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

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Message-ID: <20200617115725.GR1551@shell.armlinux.org.uk>
Date:   Wed, 17 Jun 2020 12:57:26 +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 02:38:25PM +0300, Vladimir Oltean wrote:
> On Wed, 17 Jun 2020 at 14:34, Russell King - ARM Linux admin
> <linux@...linux.org.uk> wrote:
> >
> 
> >
> > Why are you so abrasive?
> >
> > Not responding to this until you start behaving more reasonably.
> >
> > --
> > RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
> > FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!
> 
> Sorry.
> What I meant to say is: the documentation is unclear. It has been
> interpreted in conflicting ways, where the strict interpretations have
> proven to have practical limitations, and the practical
> interpretations have been accused of being incorrect. In my opinion
> there is no way to fix it without introducing new bindings, which I am
> not sure is really worth doing.

The documentation was added in 2016, many years after we have had users
of this, in an attempt to clear up some of the confusion.  It is likely
that it had to cater for existing users though - I'm sure if Florian
cares, he can comment on that.

It would be better if it made a definitive statement about it, but doing
so would likely attract pedants to try to fix everything to conform,
causing breakage in the process.

I've recently had a painful experience of this with the Atheros PHYs,
where there are lots of platforms using "rgmii" when they should have
been using "rgmii-id".  A patch changed this in the Atheros code breaking
all these platforms, breakage which persisted over several kernel
versions, needing fixes to DT files that then had to be back-ported.
That's fine if you know what happened to break it, but if you don't, and
you don't know what the fix is, you're mostly stuffed and stuck with non-
working ethernet.  That really was not nice.

This is one of the reasons why I press for any new PHY interface mode
to be documented in the phylib documentation right from the start, so
that hopefully we can avoid this kind of thing in the future.

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