Ocelot switches perform port-based ingress mirroring if
ANA:PORT:PORT_CFG field SRC_MIRROR_ENA is set, and egress mirroring if
the port is in ANA:ANA:EMIRRORPORTS.

Both ingress-mirrored and egress-mirrored frames are copied to the port
mask from ANA:ANA:MIRRORPORTS.

So the choice of limiting to a single mirror port via ocelot_mirror_get()
and ocelot_mirror_put() may seem bizarre, but the hardware model doesn't
map very well to the user space model. If the user wants to mirror the
ingress of swp1 towards swp2 and the ingress of swp3 towards swp4, we'd
have to program ANA:ANA:MIRRORPORTS with BIT(2) | BIT(4), and that would
make swp1 be mirrored towards swp4 too, and swp3 towards swp2. But there
are no tc-matchall rules to describe those actions.

Now, we could offload a matchall rule with multiple mirred actions, one
per desired mirror port, and force the user to stick to the multi-action
rule format for subsequent matchall filters. But both DSA and ocelot
have the flow_offload_has_one_action() check for the matchall offload,
plus the fact that it will get cumbersome to cross-check matchall
mirrors with flower mirrors (which will be added in the next patch).

As a result, we limit the configuration to a single mirror port, with
the possibility of lifting the restriction in the future.

Frames injected from the CPU don't get egress-mirrored, since they are
sent with the BYPASS bit in the injection frame header, and this
bypasses the analyzer module (effectively also the mirroring logic).
I don't know what to do/say about this.

Functionality was tested with:

tc qdisc add dev swp3 clsact
tc filter add dev swp3 ingress \
	matchall skip_sw \
	action mirred egress mirror dev swp1

and pinging through swp3, while seeing that the ICMP replies are
mirrored towards swp1.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>