well. Instead, track this configuration at each priority, so
that it can be adjusted flexibly.

In the following patches, as dcbnl_setbuffer is implemented, it will need
to store its own mapping, and it will also be sometimes necessary to revert
back to the original ETS mapping. Therefore track two buffer indices: the
one for chip configuration (buf_idx), and the source one (ets_buf_idx).
Introduce a function to configure the chip-level buffer index, and for now
have it simply copy the ETS mapping over to the chip mapping.

Update the ETS handler to project prio_tc to the ets_buf_idx and invoke the
buf_idx recomputation.

Now that there is a canonical place to look for this configuration,
mlxsw_sp_port_headroom_set() does not need to invent def_prio_tc to use if
DCB is compiled out.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>