Date:	Mon, 28 Dec 2015 15:39:55 -0800
From:	Dustin Byford <dustin@...ulusnetworks.com>
To:	Florian Fainelli <f.fainelli@...il.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	netdev@...r.kernel.org
Subject: Re: [PATCH RFC 00/26] Phylink & SFP support

Hi Florian,

On Sun Dec 27 18:08, Florian Fainelli wrote:
> On December 14, 2015 11:26:21 PM PST, Dustin Byford <dustin@...ulusnetworks.com> wrote:
> >On Mon Dec 07 17:35, Russell King - ARM Linux wrote:
> >> Hi,
> >
> >Hello.
> >
> >> SFP modules are hot-pluggable ethernet transceivers; they can be
> >> detected at runtime and accordingly configured.  There are a range of
> >> modules offering many different features.
> >> 
> >> Some SFP modules have PHYs conventional integrated into them, others
> >> drive a laser diode from the Serdes bus.  Some have monitoring,
> >others
> >> do not.
> >> 
> >> Some SFP modules want to use SGMII over the Serdes link, others want
> >> to use 1000base-X over the Serdes link.
> >> 
> >> This makes it non-trivial to support with the existing code
> >structure.
> >> Not wanting to write something specific to the mvneta driver, I
> >decided
> >> to have a go at coming up with something more generic.
> >> 
> >> My initial attempts were to provide a PHY driver, but I found that
> >> phylib's state machine got in the way, and it was hard to support two
> >> chained PHYs.  Conversely, having a fixed DT specified setup (via
> >> the fixed phy infrastructure) would allow some SFP modules to work,
> >but
> >> not others.  The same is true of the "managed" in-band status (which
> >> is SGMII.)
> >> 
> >> The result is that I came up with phylink - an infrastructure layer
> >> which sits between the network driver and any attached PHY, and a
> >> SFP module layer detects the SFP module, and configures phylink
> >> accordingly.
> >> 
> >> Overall, this supports:
> >> 
> >> * switching the serdes mode at the NIC driver
> >> * controlling autonegotiation and autoneg results
> >> * allowing PHYs to be hotplugged
> >> * allowing SFP modules to be hotplugged with proper link indication
> >> * fixed-mode links without involving phylib
> >> * flow control
> >> * EEE support
> >> * reading SFP module EEPROMs
> >> 
> >> Overall, phylink supports several link modes, with dynamic switching
> >> possible between these:
> >> * A true fixed link mode, where the parameters are set by DT.
> >> * PHY mode, where we read the negotiation results from the PHY
> >registers
> >>   and pass them to the NIC driver.
> >> * SGMII mode, where the in-band status indicates the speed, duplex
> >and
> >>   flow control settings of the link partner.
> >> * 1000base-X mode, where the in-band status indicates only duplex and
> >>   flow control settings (different, incompatible bit layout from
> >SGMII.)
> >
> >I've been working on some similar code to handle interactions with a
> >wide range of SFF modules, 1G to 100G, on Linux network switches for
> >some time.  For practical reasons a lot of that was in userspace but
> >I've been planning and recently working on an SFF kernel driver that
> >does some of what's done in this series.  I think the model you're
> >proposing is right on, and since you're further along in implementation
> >I'd like to help round out support for the other SFF modules if I can.
> >Then make this work on the network ASICs I have access to.
> >
> >Any concrete plans for QSFP or the new 25G modules?
> >
> >> Ethtool support is included, as well as emulation of the MII
> >registers
> >> for situations where a PHY is not attached, giving compatible
> >emulation
> >> of existing user interfaces where required.
> >> 
> >> The patches here include modification of mvneta (against 4.4-rc1, so
> >> probably won't apply to current development tips.)  It basically
> >> hooks into the places where the phylib would hook into.
> >> 
> >> DT wise, the changes needed to support SFP look like this (example
> >> taken from Clearfog):
> >> 
> >>  			ethernet@...00 {
> >> +				managed = "in-band-status";
> >>  				phy-mode = "sgmii";
> >>  				status = "okay";
> >> -
> >> -				fixed-link {
> >> -					speed = <1000>;
> >> -					full-duplex;
> >> -				};
> >>  			};
> >> ...
> >> +	sfp: sfp {
> >> +		compatible = "sff,sfp";
> >> +		i2c-bus = <&i2c1>;
> >> +		los-gpio = <&expander0 12 GPIO_ACTIVE_HIGH>;
> >> +		moddef0-gpio = <&expander0 15 GPIO_ACTIVE_LOW>;
> >> +		sfp,ethernet = <&eth2>;
> >
> >Using &eth2 is unambiguous in the this case because there's only one
> >serdes and one mac involved.  To specify the mac/serdes/cage
> >associations at the same level of detail as the gpios it might be nice
> >(at least for some devices) to point to a serdes node (or 4 in the case
> >of QSFP) instead of &eth2.  Any thoughts on that?
> 
> Using a phandle here allows for quite a lot of flexibility on how you want to associate a given SFP to its data plane partner. I do not think we need to get more strict than that strictly mandate an actual Ethernet controller node. These Marvell adapters typically have one or more " ports", each of them being backed by a netdev. The same could be true with a switch properly modeled.

