linux-kernel - Re: [RFE]: watchdog: atmel: atmel-sama5d4-wdt

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Date:   Mon, 10 Jun 2019 09:28:11 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Ken Sloat <KSloat@...pglobal.com>
Cc:     "Nicolas.Ferre@...rochip.com" <Nicolas.Ferre@...rochip.com>,
        "alexandre.belloni@...e-electrons.com" 
        <alexandre.belloni@...e-electrons.com>,
        "wim@...ana.be" <wim@...ana.be>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-watchdog@...r.kernel.org" <linux-watchdog@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFE]: watchdog: atmel: atmel-sama5d4-wdt

On Mon, Jun 10, 2019 at 03:51:52PM +0000, Ken Sloat wrote:
> Hello Nicolas,
> 
> I wanted to open a discussion proposing new functionality to allow disabling of the watchdog timer upon entering 
> suspend in the SAMA5D2/4.
> 
> Typical use case of a hardware watchdog timer in the kernel is a userspace application opens the watchdog timer and
> periodically "kicks" it. If the application hits a deadlock somewhere and is no longer able to kick it, then the watchdog
> intervenes and often resets the processor. Such is the case for the Atmel driver (which also allows a watchdog interrupt
> to be asserted in lieu of a system reset). In most use cases, upon entering a low power/suspend state, the application 
> will no longer be able to "kick" the watchdog. If the watchdog is not disabled or kicked via another method, then it will
> reset the system. This is the current behavior of the Atmel driver as of today.
> 
> The watchdog peripheral itself does have a "WDIDLEHLT" bit however, and this is enabled via the "atmel,idle-halt" dt
> property. However, this is not very useful, as it literally only makes the watchdog count when the CPU is active. This 
> results in non-deterministic triggering of the WDT and means that if a critical application were to crash, it may be
> quite a long time before the WDT would ever trigger. Below is a similar statement made in the device-tree doc for this
> peripheral:
> 
> - atmel,idle-halt: present if you want to stop the watchdog when the CPU is
> 		   in idle state.
> 	CAUTION: This property should be used with care, it actually makes the
> 	watchdog not counting when the CPU is in idle state, therefore the
> 	watchdog reset time depends on mean CPU usage and will not reset at all
> 	if the CPU stop working while it is in idle state, which is probably
> 	not what you want.
> 
> It seems to me, that it would be logical and useful to introduce a new property that would cause the Atmel WDT
> to disable on suspend and re-enable on resume. It also appears that the WDT is re-initialized anyways upon
> resume, so the only piece missing here would really be a dt flag and a call to disable.
> 
Wondering - why would this need a dt property ? That would be quite unusual. Is
there a condition where one would _not_ want the watchdog to stop on suspend ?

If anything I would suggest to drop atmel,idle-halt completely; it really looks
like it would make the watchdog unreliable.

Thanks,
Guenter