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

From: Russell King - ARM Linux
Date: Tue May 08 2018 - 08:45:15 EST

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@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> >---
> >
> >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.

RMK's Patch system:
FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps up
According to 8.21Mbps down 510kbps up