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

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Date:   Wed, 17 Jun 2020 13:08:09 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Helmut Grohne <helmut.grohne@...enta.de>
Cc:     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@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: macb: reject unsupported rgmii delays

On Wed, Jun 17, 2020 at 01:52:01PM +0200, Helmut Grohne wrote:
> On Wed, Jun 17, 2020 at 01:40:25PM +0200, Russell King - ARM Linux admin wrote:
> > > For a fixed-link, the validation function is never called. Therefore, it
> > > cannot reject PHY_INTERFACE_MODE_RGMII. It works in practice.
> > 
> > Hmm, I'm not so sure, but then I don't know exactly what code you're
> > using.  Looking at mainline, even for a fixed link, you call
> > phylink_create().  phylink_create() will spot the fixed link, and
> > parse the description, calling the validation function.  If that
> > fails, it will generate a warning at that point:
> > 
> >   "fixed link %s duplex %dMbps not recognised"
> > 
> > It doesn't cause an operational failure, but it means that you end up
> > with a zero supported mask, which is likely not expected.
> > 
> > This is not an expected situation, so I'll modify your claim to "it
> > works but issues a warning" which still means that it's not correct.
> 
> I do see that warning. I agree with your correction of my claim. Thank
> you for your attention to detail.
> 
> So we have two good reasons for not rejecting delay configuration in the
> validation function now.
> 
> The remaining open question seems to be whether configuring a delay on a
> MAC to MAC connection should cause a failure or a only warning. Do you
> have an opinion on that?
> 
> All in-tree bindings of the driver seem to use rmii when they specify a
> phy-mode.

This brings up a problem in itself - the phy interface mode is
currently defined in terms of a MAC-to-PHY setup, not a MAC-to-MAC
setup.

With a fixed link, we could be in either a MAC-to-PHY or MAC-to-MAC
setup; we just don't know.  However, we don't have is access to the
PHY (if it exists) in the fixed link case to configure it for the
delay.

In the MAC-to-MAC RGMII setup, where neither MAC can insert the
necessary delay, the only way to have a RGMII conformant link is to
have the PCB traces induce the necessary delay. That errs towards
PHY_INTERFACE_MODE_RGMII for this case.

However, considering the MAC-to-PHY RGMII fixed link case, where the
PHY may not be accessible, and may be configured with the necessary
delay, should that case also use PHY_INTERFACE_MODE_RGMII - clearly
that would be as wrong as using PHY_INTERFACE_MODE_RGMII_ID would
be for the MAC-to-MAC RGMII with PCB-delays case.

So, I think a MAC driver should not care about the specific RGMII
mode being asked for in any case, and just accept them all.

I also think that some of this ought to be put in the documentation
as guidance for new implementations.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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