linux-kernel - RE: [PATCH RFC 6/6] Modify tag_ksz.c to support other KSZ switch drivers

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Message-ID: <93AF473E2DA327428DE3D46B72B1E9FD41124E0C@CHN-SV-EXMX02.mchp-main.com>
Date:   Mon, 18 Sep 2017 23:44:42 +0000
From:   <Tristram.Ha@...rochip.com>
To:     <andrew@...n.ch>
CC:     <muvarov@...il.com>, <pavel@....cz>,
        <nathan.leigh.conrad@...il.com>,
        <vivien.didelot@...oirfairelinux.com>, <f.fainelli@...il.com>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <Woojung.Huh@...rochip.com>
Subject: RE: [PATCH RFC 6/6] Modify tag_ksz.c to support other KSZ switch
 drivers

> > In the old DSA implementation all the ports are partitioned into its own
> device
> > and the bridge joining them will do all the forwarding.  This is useful for
> quick
> > testing with some protocols like RSTP but it is probably useless for real
> > operation.
> 
> It is a good minimal driver, to get something into the kernel. You can
> then add features to it.
> 
> > The new switchdev model tries to use the switch hardware as much as
> > possible.  This offload_fwd_mark bit means the frame is forwarded by the
> > hardware switch, so the software bridge does not need to do it again.
> Without
> > this bit there will be duplicated multicast frames coming out the ports if
> internal
> > forwarding is enabled.
> 
> Correct. Once you switch driver is clever enough, you can enable
> offload_fwd_mark.
> 
> > When RSTP is used the port can be put in blocked state and so the
> forwarding
> > will stop for that port.   Currently the switch driver will check that
> membership
> > to decide whether to set that bit.
> 
> This i don't get. RSTP or STP just break loops. How does RSTP vs STP
> mean you need to set offload_fwd_mark differently?
> 

The logic of the switch driver is if the membership of the port receiving
the frame contains other ports--not counting cpu port--the bit
offload_fwd_mark is set.  In RSTP closing the blocked port is generally good
enough, but there are exceptions, so the port is removed from the
membership of other forwarding ports.  A disabled port will have its
membership completely reset so it cannot receive anything.  It does not
matter much in RSTP as the software bridge should know whether to forward
the frame or not.

We are back to square one.  Is there any plan to add this offload_fwd_mark
support to DSA driver so that it can be reported properly?  It can be set all the
time, except during port initialization or before bridge creation the forwarding
state does not reflect reality.

If not the port membership can be fixed and there is no internal switch
forwarding, leaving everything handled by the software bridge.