RE: [PATCH 2/2] net: phy: Associate device node with fixed PHY

["Madalin Bucur (OSS)" <madalin.bucur@oss.nxp.com>]

> -----Original Message-----
> From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
> Sent: 03 August 2020 17:10
> To: Madalin Bucur (OSS) <madalin.bucur@oss.nxp.com>
> Cc: Andrew Lunn <andrew@lunn.ch>; Vikas Singh
> <vikas.singh@puresoftware.com>; f.fainelli@gmail.com; hkallweit1@gmail.com;
> netdev@vger.kernel.org; Calvin Johnson (OSS) <calvin.johnson@oss.nxp.com>;
> kuldip dwivedi <kuldip.dwivedi@puresoftware.com>; Vikas Singh
> <vikas.singh@nxp.com>
> Subject: Re: [PATCH 2/2] net: phy: Associate device node with fixed PHY
> 
> On Mon, Aug 03, 2020 at 11:45:55AM +0000, Madalin Bucur (OSS) wrote:
> > > -----Original Message-----
> > > From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
> > > Sent: 03 August 2020 12:07
> > > To: Madalin Bucur (OSS) <madalin.bucur@oss.nxp.com>
> > > Cc: Andrew Lunn <andrew@lunn.ch>; Vikas Singh
> > > <vikas.singh@puresoftware.com>; f.fainelli@gmail.com;
> hkallweit1@gmail.com;
> > > netdev@vger.kernel.org; Calvin Johnson (OSS)
> <calvin.johnson@oss.nxp.com>;
> > > kuldip dwivedi <kuldip.dwivedi@puresoftware.com>; Vikas Singh
> > > <vikas.singh@nxp.com>
> > > Subject: Re: [PATCH 2/2] net: phy: Associate device node with fixed
> PHY
> > >
> > > On Mon, Aug 03, 2020 at 08:33:19AM +0000, Madalin Bucur (OSS) wrote:
> > > > > -----Original Message-----
> > > > > From: netdev-owner@vger.kernel.org <netdev-owner@vger.kernel.org>
> On
> > > > > Behalf Of Andrew Lunn
> > > > > Sent: 01 August 2020 18:11
> > > > > To: Russell King - ARM Linux admin <linux@armlinux.org.uk>
> > > > > Cc: Vikas Singh <vikas.singh@puresoftware.com>;
> f.fainelli@gmail.com;
> > > > > hkallweit1@gmail.com; netdev@vger.kernel.org; Calvin Johnson (OSS)
> > > > > <calvin.johnson@oss.nxp.com>; kuldip dwivedi
> > > > > <kuldip.dwivedi@puresoftware.com>; Madalin Bucur (OSS)
> > > > > <madalin.bucur@oss.nxp.com>; Vikas Singh <vikas.singh@nxp.com>
> > > > > Subject: Re: [PATCH 2/2] net: phy: Associate device node with
> fixed
> > > PHY
> > > > >
> > > > > On Sat, Aug 01, 2020 at 10:41:32AM +0100, Russell King - ARM Linux
> > > admin
> > > > > wrote:
> > > > > > On Sat, Aug 01, 2020 at 09:52:52AM +0530, Vikas Singh wrote:
> > > > > > > Hi Andrew,
> > > > > > >
> > > > > > > Please refer to the "fman" node under
> > > > > > > linux/arch/arm64/boot/dts/freescale/fsl-ls1046a-rdb.dts
> > > > > > > I have two 10G ethernet interfaces out of which one is of
> fixed-
> > > link.
> > > > > >
> > > > > > Please do not top post.
> > > > > >
> > > > > > How does XGMII (which is a 10G only interface) work at 1G speed?
> Is
> > > > > > what is in DT itself a hack because fixed-phy doesn't support
> 10G
> > > > > > modes?
> > > > >
> > > > > My gut feeling is there is some hack going on here, which is why
> i'm
> > > > > being persistent at trying to understand what is actually going on
> > > > > here.
> > > >
> > > > Hi Andrew,
> > > >
> > > > That platform used 1G fixed link there since there was no support
> for
> > > > 10G fixed link at the time. PHYlib could have tolerated 10G speed
> there
> > > > With a one-liner.
> > >
> > > That statement is false.  It is not a "one liner".  fixed-phy exposes
> > > the settings to userspace as a Clause 22 PHY register set, and the
> > > Clause 22 register set does not support 10G.  So, a "one liner" would
> > > just be yet another hack.  Adding Clause 45 PHY emulation support
> > > would be a huge task.
> > >
> > > > I understand that PHYLink is working to describe this
> > > > Better, but it was not there at that time. Adding the dependency on
> > > > PHYLink was not desirable as most of the users for the DPAA 1
> platforms
> > > > were targeting kernels before the PHYLink introduction (and last
> I've
> > > > looked, it's still under development, with unstable APIs so we'll
> > > > take a look at this later, when it settles).
> > >
> > > I think you need to read Documentation/process/stable-api-nonsense.rst
> > > particularly the section "Stable Kernel Source Interfaces".
> > >
> > > phylink is going to be under development for quite some time to come
> > > as requirements evolve.  For example, when support for QSFP interfaces
> > > is eventually worked out, I suspect there will need to be some further
> > > changes to the driver interface.  This is completely normal.
> > >
> > > Now, as to the stability of the phylink API to drivers, it has in fact
