em network issues

[Bill Paul]

[...]

> > Another thing that might be handy is improving the watchdog timeout
> > message so that it dumps the state of the ICR and ICM registers (and
> > maybe some other interesting driver and/or device state). The timeout
> > implies no interrupts were delivered for a Long Time (tm). If the
> > ICM register indicates interrupts have been masked, then that means
> > em_intr_fast() was triggered by and interrupt and it scheduled work,
> > but that work never executed. If that really is what happened, then
> > I can understand the watchdog error occuring. If that's _not_ what
> > happened, them something else is screwed up.
> 
> Jesse Brandeburg just did an interesting hack for the Linux driver, I
> was considering trying to code an equivalent thing up for us. We
> have evidence that on some AMD based systems there are writebacks
> that get lost, since the TX cleanup relies on the DD being set you
> are hosed when this happens. What he did was make a cleanup
> routine that ONLY uses the head and tail pointers and NOT the done
> bit. Then, in the watchdog routine, if there is evidence of this problem
> it will switch the cleanup function pointer to this alternate clean code.

Oho, I didn't realize the 8254x had producer/consumer indexes like this.
Hm. But the documentation for the Transmit Descriptor Head register
says:

 "Reading the transmit descriptor head to determine which buffers
  have been used (and can be returned to the memory pool) is not reliable."

There's a similar notation for the Receive Descriptor Head register.

I wonder what's unreliable about it.
 
> At least one user that was having a problem has reported this solved
> it. It may be one of the issues hitting us as well.

Switching from testing the descriptor completion bits to using the
consumer indexes should be pretty straightforward. It's worth a shot
at any rate.

-Bill 

--
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-Bill Paul            (510) 749-2329 | Senior Engineer, Master of Unix-Fu
                 wpaul at windriver.com | Wind River Systems
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              <adamw> you're just BEGGING to face the moose
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