Re: [RFC PATCH 0/3] watchdog: arm_smc_wdt: allow a list of SMC function ids

[mateusz.nowicki@posteo.net]

On 11/4/26 Guenter Roeck wrote:
> On 11/4/26 08:16, Mateusz Nowicki wrote:
>> My earlier patch, commit ae4d08bbe3f4 ("arm64: dts: st: fix arm,smc-id
>> of the SMC watchdog on stm32mp2"), switched the dtsi to 0xbc000000. 
>> That
>> made the watchdog work with current OP-TEE, but it broke boards that 
>> run
>> the older firmware. So today 6.18.y uses one id and 7.4 the other. 
>> When
>> the kernel and the firmware disagree, nobody pets the watchdog that 
>> the
>> bootloader started, and the board resets after 32 seconds. Marek hit
>> this even with a recovery kernel, which is about the worst place for 
>> it.
>> 
> 
> So the DT on the board is updated to use the new ID but the firmware 
> itself
> isn't ? That seems a bit odd; I would have assumed that the DT on a 
> board
> is kept in sync with its firmware. Is that not the case here ?

Not necessarily. On these boards the DTB is built from the kernel tree
and installed with the kernel, U-Boot just loads it from the boot
partition. TF-A and OP-TEE sit in the FIP and are updated separately, so
updating the kernel alone changes the ID. That's how Marek got hit: he
updated OP-TEE, but his recovery kernel still had the old DTB.

>> The idea from that thread: let arm,smc-id be a list. The driver tries
>> the ids one by one and keeps the first one the firmware accepts. Older
>> kernels only read the first cell, so they behave exactly as they do
>> now. For stm32mp2 the list is <0xbc000000 0xb200005a>. Calling an id
>> that belongs to some other firmware service would be bad, so the
>> binding says the list may only hold ids known to be this watchdog.
>> 
> 
> Would it possibly make sense to check keep a list of known IDs in the
> driver ? It already checks 0x82003D06, so it might as well check a 
> sequence.

Fine with me, and it avoids the binding issue. I'd just rather not try
the ST IDs on every platform, so how about making the list per
compatible:

   compatible = "st,stm32mp25-smc-wdt", "arm,smc-wdt";

with { 0xbc000000, 0xb200005a } as match data for it. arm,smc-id stays a
single u32 and, if present, is tried first. Older kernels just match
"arm,smc-wdt" and behave as today.

If that works for you and the DT folks, I'll send v2.

Thanks,
Mateusz