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

[Shivendra Pratap]

On 4/8/2025 8:46 PM, Lorenzo Pieralisi wrote:
> On Tue, Mar 25, 2025 at 07:33:36PM +0530, Mukesh Ojha 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?
> 
> No, you got it right (apologies for the delay in replying) - if the
> case for making reboot mode available to user space is accepted.
> 

Agree that the available modes should be exposed to usespace via sysfs interface
and we should implement it. Also #1 and #2 can be handled via some
changes in the design as mentioned in above discussion.

I have one doubt though when we implement this via reboot-mode framework.
The current patch implements PSCI ARM PSCI SYSTEM RESET2 vendor reset types.
psci driver is initialized very early at boot but potential ARM psci reboot-mode
driver will not probe at that stage and the ARM PSCI SYSTEM RESET2 vendor reset
types functionality will not be available in psci reset path until the reboot-mode
driver probes. Will this cause any limitation on usage of ARM's PSCI vendor-reset
types for early device resets?

One use-case may be an early device crash or a early reset where a vendor 
wants to use PSCI SYSTEM RESET2 vendor reset type to a reset the device to a 
specific state but may not be able to use this driver.
(eg: a kernel panic at early boot where a vendor wants to reset device 
to a specific state using vendor reset. Currently panic passes a NULL
(*arg command) while device reset but it may be explored for vendor specific
reset).

