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

Sasha Levin
2020-08-11T16:43:04Z

On Tue, Aug 11, 2020 at 12:01:08AM +0300, Vladimir Oltean wrote:
>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.

Will do, thanks!

--
Thanks,
Sasha