linux-kernel - Re: [PATCH v2 0/2] Add a watchdog driver that uses ARM Secure Monitor Calls.

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Date:   Wed, 15 Apr 2020 16:12:15 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Julius Werner <jwerner@...omium.org>
Cc:     Xingyu Chen <xingyu.chen@...ogic.com>,
        Evan Benn <evanbenn@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Anson Huang <Anson.Huang@....com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Leonard Crestez <leonard.crestez@....com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Marcin Juszkiewicz <marcin.juszkiewicz@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Olof Johansson <olof@...om.net>,
        Rob Herring <robh+dt@...nel.org>,
        Rob Herring <robh@...nel.org>, Shawn Guo <shawnguo@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Vinod Koul <vkoul@...nel.org>, Will Deacon <will@...nel.org>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        devicetree@...r.kernel.org,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@...ts.infradead.org>,
        LINUX-WATCHDOG <linux-watchdog@...r.kernel.org>,
        Jianxin Pan <jianxin.pan@...ogic.com>,
        Yonghui Yu <yonghui.yu@...ogic.com>,
        "open list:ARM/Amlogic Meson..." <linux-amlogic@...ts.infradead.org>
Subject: Re: [PATCH v2 0/2] Add a watchdog driver that uses ARM Secure
 Monitor Calls.

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