Re: [PATCH 1/1] efi_loader: stop watchdogs in ExitBootServices()

[Heinrich Schuchardt <heinrich.schuchardt@canonical.com>]

On 2/7/23 15:59, Michael Walle wrote:
>>>> Honestly, not really? Some good number of SoCs will start the watchdog
>>>> in ROM and these are also the ones that don't allow you to turn it off.
>>>
>>> I hope not, that sounds really risky. How would you debug such a platform?
>>
>> _Every single_ custom piece of industrial (as opposed to consumer-grade)
>> hardware I've worked on as a consultant has had an external,
>> always-running, gpio-petted watchdog. It's simply just something that
>> the hardware designers include, and in some cases that's even due to
>> certification requirements. So an always-running, cannot-be-turned-off,
>> watchdog is a real thing, in real hardware, and if specs don't account
>> for that, well, the spec is just paper, and we can ignore it.
> 
> I agree. But on the other hand, you cannot assume or force the OS to
> have a watchdog driver in the general case - which is as I understand
> it - one goal of EFI.
> 
> Obviously, there are watchdogs that can be disabled and some which
> cannot. I don't want to argue about the advantages and disadvantages.
> 
> For watchdogs which cannot be turned off, we can't really do anything
> anyway after the handoff to the OS - except increasing its timeout if
> thats possible.
> 
> For watchdogs that can be disabled (and are enabled in u-boot of course),
> there seems to be two use-cases:
>   (1) embedded EFI boot, that is you know exactly what you are booting, i.e.
>       self compiled kernel with a watchdog driver
>   (2) booting a general OS via EFI, think of a debian boot CD for example.
> 
> I agree, that for (1) the watchdog shouldn't be disabled. For (2) you
> cannot assume the booting OS has a driver for the watchdog, let it be an
> older version of a distribution which just haven't the SoC watchdog driver
> enabled or maybe because there is no driver for it at all (yet).
> 
> Is there a way, to have the watchdog disabled for case (2) while also
> having the possibity to use bootm/booti/bootz and keep the watchdog
> enabled? Basically I want the following:
> 
> (1) board boots with watchdog enabled
> (2) u-boot services watchdog
> (3a) booting embedded linux with booti (watchdog enabled) or
> (3b) booting generic OS with bootefi (watchdog disabled)
> 
> The missing case is booting an embedded linux with bootefi, which
> would be nice to have. But I don't really see it as a use-case for
> our board.
> 
> -michael

For SUNXI boards disabling CONFIG_WATCHDOG_AUTOSTART solved the problem 
with the very short maximum expiration time of the watchdog.

Best regards

Heinrich