Re: [PATCH RFC] Watchdog: sbsa_gwdt: Enhance timeout range

[Guenter Roeck]

On 05/05/2016 04:45 PM, Timur Tabi wrote:
> Timur Tabi wrote:
> > > A 32-bit counter is absolutely fine. Letting it run with a 400MHz clock
> > > (or was it 200 MHz ?) is the problem. A resolution of 2.5ns for a
> > > watchdog
> > > timer does not really make any sense.
> >
> > The 10 second limit is based on a 20MHz clock.
>
> No, that's not true.  I misread the code.  I knew something was wrong, but it didn't click until just now.
>
> The default timeout is 10 seconds.  The max timeout on a 20MHz system (which is what we're running) is over 200 seconds.
>
> The problem is that Pratyush's system is running at a clock that's way too fast:
>
>   [  131.187562] sbsa-gwdt sbsa-gwdt.0: Initialized with 40s timeout @ 250000000 Hz, action=1.
>
> 250MHz is unreasonable.  Pratyush, why is your system counter so high? On our ARM64 system, it's set to 20MHz.

Guess that answers my earlier question. Problem is that the specification
_permits_ those unreasonable frequencies, and quite obviously they are
being used, no matter if it makes sense or not.

With a (still unreasonable) maximum frequency of 100 MHz, the problem
would not exist. So, if anything, someone with influence on the standard
might suggest to reduce the maximum permitted frequency.

Guenter