Re: [PATCH v9 2/3] PCI: Add tango PCIe host bridge support

Mason
2017-07-04T15:19:10Z

On 04/07/2017 16:27, Peter Zijlstra wrote:

> Mason wrote:
>
>> if (IS_ENABLED(CONFIG_DEBUG_PREEMPT) && in_atomic_preempt_off()) {
>> pr_err("Preemption disabled at:");
>> print_ip_sym(preempt_disable_ip);
>> pr_cont("\n");
>> }
>>
>> BTW, why didn't print_ip_sym(preempt_disable_ip); say
>> where preemption had been disabled?
>
> It does, but it might be non-obvious. We only store the first 0->!0
> transition IP in there.

The output was:

[ 1.079483] Preemption disabled at:[ 1.082820] [< (null)>] (null)

so preempt_disable_ip was NULL, right?

Has it been already clobbered?

>> Here's the high-level view. My HW is borked and muxes
>> config space and mem space. So I need a way to freeze
>> the entire system, make the config space access, and
>> then return the system to normal. (AFAICT, config space
>> accesses are rare, so if I kill performance for these
>> accesses, the system might remain usable.)
>>
>> Is there a way to do this? Mark suggested stop_machine
>> but it seems using it in my situation is not quite
>> straight-forward.
>
> *groan*... so yeah, broken hardware demands crazy stuff... stop machine
> is tricky here because I'm not sure we can demand all PCI accessors to
> allow sleeping.
>
> And given that PCI lock is irqsave, we can't even assume IRQs are
> enabled.
>
> Does your platform have NMIs? If so, you can do yuck things like the
> kgdb sync. NMI IPI all other CPUs and have them spin-wait on your state.
>
> Then be careful not to deadlock when two CPUs do that concurrently.

My platform is arch/arm/mach-tango (Cortex A9, ARMv7-A)

This looks similar to what you described:
https://www.linaro.org/blog/core-dump/debugging-arm-kernels-using-nmifiq/
(I CCed Daniel Thompson)

Quote article:

> Note: On ARMv7-A devices that have security extensions (TrustZone)
> FIQ can only be used by the kernel if it is possible to run Linux in
> secure mode. It is therefore not possible to exploit FIQ for
> debugging and run a secure monitor simultaneously. At the end of this
> blog post we will discuss potential future work to mitigate this
> problem.

On my platform, Linux runs in non-secure mode...

Sounds like I don't have many options left for this driver :-(

Regards.