linux-kernel - Re: [PATCH v11 00/19] arm: KGDB NMI/FIQ support

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Date:	Wed, 03 Sep 2014 10:02:18 +0100
From:	Daniel Thompson <daniel.thompson@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Russell King <linux@....linux.org.uk>,
	linaro-kernel@...ts.linaro.org,
	Catalin Marinas <catalin.marinas@....com>, patches@...aro.org,
	kgdb-bugreport@...ts.sourceforge.net,
	Nicolas Pitre <nico@...aro.org>, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Anton Vorontsov <anton.vorontsov@...aro.org>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	Fabio Estevam <festevam@...il.com>,
	Colin Cross <ccross@...roid.com>, kernel-team@...roid.com,
	Dave Martin <Dave.Martin@....com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v11 00/19] arm: KGDB NMI/FIQ support

On 03/09/14 00:02, Thomas Gleixner wrote:
> On Tue, 2 Sep 2014, Daniel Thompson wrote:
> 
>> This patchset makes it possible to use kgdb's NMI infrastructure on ARM
>> platforms.
>>
>> The patches are seperated into three distinct groups:
>>
>> 1. arm specific changes; these provide multi-platform support for FIQ
>>    (including raising an IPI using FIQ to ensure effective SMP support)
>>    and extend ARM KGDB support to use the features provided.
>>
>> 2. irqchip changes; updates to the gic and vic drivers to provide
>>    support for routing certain interrupt sources to FIQ.
>>
>> 3. serial changes; driver support to allow the UART interrupt to be
>>    routed to FIQ. The already mainlined kgdb NMI infrastructure (mostly
>>    found in drivers/tty/serial/kgdb_nmi.c) will re-route the kgdb
>>    console UART's interrupt signal from IRQ to FIQ. Naturally the UART
>>    will no longer function normally and will instead be managed by kgdb
>>    using the polled I/O functions. Any character delivered to the UART
>>    causes the kgdb handler function to be called.
> 
> To be honest, what you are doing is just ass backwards.
> 
> The use case you are looking for is the most irrelevant of all. Just
> because KGDB is on some managerial "must have items" checklist does
> not make it useful.

The FIQ based interactive debugger use case is fairly common on Android,
especially for Nexus devices (they have an out-of-tree debugger similar
to kdb for this).

I think it finds favour there because during the development phases
where the console is unplugged to allow developers to go walkabout live
with a prototype phone. The interactive debugger is used for
post-morteming when something breaks. At this stage of development are
reluctant to expose/consume hardware resources (JTAG pins, RAM, FLASH)
for JTAG or kexec/kdump post-mortems.


> The only relevant use cases of FIQs are the same as those of NMIs on
> x86:
> 
>   - Watchdog to detect stuck cpus and issue stack traces

Russell put together a quick 'n dirty version of the NMI stack trace
code based on a subset of my patchset. Based on his feedback, later
revisions of my patchset are structured to simplify adding this code.


Daniel.


>   - Performace monitoring
> 
> KGDB falls into place once you solved the above.
>
> That said for the general approach, I'll have a look at the irq
> related patches in a minute.
> 
> Thanks,
> 
> 	tglx


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