Re: [PATCH v5 3/4] firmware: psci: Read and use vendor reset types

On Mon, Jun 17, 2024 at 10:18:09AM -0700, Elliot Berman wrote:

SoC vendors have different types of resets and are controlled through
various registers. For instance, Qualcomm chipsets can reboot to a
"download mode" that allows a RAM dump to be collected. Another example
is they also support writing a cookie that can be read by bootloader
during next boot. PSCI offers a mechanism, SYSTEM_RESET2, for these
vendor reset types to be implemented without requiring drivers for every
register/cookie.

Add support in PSCI to statically map reboot mode commands from
userspace to a vendor reset and cookie value using the device tree.

A separate initcall is needed to parse the devicetree, instead of using
psci_dt_init because mm isn't sufficiently set up to allocate memory.

Reboot mode framework is close but doesn't quite fit with the
design and requirements for PSCI SYSTEM_RESET2. Some of these issues can
be solved but doesn't seem reasonable in sum:
 1. reboot mode registers against the reboot_notifier_list, which is too
    early to call SYSTEM_RESET2. PSCI would need to remember the reset
    type from the reboot-mode framework callback and use it
    psci_sys_reset.
 2. reboot mode assumes only one cookie/parameter is described in the
    device tree. SYSTEM_RESET2 uses 2: one for the type and one for
    cookie.
 3. psci cpuidle driver already registers a driver against the
    arm,psci-1.0 compatible. Refactoring would be needed to have both a
    cpuidle and reboot-mode driver.

I need to think through it but when you first introduced the generic
Documentation/devicetree/bindings/power/reset/reboot-mode.yaml bindings
I also looked at drivers/power/reset/reboot-mode.c

I assumed this extension to that binding would reuse the same and
PSCI would just do reboot_mode_register(). I didn't expect to see these
changes. I might have missing something but since the bindings is still
quite generic with additional cells that act as additional cookie for
reboot call, I still think that should be possible.

What am I missing here then ?

-- 
Regards,
Sudeep