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Date: Sun, 24 May 2015 23:50:48 +0800 From: Fu Wei <fu.wei@...aro.org> To: Guenter Roeck <linux@...ck-us.net> Cc: Timur Tabi <timur@...eaurora.org>, 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 Hi Guenter, Thanks for your feedback On 24 May 2015 at 22:15, Guenter Roeck <linux@...ck-us.net> wrote: > On 05/24/2015 03:15 AM, Fu Wei wrote: >> >> Hi Guenter, >> >> On 24 May 2015 at 04:01, Guenter Roeck <linux@...ck-us.net> wrote: >>> >>> 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. >> >> >> Sorry, in Documentation/watchdog/watchdog-api.txt, I can not get the >> info about " the pretimeout concept assumes that there are two timers >> which can be set independently." >> Could you kindly point out where is the assumption. >> >> I thinks in kernel documentation, that meams "one watchdog has two >> timeout stages", maybe I miss something. Could you help me out? >> > My apologies. Terminology problem; see below. > > Note that the pretimeout, as documented, is a difference to the real > timeout, not an absolute time (which I had not realized before). > > "Note that the pretimeout is the number of seconds before the time > when the timeout will go off. It is not the number of seconds until > the pretimeout. So, for instance, if you set the timeout to 60 seconds > and the pretimeout to 10 seconds, the pretimeout will go off in 50 > seconds. Setting a pretimeout to zero disables it." yes , this patchset is designed for this pretimeout concept > >>> 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. >> >> >> If pretimeout concept assumes that there are two timers, I >> misunderstand the "pretimeout", then I will delete the pretimeout >> immediately. >> > I think I used the wrong term. I should have said something like > "two distinct timeout values". Actually, I have added my thought at the head of sbsa_gwdt.c as a comment : * * Note: This SBSA Generic watchdog driver is compatible with * the pretimeout concept of Linux kernel. * The timeout and pretimeout are set by the different REGs. * The first watch period is set by writing WCV directly, * that can support more than 10s timeout at the maximum * system counter frequency. * The second watch period is set by WOR(32bit) which will be loaded * automatically by hardware, when WS0 is triggered. * This gives a maximum watch period of around 10s at the maximum * system counter frequency. * The System Counter shall run at maximum of 400MHz. * More details: DEN0029B - Server Base System Architecture (SBSA) * * Kernel/API: P---------| pretimeout * |-------------------------------T timeout * SBSA GWDT: P--WOR---WS1 pretimeout * |-------WCV----------WS0~~~~~~~~T timeout */ > > Guenter > -- Best regards, Fu Wei Software Engineer Red Hat Software (Beijing) Co.,Ltd.Shanghai Branch Ph: +86 21 61221326(direct) Ph: +86 186 2020 4684 (mobile) Room 1512, Regus One Corporate Avenue,Level 15, One Corporate Avenue,222 Hubin Road,Huangpu District, Shanghai,China 200021 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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