> > > been very stable - it has only changed over the course of this year to
> > > support split PCS, a necessary step for DPAA2 and a few others.  It
> has
> > > been around in mainline for two years, and has been around much longer
> > > than that, and during that time it has been in mainline, the MAC
> facing
> > > interface has not changed until recently.
> > >
> > > So, I find your claim to be quite unreasonable.
> >
> > I see you agree that there were and there will be many changes for a
> while,
> > It's not a complaint, I know hot it works, it's just a decision based on
> > required effort vs features offered vs user requirements. Lately it's
> been
> > time consuming to try to fix things in this area.
> 
> No, it hasn't been time consuming.  The only API changes as far as
> drivers are concerned have been:
> 
> 1. the change to the mac_link_up() prototype to move the setup of the
>    final link parameters out of mac_config() - and almost all of the
>    updates to users were done by me.
> 
> 2. the addition of split PCS support, introducing new interfaces, has
>    had minimal impact on those drivers that updated in step (1).
> 
> There have been no other changes as far as users are concerned.
> 
> Some of the difficulty with (1) has been that users of phylink appeared
> initially with no proper review, and consequently they got quite a lot
> wrong.  The most common error has been using state->speed, state->duplex
> in mac_config() methods irrespective of the AN mode, which has _always_
> since before phylink was merged into mainline, been totally unreliable.
> 
> That leads me on to the other visible "changes" for users are concerned,
> which may be interpreted as interface changes, but are not; they have
> all been clarifications to the documentation, to strengthen things such
> as "do not use state->speed and state->duplex in mac_config() for
> various specific AN modes".  Nothing has actually changed with any of
> those clarifications.
> 
> For example, if in in-band mode, and mac_config() uses state->speed
> and state->duplex, then it doesn't matter which version of phylink
> you're using, if someone issues ethtool -s ethN ..., then state->speed
> and state->duplex will be their respective UNKNOWN values, and if you're
> using these in that situation, you will mis-program the MAC.
> 
> Again, that is not something that has changed.  Ever.  But the
> documentation has because people just don't seem to get it, and I seemed
> to be constantly repeating myself in review after review on the same
> points.
> 
> So, your assertion that the phylink API is not stable is false.  It
> has been remarkably stable over the two years that it has been around.
> It is only natural that as the technology that a piece of code supports
> evolves, so the code evolves with it.  That is exactly what has happened
> this year with the two changes I mention above.
> 
> Now, if you've found it time consuming to "fix things" (unspecified what
> "things" are) then I assert that what has needed to be fixed are things
> that NXP have got wrong.  Such as the rtnl cockups.  Such as abusing
> state->speed and state->duplex.  None of that is because the interface
> is unstable - they are down to buggy implementation on NXPs part.
> 
> Essentially, what I'm saying is that your attempt to paint phylink as
> being painful on the basis of interface changes is totally and utterly
> wrong and is just an excuse to justify abusing the fixed-link code and
> specifying things that are clearly incorrect via DT.

Thank you for the distilled phylink history, it may be easier to comprehend
with these details. I was not referring to phylink, but PHY related issues
on the DPAA 1 platforms.

> I will accept that the interface that had existed up until the
> mac_link_up() change was confusing - it clearly was due to the number
> of people getting mac_config() implementations wrong.  That is actually
> another of the reasons why the mac_link_up() change was made.  These
> problems are _only_ found by people making use of it.  If people don't
> use stuff, then problems aren't found, and nothing changes.
> 
> So, I think you can expect a NAK for the patch at the top of this
> thread on the basis that it is perpetuating an abuse not only the
> legacy fixed-link code, but also DT.  However, I will leave Andrew to
> make that call.
> 
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