linux-kernel - Re: [PATCH net-next] net: phy: sfp: handle cases where neither BR,min nor BR,max is given

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Message-ID: <20180508124456.GN16141@n2100.armlinux.org.uk>
Date:   Tue, 8 May 2018 13:44:57 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Antoine Tenart <antoine.tenart@...tlin.com>, davem@...emloft.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        thomas.petazzoni@...tlin.com, maxime.chevallier@...tlin.com,
        gregory.clement@...tlin.com, miquel.raynal@...tlin.com,
        nadavh@...vell.com, stefanc@...vell.com, ymarkman@...vell.com,
        mw@...ihalf.com
Subject: Re: [PATCH net-next] net: phy: sfp: handle cases where neither
 BR,min nor BR,max is given

On Sat, May 05, 2018 at 01:35:34PM -0700, Florian Fainelli wrote:
> On May 4, 2018 8:21:03 AM PDT, Antoine Tenart <antoine.tenart@...tlin.com> wrote:
> >When computing the bitrate using values read from an SFP module EEPROM,
> >we use the nominal BR plus BR,min and BR,max to determine the
> >boundaries. But in some cases BR,min and BR,max aren't provided, which
> >led the SFP code to end up having the nominal value for both the
> >minimum
> >and maximum bitrate values. When using a passive cable, the nominal
> >value should be used as the maximum one, and there is no minimum one
> >so we should use 0.
> >
> >Signed-off-by: Antoine Tenart <antoine.tenart@...tlin.com>
> >---
> >
> >Hi Russell,
> >
> >I'm not completely sure about this patch as this case is not really
> >specified in the specification. But the issue is there, and I've
> >discuss
> >this with others. It seemed logical (at least to us :)) to use the
> >BR,nominal values as br_max and 0 as br_min when using a passive cable
> >which only provides BR,nominal as this would be the highest rate at
> >which the cable could work. And because it's passive, there should be
> >no
> >issues using it at a lower rate.
> >
> >I've tested this with one passive cable which only reports its
> >BR,nominal (which was 10300) while it could be used when using
> >1000baseX
> >or 2500baseX modes.
> 
> Which SFP modules (vendor and model) exposed this out of curiosity?
> Russell and I already saw the Cotsworks modules having so e issues
> with checksums, so building a table of quirks would help. Thanks!

I think this is just manufacturers being lazy with their EEPROM
contents - looking around, most passive cables are specified to be
"up to" some figure, and that's definitely what's specified by the
SFP+ specification by way of the high-pass pole requirement of the
coupling capacitors.

-- 
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