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

[Quentin Schulz]

Hi Michael,

On Thu, Dec 09, 2021 at 05:32:51PM +0100, Michael Opdenacker wrote:
> 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://urldefense.proofpoint.com/v2/url?u=https-3A__lkml.org_lkml_2021_12_9_740&d=DwICaQ&c=_sEr5x9kUWhuk4_nFwjJtA&r=LYjLexDn7rXIzVmkNPvw5ymA1XTSqHGq8yBP6m6qZZ4njZguQhZhkI_-172IIy1t&m=ww7r5Dth1BLSjxhXLNnKoVNj8AucfGYrLPGtUzXCBMy77x7a-3DLDOwjZKD8McYR&s=X6oYLC_VTp7nG3HQ_bQVw4itYlKXIggXztE74rQ8LC4&e= 
> 
> 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://urldefense.proofpoint.com/v2/url?u=https-3A__elixir.bootlin.com_linux_latest_source_drivers_watchdog_at91sam9-5Fwdt.c-23L10&d=DwICaQ&c=_sEr5x9kUWhuk4_nFwjJtA&r=LYjLexDn7rXIzVmkNPvw5ymA1XTSqHGq8yBP6m6qZZ4njZguQhZhkI_-172IIy1t&m=ww7r5Dth1BLSjxhXLNnKoVNj8AucfGYrLPGtUzXCBMy77x7a-3DLDOwjZKD8McYR&s=Uj9vfn_1ctVt4xWcswqYgujwVVDW2-YXnfgRJN33tHc&e= ,
> 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.
> 

>From a quick look at the code, you need to enable CONFIG_WDT_AT91 and
the watchdog won't be disabled anymore by the SPL.

See:
https://elixir.bootlin.com/u-boot/latest/source/arch/arm/mach-at91/spl_atmel.c#L121
and
https://elixir.bootlin.com/u-boot/latest/source/arch/arm/mach-at91/spl.c#L16

Cheers,
Quentin