Re: [RFC PATCH net-next 1/2] net: dsa: allow setting port-based QoS priority using tc matchall skbedit

[Vladimir Oltean <olteanv@gmail.com>]

Hi Andrew,

On Thu, 14 Jan 2021 at 03:03, Andrew Lunn <andrew@lunn.ch> wrote:
> On Thu, Jan 14, 2021 at 02:17:59AM +0200, Vladimir Oltean wrote:
> > On Thu, Jan 14, 2021 at 12:41:28AM +0100, Andrew Lunn wrote:
> > > On Wed, Jan 13, 2021 at 05:41:38PM +0200, Vladimir Oltean wrote:
> > > > + int     (*port_priority_set)(struct dsa_switch *ds, int port,
> > > > +                              struct dsa_mall_skbedit_tc_entry *skbedit);
> > >
> > > The fact we can turn this on/off suggests there should be a way to
> > > disable this in the hardware, when the matchall is removed. I don't
> > > see any such remove support in this patch.
> >
> > I don't understand this comment, sorry. When the matchall filter
> > containing the skbedit action gets removed, DSA calls the driver's
> > .port_priority_set callback again, this time with a priority of 0.
> > There's nothing to "remove" about a port priority. I made an assumption
> > (which I still consider perfectly reasonable) that no port-based
> > prioritization means that all traffic gets classified to traffic class 0.
>
> That does not work for mv88e6xxx. Its default setup, if i remember
> correctly, is it looks at the TOS bits to determine priority
> classes. So in its default state, it is using all the available
> traffic classes.  It can also be configured to look at the VLAN
> priority, or the TCAM can set the priority class, or there is a per
> port default priority, which is what you are describing here. There
> are bits to select which of these happen on ingress, on a per port
> basis.
>
> So setting the port priority to 0 means setting the priority of
> zero. It does not mean go back to the default prioritisation scheme.
>
> I guess any switch which has a range of options for prioritisation
> selection will have a similar problem. It defaults to something,
> probably something a bit smarter than everything goes to traffic class
> 0.
>
>       Andrew

I was going through my old patches, and re-reading this conversation,
it appears one of us is misunderstanding something.

I looked at some Marvell datasheet and it has a similar QoS
classification pipeline to Vitesse switches. There is a port-based
default priority which can be overridden by IP DSCP, VLAN PCP, or
advanced QoS classification (TCAM).

The proposal I had was to configure the default port priority using tc
matchall skbedit priority. Advanced QoS classification would then be
expressed as tc-flower filters with a higher precedence than the
matchall (basically the "catchall"). PCP and DSCP, I don't know if
that can be expressed cleanly using tc. I think there's something in
the dcb ops, but I haven't studied that too deeply.

Anyway, I don't exactly understand your point, that an add/del is in
any way better than a "set". Even for Marvell, what I'm proposing here
would translate in a "set to 0" on "del" anyway. That's why this patch
set is RFC. I don't know if there's a better way to express a
port-based default priority than a matchall rule having the lowest
precedence.