netdev - Re: [PATCH] net: macb: reject unsupported rgmii delays

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Date:   Wed, 17 Jun 2020 14:32:09 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Helmut Grohne <helmut.grohne@...enta.de>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: macb: reject unsupported rgmii delays

On Wed, 17 Jun 2020 at 13:56, Russell King - ARM Linux admin
<linux@...linux.org.uk> wrote:
>
> On Tue, Jun 16, 2020 at 09:49:56AM +0200, Helmut Grohne wrote:
> > The macb driver does not support configuring rgmii delays. At least for
> > the Zynq GEM, delays are not supported by the hardware at all. However,
> > the driver happily accepts and ignores any such delays.
> >
> > When operating in a mac to phy connection, the delay setting applies to
> > the phy. Since the MAC does not support delays, the phy must provide
> > them and the only supported mode is rgmii-id.  However, in a fixed mac
> > to mac connection, the delay applies to the mac itself. Therefore the
> > only supported rgmii mode is rgmii.
>
> This seems incorrect - see the phy documentation in
> Documentation/networking/phy.rst:
>
> * PHY_INTERFACE_MODE_RGMII: the PHY is not responsible for inserting any
>   internal delay by itself, it assumes that either the Ethernet MAC (if capable
>   or the PCB traces) insert the correct 1.5-2ns delay
>
> * PHY_INTERFACE_MODE_RGMII_TXID: the PHY should insert an internal delay
>   for the transmit data lines (TXD[3:0]) processed by the PHY device
>
> * PHY_INTERFACE_MODE_RGMII_RXID: the PHY should insert an internal delay
>   for the receive data lines (RXD[3:0]) processed by the PHY device
>
> * PHY_INTERFACE_MODE_RGMII_ID: the PHY should insert internal delays for
>   both transmit AND receive data lines from/to the PHY device
>
> Note that PHY_INTERFACE_MODE_RGMII, the delay can be added by _either_
> the MAC or by PCB trace routing.
>

What does it mean "can" be added? Is it or is it not added? As a MAC
driver, what do you do?

> The individual RGMII delay modes are more about what the PHY itself is
> asked to do with respect to inserting delays, so I don't think your
> patch makes sense.
>

We all read the phy-mode documentation, but we aren't really any
smarter. That document completely fails to address the existence of
PCB traces.
Helmut's link points to some more discussion around this topic.

> --
> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
> FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!