linux-kernel - Re: [PATCH v5 3/4] wdt: Support wdt on ROHM BD9576MUF and BD9573MUF

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Date:   Wed, 11 Nov 2020 07:47:09 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     "Vaittinen, Matti" <Matti.Vaittinen@...rohmeurope.com>
Cc:     "mazziesaccount@...il.com" <mazziesaccount@...il.com>,
        "wim@...ux-watchdog.org" <wim@...ux-watchdog.org>,
        linux-power <linux-power@...rohmeurope.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "lee.jones@...aro.org" <lee.jones@...aro.org>,
        "linux-watchdog@...r.kernel.org" <linux-watchdog@...r.kernel.org>
Subject: Re: [PATCH v5 3/4] wdt: Support wdt on ROHM BD9576MUF and BD9573MUF

On Wed, Nov 11, 2020 at 03:15:17PM +0000, Vaittinen, Matti wrote:
[ ... ]
> > > > +
> > > > +	priv->wdd.info			= &bd957x_wdt_ident;
> > > > +	priv->wdd.ops			= &bd957x_wdt_ops;
> > > > +	priv->wdd.min_hw_heartbeat_ms	= hw_margin_min;
> > > > +	priv->wdd.max_hw_heartbeat_ms	= hw_margin_max;
> > > > +	priv->wdd.parent		= dev;
> > > > +	priv->wdd.timeout		= (hw_margin_max / 2) * 1000;
> > > 
> > > Hmm. Just noticed this value does not make sense, right?
> > > Maximum hw_margin is 4416 ms. If I read this correctly timeout
> > > should
> > > be in seconds -  so result is around 2 000 000 seconds here. I
> > > think it
> > > is useless value...
> > > 
> > > Perhaps
> > > 	priv->wdd.timeout		= (hw_margin_max / 2) / 1000;
> > > 	if (!priv->wdd.timeout)
> > > 		priv->wdd.timeout = 1;
> > > would be more appropriate.
> > > 
> > 
> > Yes. Good catch. Actually, since max_hw_heartbeat_ms is specified,
> > it can and should be a reasonable constant (like the usual 30
> > seconds).
> > It does not and should not be bound by max_hw_heartbeat_ms.
> 
> Thanks for confirming this Guenter. I'd better admit I didn't
> understand how the max_hw_heartbeat_ms works.
> 
> If I now read the code correctly, the "watchdog worker" takes care of
> feeding for shorter periods than the "timeout" - and only stops
> feeding max_hw_heartbeat_ms before timeout expires if userland has not
> been feedin wdg. This is really cool approach for short(ish)
> max_hw_heartbeat_ms configurations as user-space does not need to meet
> "RT requirements". WDG framework is much more advanced that I knew :)

Yes, exactly, that is the idea. Various drivers used to implement this
within the driver, so it just made sense to pull the functionality into
the watchdog core.

Thanks,
Guenter