- Shivendra

>> P.S. We appreciate Elliot for his work and follow-up on this while being
>> employed at Qualcomm.
> 
> Yes I sincerely do for his patience, thank you.
> 
> Lorenzo
> 
>>>>  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.
>>>>
>>>> Signed-off-by: Elliot Berman <elliot.berman@xxxxxxxxxxxxxxxx>
>>>> ---
>>>>  drivers/firmware/psci/psci.c | 105 +++++++++++++++++++++++++++++++++++++++++++
>>>>  1 file changed, 105 insertions(+)
>>>>
>>>> diff --git a/drivers/firmware/psci/psci.c b/drivers/firmware/psci/psci.c
>>>> index a1ebbe9b73b136218e9d9f9b8daa7756b3ab2fbe..6f8c47deaec0225f26704e1f3bcad52603127a85 100644
>>>> --- a/drivers/firmware/psci/psci.c
>>>> +++ b/drivers/firmware/psci/psci.c
>>>> @@ -80,6 +80,14 @@ static u32 psci_cpu_suspend_feature;
>>>>  static bool psci_system_reset2_supported;
>>>>  static bool psci_system_off2_hibernate_supported;
>>>>  
>>>> +struct psci_reset_param {
>>>> +	const char *mode;
>>>> +	u32 reset_type;
>>>> +	u32 cookie;
>>>> +};
>>>> +static struct psci_reset_param *psci_reset_params __ro_after_init;
>>>> +static size_t num_psci_reset_params __ro_after_init;
>>>> +
>>>>  static inline bool psci_has_ext_power_state(void)
>>>>  {
>>>>  	return psci_cpu_suspend_feature &
>>>> @@ -306,9 +314,39 @@ static int get_set_conduit_method(const struct device_node *np)
>>>>  	return 0;
>>>>  }
>>>>  
>>>> +static int psci_vendor_system_reset2(const char *cmd)
>>>> +{
>>>> +	unsigned long ret;
>>>> +	size_t i;
>>>> +
>>>> +	for (i = 0; i < num_psci_reset_params; i++) {
>>>> +		if (!strcmp(psci_reset_params[i].mode, cmd)) {
>>>> +			ret = invoke_psci_fn(PSCI_FN_NATIVE(1_1, SYSTEM_RESET2),
>>>> +					     psci_reset_params[i].reset_type,
>>>> +					     psci_reset_params[i].cookie, 0);
>>>> +			/*
>>>> +			 * if vendor reset fails, log it and fall back to
>>>> +			 * architecture reset types
>>>
>>> That's not what the code does.
>>>
>> Ack.
>>
>> -Mukesh
>>
>>>> +			 */
>>>> +			pr_err("failed to perform reset \"%s\": %ld\n", cmd,
>>>> +			       (long)ret);
>>>> +			return 0;
>>>> +		}
>>>> +	}
>>>> +
>>>> +	return -ENOENT;
>>>> +}
>>>> +
>>>>  static int psci_sys_reset(struct notifier_block *nb, unsigned long action,
>>>>  			  void *data)
>>>>  {
>>>> +	/*
>>>> +	 * try to do the vendor system_reset2
>>>> +	 * If there wasn't a matching command, fall back to architectural resets
>>>> +	 */
>>>> +	if (data && !psci_vendor_system_reset2(data))
>>>> +		return NOTIFY_DONE;
>>>> +
>>>>  	if ((reboot_mode == REBOOT_WARM || reboot_mode == REBOOT_SOFT) &&
>>>>  	    psci_system_reset2_supported) {
>>>>  		/*
>>>> @@ -795,6 +833,73 @@ static const struct of_device_id psci_of_match[] __initconst = {
>>>>  	{},
>>>>  };
>>>>  
>>>> +#define REBOOT_PREFIX "mode-"
>>>> +
>>>> +static int __init psci_init_system_reset2_modes(void)
>>>> +{
>>>> +	const size_t len = strlen(REBOOT_PREFIX);
>>>> +	struct psci_reset_param *param;
>>>> +	struct device_node *psci_np __free(device_node) = NULL;
>>>> +	struct device_node *np __free(device_node) = NULL;
>>>> +	struct property *prop;
>>>> +	size_t count = 0;
>>>> +	u32 magic[2];
>>>> +	int num;
>>>> +
>>>> +	if (!psci_system_reset2_supported)
>>>> +		return 0;
>>>> +
>>>> +	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.
>>>
>>> Thanks,
>>> Lorenzo
>>>
>>>> +
>>>> +	for_each_property_of_node(np, prop) {
>>>> +		if (strncmp(prop->name, REBOOT_PREFIX, len))
>>>> +			continue;
>>>> +		num = of_property_count_u32_elems(np, prop->name);
>>>> +		if (num != 1 && num != 2)
>>>> +			continue;
>>>> +
>>>> +		count++;
>>>> +	}
>>>> +
>>>> +	param = psci_reset_params =
>>>> +		kcalloc(count, sizeof(*psci_reset_params), GFP_KERNEL);
>>>> +	if (!psci_reset_params)
>>>> +		return -ENOMEM;
>>>> +
>>>> +	for_each_property_of_node(np, prop) {
>>>> +		if (strncmp(prop->name, REBOOT_PREFIX, len))
>>>> +			continue;
>>>> +
>>>> +		num = of_property_read_variable_u32_array(np, prop->name, magic,
>>>> +							  1, ARRAY_SIZE(magic));
>>>> +		if (num < 0) {
>>>> +			pr_warn("Failed to parse vendor reboot mode %s\n",
>>>> +				param->mode);
>>>> +			kfree_const(param->mode);
>>>> +			continue;
>>>> +		}
>>>> +
>>>> +		param->mode = kstrdup_const(prop->name + len, GFP_KERNEL);
>>>> +		if (!param->mode)
>>>> +			continue;
>>>> +
>>>> +		/* Force reset type to be in vendor space */
>>>> +		param->reset_type = PSCI_1_1_RESET_TYPE_VENDOR_START | magic[0];
>>>> +		param->cookie = num > 1 ? magic[1] : 0;
>>>> +		param++;
>>>> +		num_psci_reset_params++;
>>>> +	}
>>>> +
>>>> +	return 0;
>>>> +}
>>>> +arch_initcall(psci_init_system_reset2_modes);
>>>> +
>>>>  int __init psci_dt_init(void)
>>>>  {
>>>>  	struct device_node *np;
>>>>
>>>> -- 
>>>> 2.34.1
>>>>