Re: [PATCH] arm64: dts: ti: k3-am62-mcu: Add power-domains property for M4FSS

[Andrew Davis <afd@ti.com>]

On 9/22/26 12:19 PM, Siddharth Karanam wrote:
> Assign the power domain to the MCU M4FSS (Cortex-M4F) node by following
> the device ids info published in TI-SCI public documentation.
> 
> This will be useful in future patches that will attempt at abstracting
> ti_sci_dev_ops's calls such as get_device and put_device via pm_runtime
> calls.
> 
> Signed-off-by: Siddharth Karanam <sid9.karanam@gmail.com>
> ---
>   arch/arm64/boot/dts/ti/k3-am62-mcu.dtsi | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/boot/dts/ti/k3-am62-mcu.dtsi b/arch/arm64/boot/dts/ti/k3-am62-mcu.dtsi
> index 68e906796aefe..1ae0870893c16 100644
> --- a/arch/arm64/boot/dts/ti/k3-am62-mcu.dtsi
> +++ b/arch/arm64/boot/dts/ti/k3-am62-mcu.dtsi
> @@ -181,6 +181,7 @@ mcu_m4fss: m4fss@5000000 {
>   		      <0x00 0x5040000 0x00 0x10000>;
>   		reg-names = "iram", "dram";
>   		resets = <&k3_reset 9 1>;
> +		power-domains = <&k3_pds 7 TI_SCI_PD_EXCLUSIVE>;

The surrounding M4 subsystem's power domain is 7, but the core's power domain number
is 9 (see below in `ti,sci-dev-id`). Enabling 9 should also enable 7 as it is a
dependency. We probably should have modeled the M4 in DT the way we did the R5[0].
With the core being a subnode of the subsystem. Then 7 goes in subsystem and 9
goes in the core's subnode.

Anyway, the reason we don't use the normal "power-domains", and instead manage
the power using `ti,sci-dev-id`, is because the normal power domain framework
has a nasty habit of unconditionally powering up devices before the driver can
probe. Which in the case of remoteprocs can cause the core to start running
and executing random data. We have to do things in order,

  * Power up subsystem
  * Set reset line for core
  * Power up the core
  * Setup the core (load firmware and set boot address)
  * Release reset line for core to start it running

Andrew

[0] https://github.com/torvalds/linux/blob/master/arch/arm64/boot/dts/ti/k3-am62a-mcu.dtsi#L178

>   		firmware-name = "am62-mcu-m4f0_0-fw";
>   		ti,sci = <&dmsc>;
>   		ti,sci-dev-id = <9>;