linux-kernel - Re: [PATCH] cpufreq: amd-pstate: Register driver after static call update

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Message-ID: <145ac73d-000e-46c2-818b-d0d5deefd37a@amd.com>
Date: Mon, 28 Oct 2024 09:57:15 +0530
From: Dhananjay Ugwekar <Dhananjay.Ugwekar@....com>
To: Mario Limonciello <superm1@...nel.org>,
 "Gautham R . Shenoy" <gautham.shenoy@....com>,
 Perry Yuan <perry.yuan@....com>
Cc: "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
 <linux-kernel@...r.kernel.org>,
 "open list:CPU FREQUENCY SCALING FRAMEWORK" <linux-pm@...r.kernel.org>,
 Mario Limonciello <mario.limonciello@....com>,
 Klara Modin <klarasmodin@...il.com>
Subject: Re: [PATCH] cpufreq: amd-pstate: Register driver after static call
 update

Hello Mario,

On 10/27/2024 5:56 PM, Mario Limonciello wrote:
> From: Mario Limonciello <mario.limonciello@....com>
> 
> The driver must be registered specifically after the static call
> update.
> 
> Fixes: e238968a2087 ("cpufreq/amd-pstate: Remove the redundant amd_pstate_set_driver() call")
> Reported-and-tested-by: Klara Modin <klarasmodin@...il.com>
> Closes: https://lore.kernel.org/linux-pm/cf9c146d-bacf-444e-92e2-15ebf513af96@gmail.com/#t
> Signed-off-by: Mario Limonciello <mario.limonciello@....com>
> ---
>  drivers/cpufreq/amd-pstate.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
> index 206725219d8c9..2f0e29b05963d 100644
> --- a/drivers/cpufreq/amd-pstate.c
> +++ b/drivers/cpufreq/amd-pstate.c
> @@ -1857,12 +1857,6 @@ static int __init amd_pstate_init(void)
>  		return -ENODEV;
>  	}
>  
> -	ret = amd_pstate_register_driver(cppc_state);
> -	if (ret) {
> -		pr_err("failed to register with return %d\n", ret);
> -		return ret;
> -	}
> -
>  	/* capability check */
>  	if (cpu_feature_enabled(X86_FEATURE_CPPC)) {
>  		pr_debug("AMD CPPC MSR based functionality is supported\n");
1865                 if (cppc_state != AMD_PSTATE_ACTIVE)
1866                         current_pstate_driver->adjust_perf = amd_pstate_adjust_perf;

If we move the "amd_pstate_register_driver()" call to the below point, wont the above dereference (current_pstate_driver->) 
cause a null pointer error in MSR based systems with amd_pstate=passive/guided boot param. As we initialize 
"current_pstate_driver" in amd_pstate_register_driver().

We probably need to break up the if-else condition and handle the MSR based "if" part after the register call and "else" 
part before the "amd_pstate_register_driver()" call.

Something like,
		
	if (!cpu_feature_enabled(X86_FEATURE_CPPC)) {
                 pr_debug("AMD CPPC shared memory based functionality is supported\n");
                 static_call_update(amd_pstate_enable, shmem_enable);
                 static_call_update(amd_pstate_init_perf, shmem_init_perf);
                 static_call_update(amd_pstate_update_perf, shmem_update_perf);  	<---- static call updates done
    	}
	
	ret = amd_pstate_register_driver(cppc_state);					<---- Initializes "current_pstate_driver"
	if (ret) {
		pr_err("failed to register with return %d\n", ret);
		return ret;
	}
	
	if (cpu_feature_enabled(X86_FEATURE_CPPC)) {
		pr_debug("AMD CPPC MSR based functionality is supported\n");
                if (cppc_state != AMD_PSTATE_ACTIVE)
                         current_pstate_driver->adjust_perf = amd_pstate_adjust_perf;	<---- Now we dereference "current_pstate_driver"
	}
	

What do you think?

Thanks,
Dhananjay

> @@ -1881,6 +1875,12 @@ static int __init amd_pstate_init(void)
>  			return ret;
>  	}
>  
> +	ret = amd_pstate_register_driver(cppc_state);
> +	if (ret) {
> +		pr_err("failed to register with return %d\n", ret);
> +		return ret;
> +	}
> +
>  	dev_root = bus_get_dev_root(&cpu_subsys);
>  	if (dev_root) {
>  		ret = sysfs_create_group(&dev_root->kobj, &amd_pstate_global_attr_group);