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

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Message-ID: <70db8fea-a695-476b-83b4-5cd5787dc7de@kernel.org>
Date: Mon, 28 Oct 2024 08:34:42 -0500
From: Mario Limonciello <superm1@...nel.org>
To: Dhananjay Ugwekar <Dhananjay.Ugwekar@....com>,
 "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

On 10/27/2024 23:27, Dhananjay Ugwekar wrote:
> 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?

Makes sense, I'll send out a v2 like this, thanks for the suggestion.

> 
> 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);