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From:   Evan Benn <evanbenn@chromium.org>
Date:   Thu, 16 Apr 2020 10:46:39 +1000
Message-ID: <CAKz_xw0+gKBM1jp-Avnd+4j9vSxUix67RZBX-NNbStb0+ri4+Q@mail.gmail.com>
Subject: Re: [PATCH v2 0/2] Add a watchdog driver that uses ARM Secure Monitor Calls.
To:     Guenter Roeck <linux@roeck-us.net>
Cc:     Julius Werner <jwerner@chromium.org>,
        Xingyu Chen <xingyu.chen@amlogic.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
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        Catalin Marinas <catalin.marinas@arm.com>,
        "David S. Miller" <davem@davemloft.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Leonard Crestez <leonard.crestez@nxp.com>,
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        Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>,
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Thanks Xingyu,

Can anyone provide advice about making SMCWD_FUNC_ID a device tree
param directly, vs using the compatible to select from a table.

Please note get_timeleft erroneously returns res.a0 instead of res.a1
in this version.

Evan

On Thu, Apr 16, 2020 at 9:12 AM Guenter Roeck <linux@roeck-us.net> wrote:
>
> On Wed, Apr 15, 2020 at 03:29:29PM -0700, Julius Werner wrote:
> > > In addition, It looks more reasonable to use the "msec" as the unit of
> > > timeout parameter for the ATF fw interface with SMCWD_SET_TIMEOUT:
> > >
> > > - The fw interface will compatible with the uboot generic watchdog
> > > interface at [0], and there is no need to convert timeout from msec
> > > to sec.
> >
> > I think we're trying hard to keep this compatible to a Trusted
> > Firmware counterpart that we have already shipped, so we would prefer
> > to keep it at seconds if possible. That's what the Linux watchdog core
> > uses as well after all, so it just seems natural. I don't really see
> > how what U-Boot does would have anything to do with this.
> >
> > > - Some vendor's watchdog may be not support the "wdt_trigger_reset"
> > > reset operation, but they can use the method below to reset the system
> > > by the watchdog right now.
> > >
> > > watchdog_set_time(1);  //1ms
> > > watchdog_enable();
> >
> > They can still do that but they should do that on the Trusted Firmware
> > side. Emulating a missing reset functionality should be handled by the
> > hardware abstraction layer (in this case Trusted Firmware), not at the
> > Linux API level. So Linux would still send a PSCI_SYSTEM_RESET SMC,
> > but then Trusted Firmware can choose to implement that by setting the
> > watchdog to the smallest possible timeout (which it can because it's
> > accessing it directly, not through this SMC interface) and letting it
> > expire.
>
> I agree. Using a watchdog to implement reset functionality is always a
> means of last resort and should be avoided if possible.
>
> Guenter