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

Heinrich Schuchardt
2020-09-23T17:31:00Z

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@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.

Best regards

Heinrich

> a surprise to the user. I say this because it's a surprise to me and I
> guess answers the question of "how does x86 handle this?" I had the
> other day.
>