linux-kernel - Re: [PATCH v4 5/7] Watchdog: introduce ARM SBSA watchdog driver

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Message-ID: <CADyBb7s7NpQJFiFFGbbjCTQ0+bNCZoVMC4KwaZbWwGs_Wa=mLg@mail.gmail.com>
Date:	Wed, 10 Jun 2015 12:20:20 +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>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>, rjw@...ysocki.net
Subject: Re: [PATCH v4 5/7] Watchdog: introduce ARM SBSA watchdog driver

Hi Guenter,

On 10 June 2015 at 11:41, Fu Wei <fu.wei@...aro.org> wrote:
> Hi Guenter,
>
> On 10 June 2015 at 00:45, Guenter Roeck <linux@...ck-us.net> wrote:
>> On 06/09/2015 09:29 AM, Timur Tabi wrote:
>>>
>>> On 06/09/2015 11:22 AM, Guenter Roeck wrote:
>>>>
>>>>
>>>>
>>>> but I see your point. Essentially, the specification is broken
>>>> for all practical purposes, since, as you point out, enabling
>>>> the watchdog overwrites and explicitly sets WCV. Effectively
>>>> this means that just using WCV to program the timeout period
>>>> is not really possible.
>>>>
>>>> I am not really sure how to address this. We can either only use WOR,
>>>> and forget about pretimeout, or we can enforce a minimum pretimeout.
>>>> In the latter case, we'll have to write WCV after writing WOR.
>>>
>>>
>>> In talking with our hardware engineers, using WCV to program the timeout
>>> period is not a valid operation.  This is why I keep arguing against the
>>> pre-timeout feature, and I don't agree that servers should always use
>>> pre-timeout.
>>>
>>
>> Not sure if "not valid" is correct - after all, it is mentioned in the
>> specification. However, it is at the very least fragile.
>
> I think we should focus on SBSA spec, but not a specific chip design,
> because this is SBSA watchdog, not a driver for an IP core from a
> specific chip vendor.
> this operation is mentioned in the spec,
> and I have tested my driver on Foundation model(from ARM) and a real hardware.
>
>>
>> I tend to agree that we should just forget about pretimeout and
>> use your original approach, where the timeout value is used
>> to program WOR. Everything else is really just asking for trouble.

Another weakness of only using WOR is the timeout limited by this
32bit register.
10s @400MHz generic Timer

I don't think this limit is good for a server, once the server is in a
heavy load

>
> I don't mind if we give up pretimeout, The reason I use pretimeout is:
> this concept matches the function of two stage timeouts.
>
> but, If we give up pretimeout, could you give me a suggestion:
>
> How to config the two stage timeouts
> (1)from enabling watchdog to WS0
> (2)the time from WS1 to WS0
>
> If we only have one timeout parameter,  How to config the two stage timeouts?
> Any suggestion ?
>
> If we make the first stage timeout is timeout/2,  this violates the
> definition of timeout.
> I don't think users expect interrupt, panic or reboot at timeout/2.
>
> And WS1  definitely isn't a backup of WS0.
>
>>
>> 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



-- 
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
--
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