ARM: mach-at91: how to stop U-Boot/SPL from disabling the watchdog timer on SAMAD3 Xplained

[Michael Opdenacker <michael.opdenacker@bootlin.com>]

Greetings,

Documenting the issue and a workaround here in case other people
encounter it, but if you have tips for fixing the issue in U-Boot, I'm
interested of course!

I noticed that the SAMA5D3 Xplained board didn't have the watchdog timer
enabled in the Linux DTS.
I submitted a patch to fix this: https://lkml.org/lkml/2021/12/9/740

However, applying this patch isn't sufficient to make the watchdog
available on Linux:
at91_wdt fffffe40.watchdog: watchdog is disabled
at91_wdt: probe of fffffe40.watchdog failed with error -22

As explained on
https://elixir.bootlin.com/linux/latest/source/drivers/watchdog/at91sam9_wdt.c#L10,
this happens because the bootloader or bootstrap code already disables
the watchdog. The trouble is, unlike on SAMA5D2 and SAMAD4 (if I
understood correctly),"The Watchdog Timer Mode Register can be only
written to once".

However, I haven't managed to disable this behavior in U-Boot (including
the SPL). I have no watchdog related option turned on anywhere, and by
adding puts() messages in the code, I believe that the wdt_stop()
function doesn't get called either. Maybe the corresponding watchdog
register is written to by the code initializing the SoC, but I couldn't
find where.

A workaround is to use at91bootstrap instead of U-Boot SPL, which has a
configuration parameter to disable to watchdog:
git clone https://github.com/linux4sam/at91bootstrap.git
export CROSS_COMPILE=arm-linux- (whatever your cross-compiler is)
make sama5d3_xplainedsd_uboot_defconfig
make menuconfig (Disable CONFIG_DISABLE_WATCHDOG)
make
Copy boot.bin (under build/...) and u-boot.bin (from U-Boot sources) to
the SD card.

At least, this proves that the culprit is U-Boot SPL!

So, any advice for making it possible prevent the SPL from disabling the
watchdog on SAMA5D3 (Xplained at least)?

Cheers
Michael.

-- 
Michael Opdenacker, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com