4.4 BCM5301X ARM regression "External imprecise Data abort"

[scott.branden@broadcom.com (Scott Branden)]

Hi Rafal,

I do not work on BCM5301x SoCs but perhaps Jon Mason can comment.
A few comments inline as well.

On 16-04-03 11:13 PM, Rafa? Mi?ecki wrote:
> Hi guys,
>
> I got regression reports from Netgear R8000 (BCM4709A0) users and did
> some testing & regression tracking with Aditya.
>
> It happens that Linux 4.4 doesn't boot due to the following commits:
> bbeb920 ("ARM: 8422/1: enable imprecise aborts during early kernel startup")
> 9254970 ("ARM: 8447/1: catch pending imprecise abort on unmask")
> 937b123 ("ARM: BCM5301X: remove workaround imprecise abort fault handler")
>
> In kernel 4.3 we got that abort workaround which was resulting in:
> [    5.007128] Freeing unused kernel memory: 212K (c0435000 - c046a000)
> [    5.694632] init: Console is alive
> [    5.698169] init: - watchdog -
> [    5.701470] External imprecise Data abort at addr=0x0, fsr=0x1406 ignored.
> As you can see, this abort was happening soon after freeing unused
> memory and ignoring it *once* did the trick. It was never appearing
> again.
>
> With 4.4 similar (or the same?) abort happens earlier (during PCI host
> driver init) and doesn't get ignored:
> [    2.478461] pci 0000:00:00.0: PCI bridge to [bus 01]
> [    2.483451] pci 0000:00:00.0:   bridge window [mem 0x08000000-0x085fffff]
> [    2.599449] pcie_iproc_bcma bcma0:8: PCI host bridge to bus 0001:00
> [    2.605744] pci_bus 0001:00: root bus resource [mem 0x40000000-0x47ffffff]
> [    2.612657] pcie_iproc_bcma bcma0:8: link: UP
> [    2.617241] PCI: bus0: Fast back to back transfers disabled
> [    2.622845] pci 0001:00:00.0: bridge configuration invalid ([bus
> 00-00]), reconfiguring
> [    2.631297] PCI: bus1: Fast back to back transfers disabled
> [    2.636887] pci 0001:01:00.0: bridge configuration invalid ([bus
> 00-00]), reconfiguring
> [    2.645035] Unhandled fault: imprecise external abort (0x1406) at 0x00000000
> (see 4.4.txt for the backtrace)
>
> At first I was hoping that we simply need to re-add the removed
> workaround. I tried it but it appeared that one abort is immediately
> followed by another:
> [    2.936895] pci 0001:01:00.0: bridge configuration invalid ([bus
> 00-00]), reconfiguring
> [    2.945053] External imprecise Data abort at addr=0x0, fsr=0x1406 ignored.
> [    2.951966] Unhandled fault: imprecise external abort (0x1406) at 0x00000000
>
> So it seems that commits bbeb920 and 9254970 broke something in PCI
> host initialization (or maybe just exposed another bug?). Instead of
> getting an abort once and late we are getting now many of them and a
> bit earlier.
We do not observe such issues in Cygnus and other SoCs that use this 
PCIe driver (we do not use bcma either - I do not know if that is related).
>
> Reverting all three commits from the top of 4.4.6 gives me back a
> working & booting kernel.
>
> Do you have any idea how to fix this regression (and hopefully
> original problem as well)?
I think the proper fix is to correct the issues in the bootloader.  It 
was my understanding from Jon Mason that this is the root of the 
original problem.
>

Regards,
Scott