netdev - ethtool SFF-8472 issue

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Message-ID: <5DA85BA6.5020305@cox.net>
Date:   Thu, 17 Oct 2019 08:16:38 -0400
From:   Joe Buehler <aspam@....net>
To:     linville@...driver.com, netdev@...r.kernel.org
Subject: ethtool SFF-8472 issue

In sfpdiag.c the sff8472_calibration routine does not look correct for
optical RX power.

The code is currently calculating this:

power = a + b*x + c*x + d*x // x is the raw A/D reading

First problem is that there is another term (SFF_A2_CAL_RXPWR4 in
sfpdiag.c) in SFF-8472 that has been dropped.

The more serious issue is that this computation is not correct.  The
SFF-8472 document could be a little clearer -- which is probably what
caused this -- but I think what it intends is a least-squares polynomial
fit to convert from A/D reading to RX power.

So the formula should be:

power = a + b *x + c*x*x + d*x*x*x + e*x*x*x*x

Joe Buehler