[eepro100] mii-diag program - slight inaccuracy

[Donald Becker]

On Fri, 7 Jul 2000, John Cagle wrote:
> Donald Becker wrote:
> > On Wed, 5 Jul 2000, John Cagle wrote:
> > > With my NIC & Switch combination, the BMSR register has a "1" in the bit
> > > position for "Link Jabber" (bmsr & 0x0002).  This causes the mii-diag
> > > program to print out "*** Link Jabber! ***".
> > >
> > > However, according to the National DP83840A specification, the Link
> > > Jabber bit only has meaning in 10 Mb/s mode.  I'm running at 100 Mb/s,
> > > so this bit should be ignored.
> > 
> > Hmmm, curious.  I've seen false indication of link jabber at 100baseTx
> > before, but I don't believe that it adheres to the standard.  Does anyone
> > have a copy of the standard nearby to check?
> 
> Well I just dusted off the old IEEE 802.3u-1995 standard, and I found
> the following references to Jabber:
>  22.2.4.2.12 Jabber Detect
>   ...
>   PHYs specified for 100 Mb/s operation (100BASE-X and 100BASE-T4) do
> not incorporate a Jabber Detect function, as this function is defined to
> be performed in the repeater unit in 100 Mb/s systems.  Therefore,
> 100BASE-X [meaning 100BASE-TX and 100BASE-FX] and 100BASE-T4 PHYs shall
> always return a value of zero in  bit 1.1.
...
> So it appears that while Jabber is meaningless for 100 Mb/s PHYs, that
> the DP83840 is in violation of the standard in that it sometimes
> (always?) returns 1 for the Jabber bit instead of 0.

So much for the 83840 being the gold standard of MII implementations.

>   FYI, for the
> National PHY, you can read register 25's PMDSpeed bit to determine the
> current link speed.

People should keep in mind that doing this "doesn't count".  Reg 25 is a
vendor specific register, and that information doesn't exist on other chips.
There is no way, other than having a very large and always obsolete vendor
exception list, of using that info in a driver.

Donald Becker				becker@scyld.com
Scyld Computing Corporation		http://www.scyld.com
410 Severn Ave. Suite 210		Beowulf Clusters / Linux Installations
Annapolis MD 21403