Re: [PATCH v2 0/4] Implement vendor resets for PSCI SYSTEM_RESET2

[Elliot Berman]

On Tue, Apr 16, 2024 at 10:35:22AM +0100, Sudeep Holla wrote:
> On Sun, Apr 14, 2024 at 12:30:23PM -0700, Elliot Berman wrote:
> > The PSCI SYSTEM_RESET2 call allows vendor firmware to define additional
> > reset types which could be mapped to the reboot argument.
> >
> > Setting up reboot on Qualcomm devices can be inconsistent from chipset
> > to chipset.
> 
> That doesn't sound good. Do you mean PSCI SYSTEM_RESET doesn't work as
> expected ? Does it mean it is not conformant to the specification ?
> 

I was motivating the reason for using SYSTEM_RESET2. How to set the PMIC
register and IMEM cookie can change between chipsets. Using
SYSTEM_RESET2 alows us to abstract how to perform the reset.

> > Generally, there is a PMIC register that gets written to
> > decide the reboot type. There is also sometimes a cookie that can be
> > written to indicate that the bootloader should behave differently than a
> > regular boot. These knobs evolve over product generations and require
> > more drivers. Qualcomm firmwares are beginning to expose vendor
> > SYSTEM_RESET2 types to simplify driver requirements from Linux.
> >
> 
> Why can't this be fully userspace driven ? What is the need to keep the
> cookie in the DT ?

As Dmitry pointed out, this information isn't discoverable. I suppose
we could technically use bootconfig or kernel command-line to convey the
map although I think devicetree is the right spot for this mapping.

- Other vendor-specific bits for PSCI are described in the devicetree.
  One example is the suspend param (e.g. the StateID) for cpu idle
  states.
- Describing firmware bits in the DT isn't unprecedented, and putting
  this information outside the DT means that other OSes (besides Linux)
  need their own way to convey this information.
- PSCI would be the odd one out that reboot mode map is not described in
  DT. Other reboot-mode drivers specify the mapping in the DT. Userspace
  that runs with firmware that support vendor reset2 need to make sure
  they can configure the mapping early enough.

Thanks,
Elliot