On a switch though, the number of "ports" is often configurable.
Physically the q/sfp cage to ASIC wiring is fixed, but when you've
plugged in a breakout cable you get four "ports" for a single QSFP cage.
They act like four separate devices in most ways, the notable exception
is that they share a QSFP module eeprom and the discrete IOs to the cage
like "reset" and "interrupt"  At the MAC layer, each port gets an
independent set of resources and they act like separate netdevs.

A concrete proposal might be to add a "channel" or "lane" parameter to
sfp,ethernet with a default of 0.

sfp,ethernet = <&eth2>

is equivalent to:
sfp,ethernet = <&eth2 0>


SFP on a switch0 device with 128 channels:

sfp,ethernet = <&switch0 42>
consumes channel 42

qsfp,ethernet = <&switch0>
consumes channels 0-3

qsfp,ethernet = <&switch0 124 125 126 127>
consumes channels 124-127

alternatives:

(less explicit, assume adjacent channels)
qsfp,ethernet = <&switch0 124> // consumes 124-127

(more explicit, don't assume the same device)
qsfp,ethernet = <&switch0 124 &switch0 125 &switch0 126 &switch0 127>
or:
qsfp,ethernet0 = <&switch0 124>
qsfp,ethernet1 = <&switch0 125>
qsfp,ethernet2 = <&switch0 126>
qsfp,ethernet3 = <&switch0 127>

(move complexity to the NIC/ASIC, ensure one channel per handle on the
NIC/ASIC side)
qsfp,ethernet = <&switch0c124 &switch0c125 &switch0c126 &switch0c127>


> >Switch ASICs, and I imagine at least some NICs, are really flexible in
> >terms of how serdes are wired to a cage.  Both in the sense that the
> >board designer gets to pick which wires route to the cage based on
> >physical constraints and the user gets to pick which serdes or group of
> >serdes compose the ethernet device.  For example, using a breakout
> >cable
> >to get 4xSFP out of a QSFP or the other way around.
> >
> >Perhaps the simple case (sfp,ethernet -> &eth2) can remain simple, but
> >I'd be interested in any thoughts you have on introducing a serdes
> >layer here.
> >
> >I think adding such a layer would make it easier to 1) make serdes to
> >cage mappings part of the platform description (DT or ACPI) and 2)
> >allow
> >automatic reconfiguration of the mac based on the SFF module.  For
> >example, if a user plugs in a QSFP->4xSFP breakout cable why not
> >automatically create four netdevs instead of one?
> 
> Would this be something you expect to happen dynamically? Not that this does not seem reasonable but would these netdevs serve a different purpose than being control endpoints, or would they become real logical netdevs with separate data planes at the MAC they would be linked to?

Real netdevs with separate data planes.  Reconfiguring them dynamically
seems like a good theoretical goal but is probably impractical in most
cases.  Even if it's not dynamic I think it's a good example of why you
might want a QSFP device to have an ethernet handle that points to four
things instead of one.

> >> +		tx-disable-gpio = <&expander0 14 GPIO_ACTIVE_HIGH>;
> >> +		tx-fault-gpio = <&expander0 13 GPIO_ACTIVE_HIGH>;
> >> +	};
> >> 
> >> These DT changes are omitted from this patch set as the baseline DT
> >> file is not in mainline yet (has been submitted.)
> >
> >Cool.  Do you have a link to the DT patches?
> >
> >
> >In short, I think this is awesome, and I'd like to help where I can.
> >I'll start by having a look at the rest of the series.  I'd like to
> >apply it and see if I can make it work on one of my systems.
> >
> >Thanks,
> >
> >		--Dustin
> 
> 
> -- 
> Florian


		--Dustin
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