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

[Michael Walle]

Am 2020-09-24 10:10, schrieb 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.

I've missed that, but that will just limit the search space.
Like how can we figure out what board has both EFI_LOADER=y and
WDT=y set? If there is no such board, then we can use the
defaults above. If there are such boards we will have to put
CONFIG_SUPERVISE_OS=y in their defconfig.

If EFI_LOADER would have had no default y, then a simple
grep for EFI_LOADER=y (and WDT=y) would have been sufficient.

Are there any handy methods/tools?

-michael