Re: re(4) driver dropping packets when reading NFS files

> 
> I highly doubt it could be hardware issue.
> 
Looks like the hardware guys may be off the hook. See below.
> 
> It's job of bus_dma(9) and I don't think barrier instructions would
> be helpful here as I don't see out-of-order execution in RX
> handler.
> 
My current hunch is that something that changed between June 7 and
June 15 in head/sys has caused the chip to have difficulties doing
DMA, resulting in the Fifo overflows and approx. 10% "missed frames".

> 
> Let's kill driver bug. No one reported this kind of issue so far
> and I guess most users took it granted for the poor performance
> because they are using low end consumer grade controller.
>
I think your driver is off the hook, too.

> 
> > re0 statistics:
> > Transmit good frames : 101346
> > Receive good frames : 133390
> > Tx errors : 0
> > Rx errors : 0
> > Rx missed frames : 14394
> > Rx frame alignment errs : 0
> > Tx single collisions : 0
> > Tx multiple collisions : 0
> > Rx unicast frames : 133378
> > Rx broadcast frames : 0
> > Rx multicast frames : 12
> > Tx aborts : 0
> > Tx underruns : 0
> > rxe did 0: 14359
> 
Seeing that someone thought it had worked ok a while back, I decided to
try some old kernels I had lying about from head/-current. I found that
the one I svn`d on June 7 works well (about 7Mbytes per sec read rate) whereas one
svn`d on June 15 had the problem (about 500Kbytes per sec read rate).

So what is different between these kernels:
- if_re.c is identical
- subr_dma.c has a simple change and porting the June 7 one over didn`t make
  the June 15 one work better
- amd64`s busdma_machdep.c is identical

so it must be something else. There are a bunch of changes to amd64`s pmap.c,
which is why I`ve cc`d Alan, in case he might know if those changes could affect
PCIe DMA or similar.

Other than that, maybe someone else familiar with the PCIe DMA could look and see
if a change done to head between June 7 and 15 might explain it. (and it could
be something else, a DMA problem for the chip is just a guess)

rick
ps: Unfortunately I`ll be on the road for the next month, so I won`t be able
    to test patches until early Dec.