Re: Collisions using 3Com 3c905C-TX ethernet cards.

[Chuck Swiger <cswiger%mac.com@localhost>]

On Apr 21, 2011, at 11:34 AM, Vesa Laakso wrote:
> There is awful lot of Collision. I don't have a switch on my network, I
> am using ethernet crossover cable between my Linux and NetBSD router.

I won't try to quote your "netstat -i" output since it was poorly wrapped,
but a quick estimate shows about 1% collision rate.  That's not unusual
for a hub or crossover when running at 100Mbps; to fix it, use a switch
instead.

> dmesg is showing some errors as well:
> 
> besdi$ dmesg | grep 'ex'
> npx0: reported by CPUID; using exception 16
> ex0 at pci0 dev 10 function 0: 3Com 3c905C-TX 10/100 Ethernet with mngmt
> (rev. 0x74)
> ex0: interrupting at ioapic0 pin 17
> ex0: MAC address 00:01:02:0b:53:5e
> ukphy1 at ex0 phy 24: Generic IEEE 802.3u media interface
> ex1 at pci0 dev 12 function 0: 3Com 3c905C-TX 10/100 Ethernet with mngmt
> (rev. 0x74)
> ex1: interrupting at ioapic0 pin 18
> ex1: MAC address 00:04:76:cd:13:7e
> ukphy2 at ex1 phy 24: Generic IEEE 802.3u media interface
> ex1:  transmit underrun (9000) @20
> ex0:  transmit underrun (9000) @20
> ex1:  transmit underrun (9000) @20
> ex1:  transmit underrun (9000) @20, new threshold is 40

The 3com 905 family supported early transmission of packets before the data
were completely DMA'ed to the NIC; however, if the DMA doesn't complete before
the transmission actually goes out, then the packet is garbled and causes
a transmit underrun.

The driver for the card is auto-tuning itself to increase the byte count so that
the DMA has time to complete.

Regards,
-- 
-Chuck