linux-kernel - Re: [PATCH v2 2/8] cpuidle: psci: Transition to the faux device interface

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Message-ID: <cf4e70e4-9fe5-4697-8744-8c12c41b5ff9@nvidia.com>
Date: Thu, 1 May 2025 14:01:19 +0100
From: Jon Hunter <jonathanh@...dia.com>
To: Sudeep Holla <sudeep.holla@....com>, linux-kernel@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Lorenzo Pieralisi <lpieralisi@...nel.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Daniel Lezcano <daniel.lezcano@...aro.org>, linux-pm@...r.kernel.org,
 "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH v2 2/8] cpuidle: psci: Transition to the faux device
 interface

Hi Sudeep,

On 18/03/2025 17:01, Sudeep Holla wrote:
> The PSCI cpuidle driver does not require the creation of a platform
> device. Originally, this approach was chosen for simplicity when the
> driver was first implemented.
> 
> With the introduction of the lightweight faux device interface, we now
> have a more appropriate alternative. Migrate the driver to utilize the
> faux bus, given that the platform device it previously created was not
> a real one anyway. This will simplify the code, reducing its footprint
> while maintaining functionality.
> 
> Cc: Lorenzo Pieralisi <lpieralisi@...nel.org>
> Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> Cc: linux-pm@...r.kernel.org
> Signed-off-by: Sudeep Holla <sudeep.holla@....com>
> ---
>   drivers/cpuidle/cpuidle-psci.c | 32 ++++----------------------------
>   1 file changed, 4 insertions(+), 28 deletions(-)
> 
> diff --git a/drivers/cpuidle/cpuidle-psci.c b/drivers/cpuidle/cpuidle-psci.c
> index 2562dc001fc1de69732ef28f383d2809262a3d96..5d4d6daed36d8540ba2ce3dc54a3180731b03d22 100644
> --- a/drivers/cpuidle/cpuidle-psci.c
> +++ b/drivers/cpuidle/cpuidle-psci.c
> @@ -16,7 +16,7 @@
>   #include <linux/kernel.h>
>   #include <linux/module.h>
>   #include <linux/of.h>
> -#include <linux/platform_device.h>
> +#include <linux/device/faux.h>
>   #include <linux/psci.h>
>   #include <linux/pm_domain.h>
>   #include <linux/pm_runtime.h>
> @@ -404,14 +404,14 @@ static int psci_idle_init_cpu(struct device *dev, int cpu)
>    * to register cpuidle driver then rollback to cancel all CPUs
>    * registration.
>    */
> -static int psci_cpuidle_probe(struct platform_device *pdev)
> +static int psci_cpuidle_probe(struct faux_device *fdev)
>   {
>   	int cpu, ret;
>   	struct cpuidle_driver *drv;
>   	struct cpuidle_device *dev;
>   
>   	for_each_possible_cpu(cpu) {
> -		ret = psci_idle_init_cpu(&pdev->dev, cpu);
> +		ret = psci_idle_init_cpu(&fdev->dev, cpu);
>   		if (ret)
>   			goto out_fail;
>   	}
> @@ -431,28 +431,4 @@ static int psci_cpuidle_probe(struct platform_device *pdev)
>   	return ret;
>   }
>   
> -static struct platform_driver psci_cpuidle_driver = {
> -	.probe = psci_cpuidle_probe,
> -	.driver = {
> -		.name = "psci-cpuidle",
> -	},
> -};
> -
> -static int __init psci_idle_init(void)
> -{
> -	struct platform_device *pdev;
> -	int ret;
> -
> -	ret = platform_driver_register(&psci_cpuidle_driver);
> -	if (ret)
> -		return ret;
> -
> -	pdev = platform_device_register_simple("psci-cpuidle", -1, NULL, 0);
> -	if (IS_ERR(pdev)) {
> -		platform_driver_unregister(&psci_cpuidle_driver);
> -		return PTR_ERR(pdev);
> -	}
> -
> -	return 0;
> -}
> -device_initcall(psci_idle_init);
> +module_faux_driver(psci_cpuidle, psci_cpuidle_probe, NULL, true);
> 


I have noticed the following error messages on some of our Tegra devices ...

  ERR KERN faux psci-cpuidle: probe did not succeed, tearing down the device
  ERR KERN CPUidle PSCI: Failed to create psci-cpuidle device

I had a quick look at this and this occurs because of the following code 
in the probe cpuidle-psci driver ...

         /*
          * If no DT idle states are detected (ret == 0) let the driver
          * initialization fail accordingly since there is no reason to
          * initialize the idle driver if only wfi is supported, the
          * default archictectural back-end already executes wfi
          * on idle entry.
          */
         ret = dt_init_idle_driver(drv, psci_idle_state_match, 1);
         if (ret <= 0)
                 return ret ? : -ENODEV;


So although it could be argued that the error message is valid, I am not 
sure if there is anything that mandates that we need to have the 
idle-states present.

We are always checking for new kernel errors and so if something new 
occurs, I am trying to figure out what is the correct way to fix. For 
this case I am not sure what is best.

Thanks
Jon

-- 
nvpublic