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

[Mukesh Ojha]

On Fri, Mar 14, 2025 at 05:31:44PM +0100, Arnd Bergmann wrote:
> On Fri, Mar 14, 2025, at 12:19, Lorenzo Pieralisi wrote:
> > On Mon, Mar 03, 2025 at 01:08:31PM -0800, Elliot Berman wrote:
> >> From: Elliot Berman <elliot.berman at oss.qualcomm.com>
> >> 
> >> 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".
> 
> I think the reboot_mode predates the 'cmd' argument: linux-2.1.30
> introduced LINUX_REBOOT_CMD_RESTART2 and it already had
> the warm/cold/bios/hard options for i386 reboot. I think the
> argument went unused for a while after it got introduced though.
> 
> > 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 ?
> 
> Don't know one either.
> 
> >> 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.
> 
> Can we try to call them 'arguments' rather than 'modes' while discussing?
> I think it's way too easy to confuse them otherwise.
> 
> >> +	psci_np = of_find_matching_node(NULL, psci_of_match);
> >> +	if (!psci_np)
> >> +		return 0;
> >> +
> >> +	np = of_find_node_by_name(psci_np, "reset-types");
> >> +	if (!np)
> >> +		return 0;
> >
> > Related to my initial question above. If LINUX_REBOOT_CMD_RESTART2 *arg command,
> > is the actual reset to be issued, should we add a default mode "cold"
> > and, if SYSTEM_RESET2 is supported, a "warm" reset mode too ?
> >
> > It all boils down to what *arg represents - adding "cold" and "warm"
> > modes would remove the dependency on reboot_mode for resets issued
> > through LINUX_REBOOT_CMD_RESTART2, the question is whether this
> > is the correct thing to do.
> >
> > Comments very welcome.
> 
> It would make some sense to me to treat all psci reboot as "warm" and
> not do anything here if reboot="cold" is set: those would have to
> be backed by a hardware specific driver.

The patch implements PSCI SYSTEM_RESET2 vendor reset types which seems
independent of reboot_mode (warm/cold/etc) as per specs.  As per ARM
PSCI SPEC:
https://documentation-service.arm.com/static/60feb9bf3d73a34b640e1b67%3Ftoken%3D&ved=2ahUKEwiX7ramjZCGAxUXrlYBHaPaBX4QFnoECBUQAQ&usg=AOvVaw1L35eQniULNRstfGKX5KXp

quoted from spec: -- 

""
5.1.11 

SYSTEM_RESET2 This function extends SYSTEM_RESET.

It provides:
	• Architectural reset definitions.
	• Support for vendor-specific resets.

Bit[31] Reserved must be zero. 
	o Set to 1 for vendor-specific resets.
	o Set to 0 for architectural resets.

Bits[30:0]
	o For vendor-specific resets, the format of these bits is IMPLEMENTATION DEFINED. 
	o For architectural resets, the following values are defined:
		- 0x0 SYSTEM_WARM_RESET.
		- All other values are reserved.

""

So, for vendor specific reset types it does not specify warm or cold.

-Mukesh

> 
> A related problem is how to determine when to use UEFI reboot: at the
> moment arm64 tries the UEFI runtime interface before it even attempts
> any of the notifiers, so PSCI reboot won't ever be used if UEFI is
> present.
> 
>      Arnd