netdev - Re: Possible to use both dev_mc_sync and __dev_mc

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Date:   Mon, 21 Mar 2022 20:42:59 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Alexander Duyck <alexanderduyck@...com>
Cc:     Jakub Kicinski <kuba@...nel.org>, Jiri Pirko <jiri@...dia.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Possible to use both dev_mc_sync and __dev_mc_sync?

On Mon, Mar 21, 2022 at 06:37:05PM +0000, Alexander Duyck wrote:
> > -----Original Message-----
> > From: Vladimir Oltean <olteanv@...il.com>
> > Sent: Monday, March 21, 2022 9:32 AM
> > To: Alexander Duyck <alexanderduyck@...com>; Jakub Kicinski
> > <kuba@...nel.org>; Jiri Pirko <jiri@...dia.com>; Florian Fainelli
> > <f.fainelli@...il.com>
> > Cc: netdev@...r.kernel.org
> > Subject: Possible to use both dev_mc_sync and __dev_mc_sync?
> I hadn't intended it to work this way. The expectation was that
> __dev_mc_sync would be used by hardware devices whereas dev_mc_sync
> was used by stacked devices such as vlan or macvlan.

Understood, thanks for confirming.

> Probably the easiest way to address it is to split things up so that
> you are using __dev_mc_sync if the switch supports mc filtering and
> have your dsa_slave_sync/unsync_mc call also push it down to the lower
> device, and then call dev_mc_sync after that so that if it hasn't
> already been pushed to the lower device it gets pushed.

Yes, I have a patch with that change, just wanted to make sure I'm not
missing something. It's less efficient because now we need to check
whether dsa_switch_supports_uc_filtering() for each address, whereas
before we checked only once, before calling __dev_uc_add(). Oh well.

> The assumption is that the lower device and the hardware would be
> synced in the same way. If we can't go that route we may have to look
> at implementing a different setup in terms of the reference counting
> such as what is done in __hw_addr_sync_multiple.

So as mentioned, I haven't really understood the internals of the
reference/sync counting schemes being used here. But why are there
different implementations for dev_mc_sync() and dev_mc_sync_multiple()?