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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Wed, 29 Apr 2020 10:45:35 +0200
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Subject: Re: [PATCH net-next v3] net: phy: micrel: add phy-mode support for
 the KSZ9031 PHY
To:     Philippe Schenker <philippe.schenker@toradex.com>
Cc:     "andrew@lunn.ch" <andrew@lunn.ch>,
        "sergei.shtylyov@cogentembedded.com" 
        <sergei.shtylyov@cogentembedded.com>,
        "kernel@pengutronix.de" <kernel@pengutronix.de>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "hkallweit1@gmail.com" <hkallweit1@gmail.com>,
        "linux@armlinux.org.uk" <linux@armlinux.org.uk>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "david@protonic.nl" <david@protonic.nl>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "o.rempel@pengutronix.de" <o.rempel@pengutronix.de>,
        "f.fainelli@gmail.com" <f.fainelli@gmail.com>,
        "linux-renesas-soc@vger.kernel.org" 
        <linux-renesas-soc@vger.kernel.org>,
        Kazuya Mizuguchi <kazuya.mizuguchi.ks@renesas.com>
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Hi Philippe,

On Tue, Apr 28, 2020 at 6:16 PM Philippe Schenker
<philippe.schenker@toradex.com> wrote:
> On Tue, 2020-04-28 at 17:47 +0200, Andrew Lunn wrote:
> > On Tue, Apr 28, 2020 at 05:28:30PM +0200, Geert Uytterhoeven wrote:
> > > This triggers on Renesas Salvator-X(S):
> > >
> > >     Micrel KSZ9031 Gigabit PHY e6800000.ethernet-ffffffff:00:
> > > *-skew-ps values should be used only with phy-mode = "rgmii"
> > >
> > > which uses:
> > >
> > >         phy-mode = "rgmii-txid";
> > >
> > > and:
> > >
> > >         rxc-skew-ps = <1500>;
> > >
> > > If I understand Documentation/devicetree/bindings/net/ethernet-
> > > controller.yaml
> > > correctly:
> >
> > Checking for skews which might contradict the PHY-mode is new. I think
> > this is the first PHY driver to do it. So i'm not too surprised it has
> > triggered a warning, or there is contradictory documentation.
> >
> > Your use cases is reasonable. Have the normal transmit delay, and a
> > bit shorted receive delay. So we should allow it. It just makes the
> > validation code more complex :-(
>
> I reviewed Oleksij's patch that introduced this warning. I just want to
> explain our thinking why this is a good thing, but yes maybe we change
> that warning a little bit until it lands in mainline.
>
> The KSZ9031 driver didn't support for proper phy-modes until now as it
> don't have dedicated registers to control tx and rx delays. With
> Oleksij's patch this delay is now done accordingly in skew registers as
> best as possible. If you now also set the rxc-skew-ps registers those
> values you previously set with rgmii-txid or rxid get overwritten.
>
> We chose the warning to occur on phy-modes 'rgmii-id', 'rgmii-rxid' and
> 'rgmii-txid' as on those, with the 'rxc-skew-ps' value present,
> overwriting skew values could occur and you end up with values you do
> not wanted. We thought, that most of the boards have just 'rgmii' set in
> phy-mode with specific skew-values present.
>
> @Geert if you actually want the PHY to apply RXC and TXC delays just
> insert 'rgmii-id' in your DT and remove those *-skew-ps values. If you

That seems to work for me, but of course doesn't take into account PCB
routing.

> need custom timing due to PCB routing it was thought out to use the phy-
> mode 'rgmii' and do the whole required timing with the *-skew-ps values.

That mean we do have to provide all values again?
Using "rgmii" without any skew values makes DHCP fail on R-Car H3 ES2.0,
M3-W (ES1.0), and M3-N (ES1.0). Interestingly, DHCP still works on R-Car
H3 ES1.0.

Note that I'm not too-familiar with the actual skew values needed
(CC Mizuguchi-san).

Related commits:
  - 0e45da1c6ea6b186 ("arm64: dts: r8a7795: salvator-x: Fix
EthernetAVB PHY timing")
  - dda3887907d74338 ("arm64: dts: r8a7795: Use rgmii-txid phy-mode
for EthernetAVB")
  - 7eda14afb8843a0d ("arm64: dts: renesas: r8a77990: ebisu: Fix
EthernetAVB phy mode to rgmii")

Thanks!

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds