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

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Date:   Wed, 17 Jun 2020 12:34:10 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Vladimir Oltean <olteanv@...il.com>
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, Jun 17, 2020 at 02:32:09PM +0300, Vladimir Oltean wrote:
> 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?

I'm just stating what is documented.

> > 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.

Why are you so abrasive?

Not responding to this until you start behaving more reasonably.

-- 
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