linux-kernel - Re: [PATCH 3.17-rc4 v7 0/6] arm: Implement arch_trigger_all_cpu

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Date:	Tue, 04 Nov 2014 17:05:25 +0000
From:	Daniel Thompson <daniel.thompson@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Daniel Drake <drake@...lessm.com>
CC:	linaro-kernel@...ts.linaro.org, patches@...aro.org,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Jon Medhurst <tixy@...aro.org>,
	Marc Zyngier <marc.zyngier@....com>
Subject: Re: [PATCH 3.17-rc4 v7 0/6] arm: Implement arch_trigger_all_cpu_backtrace

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.
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