linux-kernel - Re: [PATCH v2 1/3] dt-bindings: rtc: add reset-source property

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Message-ID: <20201217181209.sibyhlfvlpjaewrv@pengutronix.de>
Date:   Thu, 17 Dec 2020 19:12:09 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        "open list:REAL TIME CLOCK (RTC) SUBSYSTEM" 
        <linux-rtc@...r.kernel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Qiang Zhao <qiang.zhao@....com>,
        Bruno Thomsen <bruno.thomsen@...il.com>
Subject: Re: [PATCH v2 1/3] dt-bindings: rtc: add reset-source property

On Thu, Dec 17, 2020 at 10:51:08AM -0600, Rob Herring wrote:
> On Fri, Dec 11, 2020 at 5:10 PM Rasmus Villemoes
> <rasmus.villemoes@...vas.dk> wrote:
> >
> > On 11/12/2020 23.30, Rob Herring wrote:
> > > On Fri, Dec 11, 2020 at 3:56 PM Rasmus Villemoes
> > > <rasmus.villemoes@...vas.dk> wrote:
> > >>
> > >> Some RTCs, e.g. the pcf2127, can be used as a hardware watchdog. But
> > >> if the reset pin is not actually wired up, the driver exposes a
> > >> watchdog device that doesn't actually work.
> > >>
> > >> Provide a standard binding that can be used to indicate that a given
> > >> RTC can perform a reset of the machine, similar to wakeup-source.
> > >
> > > Why not use the watchdog 'timeout-sec' property?
> >
> > Wouldn't that be overloading that property? AFAIU, that is used to ask
> > the kernel to program an initial timeout value into the watchdog device.
> > But what if one doesn't want to start the watchdog device at kernel
> > boot, but just indicate that the RTC has that capability?
> 
> Yeah, I guess you're right.

I agree, too. The initial suggestion looks fine.

> > It's quite possible that if it can act as a watchdog device (and
> > has-watchdog was also suggested), one would also want timeout-sec and
> > other watchdog bindings to apply. But that can be added later, by those
> > who actually want that.
> >
> > For now, I'd really like to get my board booting again (or rather, not
> > get reset by the real watchdog just because the pcf2127 driver now
> > exposes something as /dev/wathdog0, pushing the real one to
> > /dev/wathcdog1 which doesn't get pinged from userspace).
> 
> I'm wondering how you solve which wdog to ping when there are multiple
> without relying on numbering. I guess 'reset-source' will solve that
> even if that's not your current fix. So I guess I'm fine with this.

I guess you'd need some udev magic that ensures that the right watchdog
always gets the same number.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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