linux-kernel - Re: [PATCH] arm: dts: uDPU: switch PHY operation mode to 2500base-x

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Date:   Wed, 22 Apr 2020 16:37:13 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Jakov Petrina <jakov.petrina@...tura.hr>, jason@...edaemon.net,
        devicetree@...r.kernel.org, gregory.clement@...tlin.com,
        linux-kernel@...r.kernel.org, vladimir.vid@...tura.hr,
        robh+dt@...nel.org, linux-arm-kernel@...ts.infradead.org,
        sebastian.hesselbarth@...il.com
Subject: Re: [PATCH] arm: dts: uDPU: switch PHY operation mode to 2500base-x

On Wed, Apr 22, 2020 at 05:24:39PM +0200, Andrew Lunn wrote:
> On Wed, Apr 22, 2020 at 05:09:15PM +0200, Jakov Petrina wrote:
> > This resolves issues with certain SPF modules.
> 
> Hi Jakov
> 
> Please could you explain this some more. PHYLINK should be setting the
> mode appropriately, based on the capabilities of the SFP module. Is
> the real problem that the SFP module is indicating the wrong baud
> rate?

The issue is way more complex than that, and this is just a sticky
plaster over the problem, and I'm really unconvinced that the issue
has really been solved.

There are some GPON modules that support 2.5G and 1G, try to guess
the speed of the host somehow.  How that happens is not really
known, and I never got the impression that even Scott at Telus worked
it out - he just played around until he got stuff to work, and this
patch is the result.

This patch just works around the problem because it _may_ cause the
interface to be at 2.5G at boot, but it won't be at 2.5G after a
1G module has previously been plugged in, and one of these GPON
modules is subsequently inserted.

So, this is just a bodge that works in one particular situation for a
problem with modules playing their own games.

So, it has to be a NAK.

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