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..., 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.