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

[Mark Kettenis]

> Date: Thu, 24 Sep 2020 09:33:50 +0200
> From: Michael Walle <michael at walle.cc>
> 
> Am 2020-09-23 19:35, schrieb 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
> 
> I agree with "current and historical behavior" but not with "expected
> behavior".
> 
> I was thinking about something like
> 
> +choice
> +       prompt "Watchdog behavior"
> +       default WATCHDOG_SUPERVISE_U_BOOT if EFI_LOADER
> +       default WATCHDOG_SUPERVISE_OS if !EFI_LOADER
> +       depends on WDT
> 
> Unfortunately, EFI_LOADER is default y for any architecture != ARM.
> Therefore, it is likely we are changing the behavior of some boards
> and I agree this isn't what we want.

I think you are misreading that.  The following stanza:

        depends on OF_LIBFDT && ( \
                ARM && (SYS_CPU = arm1136 || \
                        SYS_CPU = arm1176 || \
                        SYS_CPU = armv7   || \
                        SYS_CPU = armv8)  || \
                X86 || RISCV || SANDBOX)

means that EFI_LOADER is onlu ever defined on (newish) ARM, X86, RISCV
and SANDBOX.  Which makes sense since there is no EFI calling
convention defined for other architectures like MIPS and PPC.

> >> 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.
> 
> The watchdog is only disabled in the "supervise u-boot" mode, why
> would we need to inform the user? It was the users choice to have
> the timer only enabled in u-boot.
> 
> Or do you mean if for example the vendor chooses that option and
> in this case the user doesn't know anything about it? The mode
> is indicated in the "WDT:" output.
> 
> -michael
>