linux-kernel - RE: [PATCH v2 3/4] dt-bindings: watchdog: da9062: add watchdog timeout mode

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Date:   Tue, 30 Nov 2021 17:46:23 +0000
From:   Adam Thomson <Adam.Thomson.Opensource@...semi.com>
To:     Guenter Roeck <linux@...ck-us.net>,
        Adam Thomson <Adam.Thomson.Opensource@...semi.com>,
        Andrej Picej <andrej.picej@...ik.com>
CC:     Support Opensource <Support.Opensource@...semi.com>,
        "wim@...ux-watchdog.org" <wim@...ux-watchdog.org>,
        "linux-watchdog@...r.kernel.org" <linux-watchdog@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        "festevam@...il.com" <festevam@...il.com>,
        "linux-imx@....com" <linux-imx@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: RE: [PATCH v2 3/4] dt-bindings: watchdog: da9062: add watchdog
 timeout mode

On 30 November 2021 16:40, Guenter Roeck wrote:

> >> Why does it need a value ? Why not just bool ?
> >
> > One argument might be that if the property isn't provided then the OTP
> > configured value can persist without needing a FW change around this DT
> binding.
> >
> > My belief though is that the majority of users would have this property set to 0
> > by default in OTP, so a boolean would be OK I think here to enable watchdog
> > shutdown.
> >
> 
> Sorry, you lost me.
> 	dlg,wdt-sd = <0>;
> is the current situation, and identical to not having the property in
> the first place.
> 	dlg,wdt-sd = <1>;
> is new. I don't see the difference to
> 	dlg,wdt-sd;
> vs. not having the property at all (which is, again, the current situation).
> Since it has to be backward compatible,
> 	dlg,wdt-sd = <0>;
> will always be identical to not having the property at all.
> I can not find a situation where an integer would have any benefits over a
> boolean.

So if you have a binary DT binding, it's either there or it isn't which implies
the bit to be set to 0/1 in this case. If you have a binding which has a value,
there can be 3 outcomes in this discussion:

 1) Binding = 0, bit is set to 0
 2) Binding = 1, bit is set to 1
 3) Binding NOT present in DT, OTP default value in HW remains untouched

Say a platform updates to a later kernel version, but sticks with existing DT
FW (i.e. the new boolean binding isn't present in FW), then the following could
happen:

 1) OTP for DA9061/2 has this bit set to 1, system expectation is that watchdog
    triggers SHUTDOWN.
 2) New driver checks existance of 'dlg,wdt-sd' but it's obviously not there so
    assumes the bit should be set to 0 and does so
 3) When the watchdog fires, it will no longer trigger SHUTDOWN but instead
    POWER-DOWN due to binary handling of new boolean binding.