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some more explicit text about the cost of LANCE_REVC_BUG

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fair 1998-06-19 07:43:19 +00:00
parent 56fac275e8
commit 0d96992f01
1 changed files with 35 additions and 15 deletions
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50
share/man/man4/man4.sun3/le.4
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@ -35,7 +35,7 @@
.\"
.\" from: Header: le.4,v 1.2 92/10/13 05:31:33 leres Exp
.\" from: @(#)le.4 8.1 (Berkeley) 6/9/93
.\" $NetBSD: le.4,v 1.4 1998/06/17 19:53:22 fair Exp $
.\" $NetBSD: le.4,v 1.5 1998/06/19 07:43:19 fair Exp $
.\"
.Dd June 9, 1993
.Dt LE 4 sun3
@ -53,7 +53,7 @@ interface provides access to a 10 Mb/s
.Tn Ethernet
network via the
.Tn AMD
7990
Am7990 and Am79C90 (CMOS, pin-compatible)
.Tn LANCE
(Local Area Network Controller - Ethernet) chip set.
.Pp
@ -63,26 +63,45 @@ is specified at boot time with an
.Xr ioctl 2 .
The
.Nm
interface employs the address resolution protocol described in
interface employs the Address Resolution Protocol (ARP) described in
.Xr arp 4
to dynamically map between Internet and Ethernet addresses on the local
network.
to dynamically map between Internet and
.Tn Ethernet
addresses on the local network.
.Sh BUGS
The old
.Tn LANCE
Revision C chips have a bug which causes garbage to be inserted in
front of the received packet occasionally.
The Am7990 Revision C chips have a bug which causes garbage to
be inserted in front of the received packet occasionally.
The work-around is to ignore packets with an invalid destination
address (garbage will usually not match), by double-checking the
destination address of every packet in the driver.
This work-around is enabled with the
.Cd LANCE_REVC_BUG
.Dv LANCE_REVC_BUG
kernel option.
It is unfortunately computationally costly, and
this precludes multicast support on Ethernet interfaces with that
chip in them.
.Pp
All sun3 systems are presumed to have this revision of the AM7990,
When
.Dv LANCE_REVC_BUG
is enabled, the
.Nm
driver executes one or two calls to an inline ethernet address
comparison function for every received packet.
On the
.Tn mc68000
it is exactly eight instructions of 16-bits each.
There is one comparison for each unicast packet, and two
comparisons for each broadcast packet.
.Pp
In summary, the cost of the LANCE_REVC_BUG option is:
.Bl -enum -compact
.It
loss of multicast support, and
.It
eight extra
.Tn CPU
instructions per received packet, sometimes sixteen,
depending on both the processor, and the type of packet.
.El
.Pp
All sun3 systems are presumed to have bad this revision of the Am7990,
until proven otherwise.
Alas, the only way to prove what revision of the chip is in a particular
system is inspection of the date code on the chip package, with a list of
@ -110,7 +129,8 @@ but will not automatically retry transmission if there is a collision.
Ethernet extremely busy or jammed,
outbound packets dropped after 16 attempts to retransmit.
.Pp
TDR is
.Sy TDR
is
.Qq Time Domain Reflectometry .
The
.Tn LANCE