[eepro100] Intel HPQ and MAC Setting question

Donald Becker becker@scyld.com
Mon Nov 26 14:44:01 2001

On Mon, 26 Nov 2001, Markus Fraedrich wrote:

> Due to a project im studying for the Timing of Fast Ethernet NICs to
> generate a QoS.
> Unfortunately I didnīt get the User Manual for the 82557.

That's usual -- Intel rarely provides complete programming information.
I haven't gotten an update from Intel for years.

> Does anyone know if its possible to change/increase  the IFS setting of the
> NIC, and knows the Register Adress.

Yes, it's at offset 2 in the configuration block.  The extended IFS is
tied to the PCI bus speed, with the IFS never being less than the
standard Ethernet IFS.  If you are running a 33Mhz PCI bus, a setting of
'4' correesponds to the 960ns minimum IFS.  A setting of '5' results in
an IFS of 5*8*30ns = 1200ns.

Changing the IFS on the 8255* chips requires sending a configure
command.  This overhead makes it impractical to change the IFS

Note that changing the IFS is not a good solution to QoS.  You will
change the fairness of the contention-based network, but the change is
very difficult to predict.  If you have a switch-based network and want
to experiment with QoS you should really be looking at real-time packet
scheduling and rate limiting.

Donald Becker				becker@scyld.com
Scyld Computing Corporation		http://www.scyld.com
410 Severn Ave. Suite 210		Second Generation Beowulf Clusters
Annapolis MD 21403			410-990-9993