[PATCH 3.17-rc4 v7 0/6] arm: Implement arch_trigger_all_cpu_backtrace

[Daniel Thompson]

On 16/10/14 10:33, Daniel Thompson wrote:
>> The blocker on this work right now is the annoying Versatile Express
>> platform, which pretty much means that we currently can't push the
>> code into the GIC to support FIQs.  As long as adding FIQ support to
>> the GIC results in the Versatile Express becoming non-bootable, the
>> idea of using FIQs is a total non-starter.
>>
>> Or we decide that we dump the platform completely (which will upset
>> a number of developers.)
>>
>> I have patches I'm using for trigger_all_cpu_backtrace() which I'm
>> maintaining privately in my tree until we can get the FIQ situation
>> sorted.
> 
> I do hope to gain (remote) access to a vexpress at some point just to
> pick at this issue a little.

This week with the help of one of my colleagues (thanks Tixy) I have
been able to run some tests and figure out what it going on on vexpress-a9.

The summary is that on some GICv1 implementations the bit to enable
group 1 interrupts cannot be accessed using secure memory accesses. More
specifically the presence/absence of the EnableGrp1 bit in the secure
version GICD_CTRL register is implementation defined.

My original patches overlooked this and as a result the existing code
will migrate all interrupts to group but then cannot enable delivery of
group 1 interrupts.

I'm planning to respin the code so it will automatically disable FIQ
support when the EnableGrp1 bit is not implemented. This means
vexpress-a9 will not benefit from FIQ support but will also not be
harmed by it.