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

[Mateusz Nowicki <mateusz.nowicki@posteo.net>]

Hi all,

this follows up on Marek's report about the stm32mp2 watchdog [1].

A quick recap: the arm,smc-wdt driver talks to the firmware through one
SMC function id that it reads from the device tree, and there is no way
to ask the firmware which id it uses. On the STM32MP2 SoCs that id
changed over time. ST's OP-TEE from ecosystem v5 expects 0xb200005a,
their 4.0.0 fork accepts both, and the 4.8.0 fork as well as upstream
OP-TEE only accept 0xbc000000.

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.

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.

Why RFC: arm,smc-id already exists in firmware/arm,scmi.yaml as a plain
uint32, and dt_binding_check complains when another binding turns it
into an array:

  arm-smc-wdt.yaml: arm,smc-id: multiple incompatible types: uint32-array
  arm,scmi.yaml: arm,smc-id: multiple incompatible types: uint32

DT folks, is there a way to keep the list in arm,smc-id that you would
be fine with? If not, I can move the extra ids into a new property,
something like:

  arm,smc-id = <0xbc000000>;
  arm,smc-alt-ids = <0xb200005a>;

The driver side would stay the same, it would just read the fallback
ids from the other property.

One more question: should patches 2 and 3 go to stable as well? Then
6.18.y would also work with both firmware generations. The 6.18.y
version of patch 3 would turn its 0xb200005a into the same list.

[1] https://lore.kernel.org/all/1b14e1e4-962f-4391-91fd-b09143641643@nabladev.com/

Mateusz Nowicki (3):
  dt-bindings: watchdog: arm-smc-wdt: allow a list of SMC ids
  watchdog: arm_smc_wdt: try every id of arm,smc-id
  arm64: dts: st: list both watchdog SMC ids on stm32mp23 and stm32mp25

 .../bindings/watchdog/arm-smc-wdt.yaml         |  9 +++++++--
 arch/arm64/boot/dts/st/stm32mp231.dtsi         |  2 +-
 arch/arm64/boot/dts/st/stm32mp251.dtsi         |  2 +-
 drivers/watchdog/arm_smc_wdt.c                 | 18 ++++++++++++------
 4 files changed, 21 insertions(+), 10 deletions(-)


base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
-- 
2.56.0