linux-kernel - Re: [RFC PATCH v3 net-next 10/10] docs: devicetree: add documentation for the VSC7512 SPI device

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Date:   Sat, 14 Aug 2021 22:08:54 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Colin Foster <colin.foster@...advantage.com>
Cc:     andrew@...n.ch, vivien.didelot@...il.com, f.fainelli@...il.com,
        davem@...emloft.net, kuba@...nel.org, robh+dt@...nel.org,
        claudiu.manoil@....com, alexandre.belloni@...tlin.com,
        UNGLinuxDriver@...rochip.com, hkallweit1@...il.com,
        linux@...linux.org.uk, netdev@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v3 net-next 10/10] docs: devicetree: add
 documentation for the VSC7512 SPI device

On Sat, Aug 14, 2021 at 11:40:40AM -0700, Colin Foster wrote:
> On Sat, Aug 14, 2021 at 02:47:21PM +0300, Vladimir Oltean wrote:
> > On Fri, Aug 13, 2021 at 07:50:03PM -0700, Colin Foster wrote:
> > > +* phy_mode = "sgmii": on ports 0, 1, 2, 3
> > 
> > > +			port@0 {
> > > +				reg = <0>;
> > > +				ethernet = <&mac>;
> > > +				phy-mode = "sgmii";
> > > +
> > > +				fixed-link {
> > > +					speed = <100>;
> > > +					full-duplex;
> > > +				};
> > > +			};
> > 
> > Your driver is unconditionally setting up the NPI port at gigabit and
> > you claim it works, yet the device tree sees a 100Mbps fixed-link? Which
> > one is right, what speed does the port operate at?
> 
> Good catch!
> 
> I made the change to ocelot_spi_vsc7512 yesterday to set it up as
> gigabit, tested it, and it still works. Previously for my testing I'd
> had it hard-coded to 100, because the Beaglebone I'm using only supports
> 100Mbps on eth0.
> 
> # phytool print swp1/0

Why are you showing the PHY registers of swp1? Why are these relevant at all?

> 
> ieee-phy: id:0x00070540
> 
>    ieee-phy: reg:BMCR(0x00) val:0x1040
>       flags:          -reset -loopback .[1m+aneg-enable.[0m -power-down -isolate -aneg-restart -collision-test
>       speed:          1000-half

Also, 1000/half sounds like an odd speed to end negotiation at.

> 
>    ieee-phy: reg:BMSR(0x01) val:0x796d
>       capabilities:   -100-b4 .[1m+100-f.[0m .[1m+100-h.[0m .[1m+10-f.[0m .[1m+10-h.[0m -100-t2-f -100-t2-h
>       flags:          .[1m+ext-status.[0m .[1m+aneg-complete.[0m -remote-fault .[1m+aneg-capable.[0m .[1m+link.[0m -jabber .[1m+ext-register.[0m
> 
> 
> Of course I understand that "it works" is not the same as "it's correct"
> 
> What I wanted to accomplish was to use the speed parameter and set up
> the link based on that. I looked through all the DSA drivers and
> couldn't find anything that seems to do that. The closest thing I saw
> was in mt7531_cpu_port_config where they set the speed to either 2500 or
> 1000 based on the interface. But nothing that I saw would explicitly set
> the speed based on this parameter.

As I mentioned in the other email, .phylink_mac_link_up is the function
you are looking for. Phylink parses the fixed-link and calls that
function for fixed-link ports with the speed and duplex specified. Check
and see if felix_phylink_mac_link_up is not in fact called with
link_an_mode == MLO_AN_FIXED, speed == SPEED_100 and duplex == DUPLEX_FULL,
then what you are doing with that and if it makes sense for what you are
trying to do.

> 
> So I think there's something I'm missing. The fixed-link speed should apply to 
> the CPU port on the switch (port@0)?

Is this a question? It is under port@0, the port with the 'ethernet'
property i.e. the CPU port, so why should it not?

> Then eth0 can be manually set to a specific speed, but if it doesn't
> match the fixed-link speed I'd be out of luck? Or should an ip link or
> ethtool command to eth0 modify the speeds of both sides of the
> connection? It feels like setting port@0 to the fastest speed and
> letting it negotiate down to eth0 makes sense...
> 
> To ask the same question a different way:
> 
> I can currently run "ethtool -s eth0 speed 10 duplex full autoneg on" 
> and the link at eth0 drops to 10Mbps. Pinging my desktop jumps from 
> about 400us to about 600us when I do that.

If eth0 is also a fixed-link, you should not be able to do that, no.
But the fact that you are able to do that means it's not a fixed-link,
you have a pair of PHYs that freely auto-negotiate the speed between the
BeagleBone and the switch.

> 
> Should I not be able to do that? It should be fixed at 100Mbps without
> autoneg, end of story? Because in the current configuration it feels
> like the fixed-link settings are more a suggestion than a rule...
> 

It should describe the hardware configuration, of course. It is
incorrect to describe one side of a copper PHY connection as fixed-link
and the other as having a phy-handle, and it sounds like this is what
you're doing. We need to see the device tree binding for eth0, and
maybe a picture of your setup if that is possible. How do you connect
the switch board to the BeagleBone? Is it an RJ45 cable or some sort of
PCIe-style connector with fingers for an SGMII SERDES lane, in which the
board is plugged?

The device tree says SGMII, the behavior says RJ45.