Re: [PATCH 2/2] watchdog: gpio: add configurable minimum interval

[Rob Herring <robh@kernel.org>]

On Mon, Jul 04, 2022 at 01:05:04PM +0200, Isaac True wrote:
> On Fri, 1 Jul 2022 at 19:48, Guenter Roeck <linux@roeck-us.net> wrote:
> >
> > On 7/1/22 10:18, Rob Herring wrote:
> > > On Wed, Jun 29, 2022 at 01:06:26PM +0200, Isaac True wrote:
> > >> Add the "min_hw_margin_ms" parameter to gpio_wdt devices, allowing a
> > >> minimum interval to be specified, stopping watchdog devices from being
> > >> fed too quickly if they require a certain interval between feeds.
> > >
> > > I assume there is some real platform with a real problem you are trying
> > > to solve? Details please.
> > >
> >
> > Agreed, this should be explained in more detail.
> 
> Yes this is a real platform using a TI TPS3850 watchdog chip. With
> this chip you can configure a "window" which can detect early faults
> (i.e. too frequent) in addition to the standard watchdog features. I
> needed to add this minimum timeout to avoid watchdog resets in
> situations such as where first U-Boot and then the Linux kernel feed
> the watchdog with too short of an interval between them, or when
> systemd was configured to use the watchdog device and was feeding it
> too soon after taking over from the kernel.
> 
> > > Can you just hardcode some min? Maybe 10% of the max or something. Is
> > > there a downside to a larger than necessary min?
> > >
> >
> > That would result in extra overhead in the watchdog core which would not
> > be required for all other hardware using this driver. I'd rather avoid that.
> >
> 
> In the case of the TI TPS3850, the minimum timeout is configurable, so
> I didn't want to add a hard-coded value to the driver.
> 
> > > Wouldn't be better to fix this without requiring a DT change and that
> > > could work on stable kernels if needed.
> > >
> >
> > Presumably that is some new hardware. Most of the watchdog drivers
> > needing this value can derive it from the compatible property. The
> > gpio watchdog driver is a bit different since it is supposed to work
> > on a variety of hardware using gpio pins for watchdog control.
> >
> 
> Yes this is new hardware. This use case is also not very common as
> most watchdog chips don't have this window function or a minimum
> interval, at least in my experience, so I did not want to make it the
> default for everything.

Okay. However the existing property you copied has 2 problems. It uses 
underscores rather than hypens and doesn't use a standard unit suffix. 
So 'min-hw-margin-ms'.

Though maybe a new property instead:

timeout-range-ms = <min max>;

That's somewhat aligned to 'timeout-sec', and IMO, clearer meaning than 
'hw margin'.

Rob