Date:   Thu, 6 Jan 2022 21:56:56 +0100
From:   Ansuel Smith <ansuelsmth@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [net-next PATCH RFC v6 00/16] Add support for qca8k mdio rw in
 Ethernet packet

On Fri, Dec 17, 2021 at 01:38:12AM +0200, Vladimir Oltean wrote:
> On Wed, Dec 15, 2021 at 05:34:45PM +0100, Ansuel Smith wrote:
> > > > I tested this with multicpu port and with port6 set as the unique port and
> > > > it's sad.
> > > > It seems they implemented this feature in a bad way and this is only
> > > > supported with cpu port0. When cpu port6 is the unique port, the switch
> > > > doesn't send ack packet. With multicpu port, packet ack are not duplicated
> > > > and only cpu port0 sends them. This is the same for the MIB counter.
> > > > For this reason this feature is enabled only when cpu port0 is enabled and
> > > > operational.
> > > 
> > > Let's discuss this a bit (not the hardware limitation, that one is what
> > > it is). When DSA has multiple CPU ports, right now both host-side
> > > Ethernet ports are set up as DSA masters. By being a DSA master, I mean
> > > that dev->dsa_ptr is a non-NULL pointer, so these interfaces expect to
> > > receive packets that are trapped by the DSA packet_type handlers.
> > > But due to the way in which dsa_tree_setup_default_cpu() is written,
> > > by default only the first CPU port will be used. So the host port
> > > attached to the second CPU port will be a DSA master technically, but it
> > > will be an inactive one and won't be anyone's master (no dp->cpu_dp will
> > > point to this master's dev->dsa_ptr). My idea of DSA support for
> > > multiple CPU ports would be to be able to change the dp->cpu_dp mapping
> > > through rtnetlink, on a per user port basis (yes, this implies we don't
> > > have a solution for DSA ports).
> > 
> > I have a similar implementation that was proposed as RFC many times ago.
> 
> Yes, well, how to assign a user port to a CPU port seems not to be the
> biggest problem that needs to be solved before support for multiple CPU
> ports can fully go in.
>

Hi,
sorry for the delay.

I honestly think a start for multicpu that would improve the state would
be start adding support to iproute for changing the master port.

> > > My second observation is based on the fact that some switches support a
> > > single CPU port, yet they are wired using two Ethernet ports towards the
> > > host. The Felix and Seville switches are structured this way. I think
> > > some Broadcom switches too.
> > > Using the rtnetlink user API, a user could be able to migrate all user
> > > ports between one CPU port and the other, and as long as the
> > > configuration is valid, the switch driver should accept this (we perform
> > > DSA master changing while all ports are down, and we could refuse going
> > > up if e.g. some user ports are assigned to CPU port A and some user
> > > ports to CPU port B). Nonetheless, the key point is that when a single
> > > CPU port is used, the other CPU port kinda sits there doing nothing. So
> > > I also have some patches that make the host port attached to this other
> > > CPU port be a normal interface (not a DSA master).
> > > The switch side of things is still a CPU port (not a user port, since
> > > there still isn't any net device registered for it), but nonetheless, it
> > > is a CPU port with no DSA tagging over it, hence the reason why the host
> > > port isn't a DSA master. The patch itself that changes this behavior
> > > sounds something like "only set up a host port as a DSA master if some
> > > user ports are assigned to it".
> > > As to why I'm doing it this way: the device tree should be fixed, and I
> > > do need to describe the connection between the switch CPU ports and the
> > > DSA masters via the 'ethernet = <&phandle>;' property. From a hardware
> > > perspective, both switch ports A and B are CPU ports, equally. But this
> > > means that DSA won't create a user port for the CPU port B, which would
> > > be the more natural way to use it.
> > > Now why this pertains to you: Vivien's initial stab at management over
> > > Ethernet wanted to decouple a bit the concept of a DSA master (used for
> > > the network stack) from the concept of a host port used for in-band
> > > management (used for register access). Whereas our approach here is to
> > > keep the two coupled, due to us saying "hey, if there's a direct
> > > connection to the switch, this is a DSA master anyway, is it not?".
> > > Well, here's one thing which you wouldn't be able to do if I pursue my
> > > idea with lazy DSA master setup: if you decide to move all your user
> > > ports using rtnetlink to CPU port 6, then the DSA master of CPU port 0
> > > will cease to be a DSA master. So that will also prevent the management
> > > protocol from working.
> > 
> > About the migration problem, wonder if we can just use a refcount that
> > would represent the user of the master port. The port won't be DSA
> > master anymore if no user are connected. A switch can increase this ref
> > if the port is mandatory for some operation. (qca8k on state change
> > operational would increase the ref and decrease and then the port can be
> > removed from a DSA master) That should handle all the other switch and
> > still permit a driver to ""bypass"" this behaviour.
> 
> Maybe. Although not quite like the way in which you propose. Remember
> that the idea is for a DSA master to be a regular interface until it
> gains a user. So there's the chicken and egg problem if you want to
> become a user on ->master_state_change()... because it's not a master.
> You'd have to specify upfront.
> 

