linux-kernel - Re: [PATCH] watchdog: wdat_wdt: Set the min and max timeout values properly

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Message-ID: <20220919113335.18cebc74@endymion.delvare>
Date:   Mon, 19 Sep 2022 11:33:35 +0200
From:   Jean Delvare <jdelvare@...e.de>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     linux-watchdog@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH] watchdog: wdat_wdt: Set the min and max timeout values
 properly

Hi Guenter,

A few questions from an old discussion:

On Mon, 8 Aug 2022 04:36:42 -0700, Guenter Roeck wrote:
> On 8/5/22 15:07, Jean Delvare wrote:
> > To be honest, I'm not sold to the idea of a software-emulated
> > maximum timeout value above what the hardware can do, but if doing
> > that makes sense in certain situations, then I believe it should be
> > implemented as a boolean flag (named emulate_large_timeout, for
> > example) to complement max_timeout instead of a separate time value.
> > Is there a reason I'm missing, why it was not done that way?
>
> There are watchdogs with very low maximum timeout values, sometimes less than
> 3 seconds. gpio-wdt is one example - some have a maximum value of 2.5 seconds.
> rzn1_wd is even more extreme with a maximum of 1 second. With such low values,
> accuracy is important, second-based limits are insufficient, and there is an
> actual need for software timeout handling on top of hardware.

Out of curiosity, what prevents user-space itself from pinging
/dev/watchdog every 0.5 second? I assume hardware using such watchdog
devices is "special" and would be running finely tuned user-space, so
the process pinging /dev/watchdog could be given higher priority or
even real-time status to ensure it runs without delays. Is that not
sufficient?

> At the same time, there is actually a need to make timeouts milli-second based
> instead of second-based, for uses such as medical devices where timeouts need
> to be short and accurate. The only reason for not implementing this is that
> the proposals I have seen so far (including mine) were too messy for my liking,
> and I never had the time to clean it up. Reverting milli-second support would
> be the completely wrong direction.

I might look into this at some point (for example as a SUSE Hackweek
project). Did you post your work somewhere? I'd like to take a look.

Thanks,
-- 
Jean Delvare
SUSE L3 Support