Re: [PATCH v9 2/5] firmware: psci: Read and use vendor reset types

[Mukesh Ojha]

On Tue, Mar 25, 2025 at 7:33 PM Mukesh Ojha
<mukesh.ojha@xxxxxxxxxxxxxxxx> wrote:
>
> On Fri, Mar 14, 2025 at 12:19:31PM +0100, Lorenzo Pieralisi wrote:
> > On Mon, Mar 03, 2025 at 01:08:31PM -0800, Elliot Berman wrote:
> > > From: Elliot Berman <elliot.berman@xxxxxxxxxxxxxxxx>
> > >
> > > 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.
> >
> > I have managed to discuss a little bit this patchset over the last
> > few days and I think we have defined a plan going forward.
> >
> > A point that was raised is:
> >
> > https://man7.org/linux/man-pages/man2/reboot.2.html
> >
> > LINUX_REBOOT_CMD_RESTART2 *arg command, what is it supposed to
> > represent ?
> >
> > Is it the mode the system should reboot into OR it is the
> > actual command to be issued (which is what this patchset
> > implements) ?
> >
> > LINUX_REBOOT_CMD_RESTART "..a default restart..."
> >
> > It is unclear what "default" means. We wonder whether the
> > reboot_mode variable was introduced to _define_ that "default".
> >
> > So, in short, my aim is trying to decouple reboot_mode from the
> > LINUX_REBOOT_CMD_RESTART2 *arg command.
> >
> > I believe that adding a sysfs interface to reboot-mode driver
> > infrastructure would be useful, so that the commands would
> > be exposed to userspace and userspace can set the *arg command
> > specifically to issue a given reset/mode.
> >
> > I wonder why this is not already in place for eg syscon-reboot-mode
> > resets, how does user space issue a command in those systems if the
> > available commands aren't exposed to userspace ?
> >
> > Is there a kernel entity exposing those "modes" to userspace, somehow ?
> >
> > > 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.
> >
> > This can be changed and I think it should, so that the reboot modes
> > are exposed to user space and PSCI can use that.
> >
> In the case of a regular reboot or panic, the reboot/panic notifiers run
> first, followed by the restart notifiers. The PSCI reset/reset2 should
> be the last call from Linux, and ideally, this call should not fail.
>
> Reboot mode notifiers => restart notifiers or Panic notifiers => restart
> notifiers
>
> So, if I understand correctly, you mean that we can change the reboot
> mode framework to expose the arguments available to user space. We can
> extend it to accept magic and cookies, save them in the reboot
> framework, and retrieve them via a call from PSCI during a regular
> reboot or panic based on the current arguments. Is this leading towards
> writing an ARM-specific PSCI-reboot-mode driver, which in its reboot
> notifier callback saves the magic and cookies, and these magic and
> cookies will be used during psci_sys_reset2()? Or is there something
> wrong with my understanding?

In case this mail got lost in your inbox, I wanted to float this up
again as this is a very important ARM feature and
its future course of action depends on the common understanding.

-Mukesh