I mean we can really think of adding an option or a flag for the port
that will be used to declare a cpu port as to be ignored by any disable
procedure. From what I can remember some broadcom switch have some
management port that can't be disabled for example so I can see an use
where a flag of this kind would be useful. Some thing like declaring a
port as a managament port and with this case any ""cleanup"" function
will be ignored? That would solve the chicken-egg problem and dts won't
have to be changed.

> > > I don't want to break your use case, but then again, I'm wondering what
> > > we could do to support the second CPU port working without DSA tagging,
> > > without changing the device trees to declare it as a user port (which in
> > > itself isn't bad, it's just that we need to support all use cases with a
> > > single, unified device tree).
> > 
> > Just some info about the secondary CPU port.
> > From Documentation the second cpu port in sgmii mode can be used also for
> > other task so yes we should understand how to handle this. (base-x, mac
> > and phy) This mode is set based on the phy mode and if the dsa port is a
> > cpu port. Device tree changes can be accepted as AFAIK due to DSA not
> > supporting multi cpu, CPU port 6 was never used/defined. (But I'm
> > not sure... that is the case for all the device we have on openwrt)
> 
> What do you mean exactly by "other tasks"?
> 

I never notice a device with this (actually we just find one that has 2
qca8k switch that seems to be connected with the port6 port) but other
task are for example interconnecting 2 switch or attach some external
port like sfp. (I assume?)

> > Considering that introducing multicpu port would require a
> > bit of rework, wonder if we should introduce some new bindings/node and
> > fallback to a legacy (aka force first cpu port as the unique cpu port
> > and ignore others) in the absence of this new implementation. (Hoping I
> > didn't get all wrong with the main problem here)
> 
> The defaults would stay the same. (I've no idea why we would introduce
> new device tree bindings? the only device tree change IMO would be to
> declare the link between the second CPU port and its DSA master, if you
> haven't done that already) But my key point was that, to some extent,
> some change to the current behavior will still be required. Like right
> now, a kernel 5.15 when it sees a device tree with 2 CPU ports will have
> 2 DSA masters. Maybe kernel 5.17 will only start off with the first port
> as a DSA master, and the other just a candidate. I'm hoping this won't

IMHO that would be the correct way. Just offer a secondary port and
leave the user decide to use it or not. (Single CPU port by default and
leave the user the choice to use the second with an init script)

> change observable behavior for the worse for anyone, because device
> trees are supposed to be there to stay, not change on a whim. My hope is
> based on the fact that as far as I can see, that second DSA master is
> effectively useless. Which in fact creates the second problem: exactly
> because the second host port is useless with the current code structure,
> I can see people describing it as a user rather than CPU port in current
> device trees, just to make some use out of it. But that restricts the
> potential user base (and therefore appeal) of my change of behavior or
> of multi-CPU port support in general, and this is a bit sad. I think we
> should be all spending a bit more time with current-generation kernels
> on "updated" device trees with multiple CPU ports defined, and see
> what's broken and what could be improved, because otherwise we could be
> leaving behind a huge mess when the device trees get updated, and we
> need to run the occasional old kernel on them.
> 

Anyway I think I didn't understand you from the start. Your problem was
with user declaring any additional cpu port as an user port (that is not
usable?) and not declaring the needed master.

Something like

port@6 {
				reg = <6>;
				label = "swp6";
				phy-mode = "sgmii"
			};

instead of (current way we use to declare secondary port on qca8k)

port@6 {
				reg = <6>;
				label = "cpu";
				ethernet = <&gmac2>;
				phy-mode = "sgmii";

				fixed-link {
					speed = <1000>;
					full-duplex;
				};
			};

> > But if I'm not wrong there was at the start some years ago an idea of a
> > node used to declare master port separate from the generic port node but
> > it was rejected?
> 
> I don't know anything about this.

One of the first RFC for multicpu port (somethin many years ago) from
some Marvell devs was to declare an additional node (separate from the
current one) that would declare cpu ports. But it was a bit useless and
was dropped after some version.

Aside from these concern how should we proceed with this series? Should
we first understand the multicpu problem?

Again sorry for the dealy.
-- 
	Ansuel

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