linux-kernel - Re: [PATCH v2 6/7] Watchdog: introduce ARM SBSA watchdog driver

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Date:	Sat, 23 May 2015 13:01:58 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Timur Tabi <timur@...eaurora.org>, Fu Wei <fu.wei@...aro.org>
CC:	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
	Linaro ACPI Mailman List <linaro-acpi@...ts.linaro.org>,
	linux-watchdog@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	Wei Fu <tekkamanninja@...il.com>,
	G Gregory <graeme.gregory@...aro.org>,
	Al Stone <al.stone@...aro.org>,
	Hanjun Guo <hanjun.guo@...aro.org>,
	Ashwin Chaugule <ashwin.chaugule@...aro.org>,
	Arnd Bergmann <arnd@...db.de>, vgandhi@...eaurora.org,
	wim@...ana.be, Jon Masters <jcm@...hat.com>,
	Leo Duran <leo.duran@....com>, Jon Corbet <corbet@....net>,
	Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v2 6/7] Watchdog: introduce ARM SBSA watchdog driver

On 05/23/2015 12:40 PM, Timur Tabi wrote:
[ ... ]
>
> I use emergency_restart(), because the watchdog-api.txt documentation says this:
>
> "If userspace fails (RAM error, kernel bug, whatever), the
> notifications cease to occur, and the hardware watchdog will reset the
> system (causing a reboot) after the timeout occurs."
>
> Maybe I'm reading this too literally, but to me this means that when the timeout expires, the system has to reset immediately.
>
> However, maybe panic() is better, since it can do the same thing and more.
>

I have a specific requirement at work to have watchdog expiration
(not this watchdog, this is different HW) result in a panic, specifically
to enable crashdump support and thus post-mortem analysis.

I had not thought about this use case myself, and I had always wondered
why watchdog driver implementers would choose to call panic() after an
interrupt or NMI. But we live and learn, so now I finally understand.

In the pretimeout/timeout world, the pretimeout would (typically)
result in a panic, and the timeout would result in a reset. So one
would set the timer register to 10s for 10s pretimeout and 20s timeout.

However, the pretimeout concept assumes that there are two timers
which can be set independently. As you had pointed out earlier,
and as the specification seems to confirm, that is not the case here.
As such, I don't really understand why and how the pretimeout / timeout
concept would add any value here and not just make things more
complicated than necessary. Maybe I am just missing something.

Thanks,
Guenter

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