netdev - Re: [PATCH] net: dsa: allow XAUI phy interface mode

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Date:   Fri, 8 Dec 2017 16:42:02 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Vivien Didelot <vivien.didelot@...oirfairelinux.com>,
        Florian Fainelli <f.fainelli@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH] net: dsa: allow XAUI phy interface mode

On Fri, Dec 08, 2017 at 05:26:43PM +0100, Andrew Lunn wrote:
> On Fri, Dec 08, 2017 at 04:04:59PM +0000, Russell King wrote:
> > XGMII is a 32-bit bus plus two clock signals per direction.  XAUI is
> > four serial lanes per direction.  The 88e6190 supports XAUI but not
> > XGMII as it doesn't have enough pins.  The same is true of 88e6176.
> > 
> > Match on PHY_INTERFACE_MODE_XAUI for the XAUI port type, but keep
> > accepting XGMII for backwards compatibility.
> 
> Hi Russell
> 
> The backwards compatibility is for the ZII devel C, the DSA link
> between the two switches? I don't think there are any other boards
> using this.
> 
> I will ask around, but i don't think we need to worry about backwards
> compatibility. There is no region of flash set aside for the DT blob,
> etc. I always TFTP boot using the one from the kernel, etc.

Well, there's also kexec to consider, where it's better not to specify
the --dtb option because that wipes out the modifications (such as the
memory information) that the boot loader made to the pristine kbuild
generated dtb.  So, with kexec its preferable to use the existing dtb
even with a newer kernel, or if not go through a full reboot to use a
new dtb.

Whether anyone uses kexec on this platform though would be an entirely
separate question, but I'm generally not in favour of breaking
compatibility with already merged DT without good reason.

You do seem to be correct that this only applies (in mainline) to the
ZII rev C board, so I guess including a patch to update its dts for
the inter-switch connection would at least be sensible.

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