linux-kernel - Re: [PATCH RFC] Watchdog: sbsa

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Message-ID: <572BE2CC.4020606@roeck-us.net>
Date:	Thu, 5 May 2016 17:18:20 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Timur Tabi <timur@...eaurora.org>,
	Pratyush Anand <panand@...hat.com>
Cc:	fu.wei@...aro.org, Suravee.Suthikulpanit@....com, wim@...ana.be,
	linux-arm-kernel@...ts.infradead.org,
	linux-watchdog@...r.kernel.org,
	open list <linux-kernel@...r.kernel.org>,
	Dave Young <dyoung@...hat.com>, kexec@...ts.infradead.org
Subject: Re: [PATCH RFC] Watchdog: sbsa_gwdt: Enhance timeout range

On 05/05/2016 04:45 PM, Timur Tabi wrote:
> Timur Tabi wrote:
>>
>>> A 32-bit counter is absolutely fine. Letting it run with a 400MHz clock
>>> (or was it 200 MHz ?) is the problem. A resolution of 2.5ns for a
>>> watchdog
>>> timer does not really make any sense.
>>
>> The 10 second limit is based on a 20MHz clock.
>
> No, that's not true.  I misread the code.  I knew something was wrong, but it didn't click until just now.
>
> The default timeout is 10 seconds.  The max timeout on a 20MHz system (which is what we're running) is over 200 seconds.
>
> The problem is that Pratyush's system is running at a clock that's way too fast:
>
>   [  131.187562] sbsa-gwdt sbsa-gwdt.0: Initialized with 40s timeout @ 250000000 Hz, action=1.
>
> 250MHz is unreasonable.  Pratyush, why is your system counter so high? On our ARM64 system, it's set to 20MHz.
>

Guess that answers my earlier question. Problem is that the specification
_permits_ those unreasonable frequencies, and quite obviously they are
being used, no matter if it makes sense or not.

With a (still unreasonable) maximum frequency of 100 MHz, the problem
would not exist. So, if anything, someone with influence on the standard
might suggest to reduce the maximum permitted frequency.

Guenter