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

[Sasha Levin]

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@xxxxxxx>
> >
> > [ 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@xxxxxxx>
> > Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
> > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> > ---
>
> 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