Re: [PATCH net-next 5/6] arm64: dts: marvell: mcbin: enable the fourth network interface

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Thu, Dec 28, 2017 at 11:04:16AM +0100, Antoine Tenart wrote:
> Hi Russell,
> 
> On Wed, Dec 27, 2017 at 11:20:00PM +0000, Russell King - ARM Linux wrote:
> > On Wed, Dec 27, 2017 at 11:42:52PM +0100, Antoine Tenart wrote:
> > > 
> > > What do you suggest to describe this in the dt, to enable a port using
> > > the current PPv2 driver?
> > 
> > I don't - I'm merely pointing out that you're bodging support for the
> > SFP cage rather than productively discussing phylink for mvpp2.
> > 
> > As far as I remember, the discussion stalled at this point:
> > 
> > - You think there's modes that mvpp2 supports that are not supportable
> >   if you use phylink.
> > 
> > - I've described what phylink supports, and I've asked you for details
> >   about what you can't support.
> 
> That's not what I remembered. You had some valid points, and others
> related to PHY modes the driver wasn't supporting before the phylink
> transition. My understanding of this was that you wanted a full
> featured support while I only wanted to convert the already supported
> modes.

You are mistaken - you can get a full refresher on where things were
at via https://patchwork.kernel.org/patch/9963971/

There are two points in that thread where discussion stopped awaiting
input:

1. I asked for details about what mvpp2.c supports that phylink does
   not (as you indicated that there were certain things that mvpp2
   supports that phylink does not.)  I'm still awaiting a response.

2. 25th Sept, you indicated that you would get someone to test
   an issue related to in-band AN. No results of that testing have
   been forthcoming.

Consequently, the ball is in your court on both these issues.

I am not after a full featured support, what I'm after is ensuring
that phylink is (a) used correctly and (b) implementations using it
are correct.  Part of that is ensuring that users don't introduce
unexpected failure conditions.

So, when you do this in the validate() callback:

 +       phylink_set(mask, 1000baseX_Full);

and then do this in the mac_config() callback:

 +       if (!phy_interface_mode_is_rgmii(port->phy_interface) &&
 +           port->phy_interface != PHY_INTERFACE_MODE_SGMII)
 +               return;

and this in the link_state() callback:

 +       if (!phy_interface_mode_is_rgmii(port->phy_interface) &&
 +           port->phy_interface != PHY_INTERFACE_MODE_SGMII)
 +               return 0;

the result is that phylink thinks that you support 1000base-X modes,
and it will call mac_config() asking for 1000base-X, but you silently
ignore that, leaving the hardware configured in whatever state it was.
That leads to a silent failure as far as the user is concerned.

So, if you do not intend to support 1000base-X initially, don't
allow it in the validate callback until you do.

It gets worse, because the return in link_state() means that phylink
thinks that the link is up if it has requested 1000base-X, which it
won't be unless you've properly configured it.

It's this kind of unreliability that I was concerned about in your
patch.  I'm not demanding "full featured implementation" but I do
want you to use it correctly.

> You're probably right about not wanting this dt patch. The non-dt
> patches still are relevant regardless of phylink being supported in the
> PPv2 driver. I'll send a v2 without the dt parts.

Thanks.

> > What I'm most concerned about, given the bindings for comphy that
> > have been merged, is that Free Electrons is pushing forward seemingly
> > with no regard to the requirement that the serdes lanes are dynamically
> > reconfigurable, and that's a basic requirement for SFP, and for the
> > 88x3310 PHYs on the Macchiatobin platform.
> 
> The main idea behind the comphy driver is to provide a way to
> reconfigure the serdes lanes at runtime. Could you develop what are
> blocking points to properly support SFP, regarding the current comphy
> support?

If it supports serdes lane mode reconfiguration (iow, switching between
1000base-X, 2500base-X, SGMII, 10G-KR), then that's all that's required.
Note that you need comphy to switch between SGMII / 10G-KR to support
the 88x3310 fully too.

Having looked deeper at this, it seems it does have the capability of
doing what's required, sorry for the noise.

-- 
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