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

[Mario Limonciello]

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@xxxxxxx>
> >
> > 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@xxxxxxxxx>
> > Closes: https://lore.kernel.org/linux-pm/cf9c146d-bacf-444e-92e2-15ebf513af96@xxxxxxxxx/#t
> > Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxx>
> > ---
> >   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);