On Friday, May 04, 2012 6:18:19 pm Konstantin Belousov wrote:
> On Fri, May 04, 2012 at 11:30:22AM -0400, John Baldwin wrote:
> > On Tuesday, May 01, 2012 12:21:21 pm Konstantin Belousov wrote:
> > > On Thu, Apr 12, 2012 at 09:38:49PM +0300, Konstantin Belousov wrote:
> > > > On Mon, Apr 09, 2012 at 12:19:39PM -0400, John Baldwin wrote:
> > > > > On Sunday, April 08, 2012 1:11:25 am Konstantin Belousov wrote:
> > > > > > On Sat, Apr 07, 2012 at 04:22:07PM -0700, Jack Vogel wrote:
> > > > > > > Make sure you have any firmware up to the latest available, if
> > > > > > > that
> > doesn't
> > > > > > > help
> > > > > > > let me know and I'll check internally to see if there are any
> > outstanding
> > > > > > > issues
> > > > > > > in shared code, that will be after the weekend.
> > > > > >
> > > > > > I had BIOS rev. 151, after you hint I found rev. 154 on the site.
> > > > > > Now BIOS reports itself as MTCDT10N.86A.0154.2012.0323.1601,
> > > > > > March 23.
> > > > > >
> > > > > > Unfortunately, upgrade did not changed anything in regard of hanging
> > > > > > interface.
> > > > >
> > > > > Does reverting 233708 make any difference? Have you tried futzing
> > around with
> > > > > kgdb when it is hung to see what state the device is in (software
> > > > > state
> > at
> > > > > least)?
> > > > It does, in a sense that without r233708 the interface becomes stuck
> > > > almost immediately. I just upgraded to the e1000@r234154, which does not
> > > > change much.
> > > >
> > > > I fiddled with the adapter state after the hang in kgdb more, and I
> > > > noted something interesting. Apparently, tx works. When I ping the
> > > > remote
> > > > host from my suffering atom machine, remote host sees the packet. Also
> > > > remote machine sees some udp traffic originating from the tom, like
> > > > ntp queries.
> > > >
> > > > And, on receive, the atom board does receive interrupts, em0:rx 0
> > > > counter
> > > > in vmstat -i increases. Even more fun, the sysctl dev.em.0.debug
> > > > shows increasing hw rdh (as I understand, this is hardware 'last
> > > > received' packet pointer for rx ring). So I looked at the packet
> > > > descriptor at hw rdt index, and there I see
> > > > (kgdb) p/x ((struct adapter *)0xffffff80010e4000)->rx_rings->rx_base[78]
> > > > $11 = {buffer_addr = 0x12a128800, length = 0x5ea, csum = 0x3c2b, status
> > > > =
> > 0x0,
> > > > errors = 0x0, special = 0x0}
> > > >
> > > > Apparently, the Descriptor Done bit is clear, so the em_rxeof() function
> > > > breaks from the loop, not consuming the current packet. Also, it returns
> > > > false due to DD bit clear. This prevents em_msix_rx() from scheduling
> > > > taskqueue for processing. So apparent cause for the hang is missing
> > > > DD bit in descriptor.
> > > >
> > > > I am not sure isn't all this is obvious for anybody who knows em
> > > > internals, and were to go from there.
> > >
> > > Ok, nobody cares.
> > >
> > > Below is the workaround I use to prevent the interface wedging.
> > > It seems that the sole PCI register read (namely, the rx ring head read)
> > > and consequent recheck of the descriptor status greatly reduce the
> > > likelihood of the issue. Unfortunately, the read does not eliminate
> > > the hang completely. So it is not some PCIe coherency problem.
> > >
> > > With the patch applied, I am able to copy around blu-ray images, while
> > > previously the interface hang in 20-30 seconds of 100Mbit/s traffic.
> > > Sometimes the messages are printed:
> > > em0: Workaround: head 1018 tail 1002 cur 1010
> > > em0: Workaround: head 976 tail 973 cur 974
> > > em0: Workaround: head 950 tail 939 cur 946
> > > em0: Workaround: head 435 tail 419 cur 426
> > >
> > > Machine is still dead due to random memory corruption which I see, in
> > > particular, pmap sometimes read garbage from PTEs. I have no idea is
> > > it related to em0 rx descriptor missed writes, or is a different issue.
> >
> > Humm, so if I'm reading this correctly, the card "skips" a receive
> > descriptor and stores a packet at the next descriptor? That's just
> > bizarre.
> Either this, or it does store the packet but 'forgots' to update the
> rx descriptor. I think that your interpretation is closer to reality,
> since I get sustained 20MB/s over ssh with the patch even when workaround
> activates. The lost packets probably should cause retransmit and speed
> drop.
This is just weird. I wonder if there is a known errata for this?
This really seems to be broken hardware and not a driver issue.
--
John Baldwin
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