Re: [PATCH AUTOSEL 5.7 03/60] net: mscc: ocelot: fix encoding destination ports into multicast IPv4 address

Vladimir Oltean
2020-08-10T21:01:14Z

Hi Sasha,

On Mon, Aug 10, 2020 at 03:09:31PM -0400, Sasha Levin wrote:
> From: Vladimir Oltean <vladimir.oltean@nxp.com>
>
> [ Upstream commit 0897ecf7532577bda3dbcb043ce046a96948889d ]
>
> The ocelot hardware designers have made some hacks to support multicast
> IPv4 and IPv6 addresses. Normally, the MAC table matches on MAC
> addresses and the destination ports are selected through the DEST_IDX
> field of the respective MAC table entry. The DEST_IDX points to a Port
> Group ID (PGID) which contains the bit mask of ports that frames should
> be forwarded to. But there aren't a lot of PGIDs (only 80 or so) and
> there are clearly many more IP multicast addresses than that, so it
> doesn't scale to use this PGID mechanism, so something else was done.
> Since the first portion of the MAC address is known, the hack they did
> was to use a single PGID for _flooding_ unknown IPv4 multicast
> (PGID_MCIPV4 == 62), but for known IP multicast, embed the destination
> ports into the first 3 bytes of the MAC address recorded in the MAC
> table.
>
> The VSC7514 datasheet explains it like this:
>
> 3.9.1.5 IPv4 Multicast Entries
>
> MAC table entries with the ENTRY_TYPE = 2 settings are interpreted
> as IPv4 multicast entries.
> IPv4 multicasts entries match IPv4 frames, which are classified to
> the specified VID, and which have DMAC = 0x01005Exxxxxx, where
> xxxxxx is the lower 24 bits of the MAC address in the entry.
> Instead of a lookup in the destination mask table (PGID), the
> destination set is programmed as part of the entry MAC address. This
> is shown in the following table.
>
> Table 78: IPv4 Multicast Destination Mask
>
> Destination Ports Record Bit Field
> ---------------------------------------------
> Ports 10-0 MAC[34-24]
>
> Example: All IPv4 multicast frames in VLAN 12 with MAC 01005E112233 are
> to be forwarded to ports 3, 8, and 9. This is done by inserting the
> following entry in the MAC table entry:
> VALID = 1
> VID = 12
> MAC = 0x000308112233
> ENTRY_TYPE = 2
> DEST_IDX = 0
>
> But this procedure is not at all what's going on in the driver. In fact,
> the code that embeds the ports into the MAC address looks like it hasn't
> actually been tested. This patch applies the procedure described in the
> datasheet.
>
> Since there are many other fixes to be made around multicast forwarding
> until it works properly, there is no real reason for this patch to be
> backported to stable trees, or considered a real fix of something that
> should have worked.
>
> Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
> Signed-off-by: David S. Miller <davem@davemloft.net>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---

Could you please drop this patch from the 'stable' queues for 5.7 and
5.8? I haven't tested it on older kernels and without the other patches
sent in that series. I would like to avoid unexpected regressions if
possible.

Thanks,
-Vladimir