Re: [PATCH] at91sam9_wdt: Allow watchdog to reset device at early boot

[Guenter Roeck <linux@roeck-us.net>]

On Fri, Dec 05, 2014 at 08:42:07PM +0200, Timo Kokkonen wrote:
> On 05.12.2014 16:12, Boris Brezillon wrote:
> >Hi Timo,
> >
> >On Fri, 05 Dec 2014 14:57:05 +0200
> >Timo Kokkonen <timo.kokkonen@offcode.fi> wrote:
> >
> >>On 27.11.2014 21:00, Boris Brezillon wrote:
> >>>Hi Timo,
> >>>
> >>>Sorry for the late reply.
> >>>
> >>>On Fri, 21 Nov 2014 14:23:30 +0200
> >>>Timo Kokkonen <timo.kokkonen@offcode.fi> wrote:
> >>>
> >>>>By default the driver will start a kernel timer which keeps on kicking
> >>>>the watchdog HW until user space has opened the watchdog
> >>>>device. Usually this is desirable as the watchdog HW is running by
> >>>>default and the user space may not have any watchdog daemon running at
> >>>>all.
> >>>>
> >>>>However, on production systems it may be mandatory that also early
> >>>>crashes and lockups will lead to a watchdog reset, even if they happen
> >>>>before the user space has opened the watchdog device.
> >>>>
> >>>>To resolve the issue, add a new device tree property
> >>>>"enable-early-reset" which will prevent the kernel timer from pinging
> >>>>the watchdog HW on behalf of user space. The default is still to use
> >>>>kernel timer, but more strict behavior can be enabled via the device
> >>>>tree property.
> >>>>
> >>>>Signed-off-by: Timo Kokkonen <timo.kokkonen@offcode.fi>
> >>>>---
> >>>>   Documentation/devicetree/bindings/watchdog/atmel-wdt.txt | 4 ++++
> >>>>   drivers/watchdog/at91sam9_wdt.c                          | 6 +++++-
> >>>>   2 files changed, 9 insertions(+), 1 deletion(-)
> >>>>
> >>>>diff --git a/Documentation/devicetree/bindings/watchdog/atmel-wdt.txt b/Documentation/devicetree/bindings/watchdog/atmel-wdt.txt
> >>>>index f90e294..a0b7b75 100644
> >>>>--- a/Documentation/devicetree/bindings/watchdog/atmel-wdt.txt
> >>>>+++ b/Documentation/devicetree/bindings/watchdog/atmel-wdt.txt
> >>>>@@ -28,6 +28,10 @@ Optional properties:
> >>>>   	entering idle state.
> >>>>   - atmel,dbg-halt : Should be present if you want to stop the watchdog when
> >>>>   	entering debug state.
> >>>>+- enable-early-reset : Should be present if you want to let the
> >>>>+	watchdog timer to expire even before user space has opened the
> >>>>+	device. If not set, a kernel timer will keep on pinging the
> >>>>+	watchdog until it is opened.
> >>>
> >>>If you want to make this property generic, maybe you should document it
> >>>in a generic binding doc
> >>>(Documentation/devicetree/bindings/watchdog/watchdog.txt ?).
> >>>Once you're at it, maybe you could document the generic timeout-sec
> >>>property in this file.
> >>>Moreover, you might want to parse this property in watchdog_core.c and
> >>>store the information in the watchdog_device struct.
> >>
> >>I gave a little thought about this today and we could maybe have it like
> >>watchdog_init_timeout() is today. But I can't really think of any
> >>generic handling for this property, anything else except storing the
> >>parsed value to a variable in watchdog_device struct. Everything else is
> >>HW specific, except how we read the variable.. If we had some logic or
> >>checking for this variable (other than to check it is not negative)
> >>maybe then it would make sense.
> >
> >Okay, I'm fine with keeping this DT property parsing out of the core.
> 
> Ok, good.
> 
> >>
> >>So I could write a patch to document generic watchdog properties in
> >>Documentation/devicetree/bindings/watchdog/watchdog.txt (anything else
> >>generic that would go there except timeout-sec and enable-early-reset?)
> >
> >I'm not sure you and Guenter agreed on the 'early-keepalive-sec'
> >property (and this one won't be used in atmel driver since you can't
> >change the timeout once set), so I'd say the timeout-sec and
> >enable-early-reset are the only common properties for now.
> 
> Actually it can be done in Atmel driver as well. It already has a
> timer that keeps on pinging the watchdog until user space opens it.
> We just modify the code so that it keeps on pinging the watchdog
> until early-keepalive-sec has expired. No need to change the HW
> timeouts at all. And I rather have it implemented that way anyway,
> the 16 second timeout is a little tight if there happens to be a lot
> of stuff already in the early user space.
> 
> Yeah, I didn't get anything back from Guenter about my
> early-keepalive-sec property proposal. Frankly, I already forgot
> about it my self already. I guess I'll get back to you with v3 set
> once I get time for it. Probably easiest to continue from there.
> 
Sorry for that, folks. I have been a bit overwhelmed in the last cycle.

early-timeout-sec seems to be the best property name to me right now.
It is aligned with the existing timeout-sec. early-keepalive-sec seems
kind of different just for the purpose of being different.

Not sure about how to name enable-early-reset. I'd prefer to have something
generic, even if only implemented in a single driver for now, but I don't
really know right now what that might/should look like. Maybe just
"enable-early" to indicate that the watchdog should be enabled during init ?

Guenter