[vortex] Why so many IRQ/sec with 3c905c (Tornado) card?

[Richard Ellis]

I just recently (well, last night) installed a 3c905c Tornado PCI nic in
my machine.  I had been, for about the last week, using an old 3c515 ISA
10/100 3com card because of crashes I was having with another clone dec
tuilp PCI nic.  One of the effects I noticed when I swapped the clone
tulip for the 3c515 isa card was that although the isa card was slower
(max of about 1.1 to 1.4 MB/sec) it's interrupt usage per byte transferred
was much lower than with the tulip clone card.  Therefore I obtained a
905c off ebay for cheap thinking it would also have lower irq's/byte
transferred, given that the Tornado chip (from what I read from the
drivers sources) is a later variant of the chip on the 3c515 card.  Well,
I was somewhat surprised and a little disappointed that the new 3c905c
card is running with just about the same rate of irq/byte transferred as
the cheap tulip clone, and way more than the isa 3c515 card.

On my setup, the Tornado card maxes out at about 11.8 MB/sec of receive
data, and while receiving at this rate, it's generating about 60000
irq's/second.  That works out to about 206 bytes per interrupt.

The old isa 515 card maxed out at about 1.1 to 1.4 MB/sec, and when
running that, it was only causing about 750 irq's/sec.  That works out to
one irq per 1537 bytes, which looks suspiciously like 1 irq per ethernet
frame received.

So my question is, why does the 3c905c card generate so many more
interrupts than the old '515 card generated?  If it generated them at the
same rate as the '515, then 11.8MB/sec would only be about 8000 irq's/sec,
not 60000 irq's/sec.

Here's the log output from the 3c905c driver when it loads:

 3c59x.c:v0.99W 6/5/2002 Donald Becker, becker@scyld.com
   http://www.scyld.com/network/vortex.html
 eth0: 3Com 3c905C Tornado at 0xec00,  00:01:02:6e:49:76, IRQ 11
   8K buffer 5:3 Rx:Tx split, Autonegotiate interface.
   MII transceiver found at address 24, status 782d.
   Using bus-master transmits and whole-frame receives.

The machine is a dual PII box running kernel 2.4.17.

Thanks

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