[PATCH 2/2] watchdog: add watchdog behavior configuration

[Tom Rini]

On Wed, Sep 23, 2020 at 07:31:00PM +0200, Heinrich Schuchardt wrote:
> On 9/23/20 7:14 PM, Tom Rini wrote:
> > On Wed, Sep 23, 2020 at 07:01:54PM +0200, Mark Kettenis wrote:
> >>> From: Michael Walle <michael at walle.cc>
> >>> Date: Wed, 23 Sep 2020 18:45:27 +0200
> >>>
> >>> Let the user choose between three different behaviours of the watchdog:
> >>>  (1) Keep the watchdog disabled
> >>>  (2) Supervise u-boot
> >>>  (3) Supervise u-boot and the operating systen (default)
> >>>
> >>> Option (2) will disable the watchdog right before handing control to the
> >>> operating system. This is useful when the OS is not aware of the
> >>> watchdog. Option (3) doesn't disable the watchdog and assumes the OS
> >>> will continue servicing.
> >>
> >> (3) can't be the default, at least for EFI
> >>
> >> The UEFI standard explicitly says that upon calling
> >> ExitBootServices(), the watchdog timer is disabled.
> >>
> >> In general, you can't expect an OS to have support for a particular
> >> watchdog timer.  So (3) only makes sense in cases where U-Boot is
> >> bundled with an OS image.
> >
> > We need to be careful here then.  The current and historical / generally
> > expected behavior is if we've enabled the watchdog we supervise it and
> > leave it enabled for the OS.  Given what UEFI requires I'd like to see
> > that case handled with a print about disabling the watchdog so it's not
> 
> Not printf(), maybe log_info().
> 
> The disabling has to occur in ExitBootServices() (aka.
> efi_exit_boot_services()). Here we are in the middle of an executing
> UEFI application. Printing anything on the screen may mess up the output
> of the UEFI application.
> 
> So, please, don't output anything.

We need to find a good way to inform the user we're disabling their
watchdog.  Maybe before we fully jump in to UEFI note that it will be
disabled before entering the OS?  Or something a bit more generally
understood than ExitBootServices() having been called.  I don't know
_where_ the best place is, but I think it's important to inform the
user.

-- 
Tom
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