Re: [PATCH] ACPI: CPPC: Skip writes to unsupported performance controls

[Christian Loehle <christian.loehle@arm.com>]

On 7/24/26 11:57, Sudeep Holla wrote:
> On Fri, Jul 24, 2026 at 11:40:42AM +0100, Christian Loehle wrote:
>> MIN_PERF and MAX_PERF are optional CPPC controls. DESIRED_PERF is also
>> optional with CPPC2 when autonomous selection is supported.
>>
>> The cppc-cpufreq target callbacks populate both limits for every request
>> without checking whether the controls are implemented. cppc_set_perf()
>> consequently passes NULL register descriptors to cpc_write(). The writes
>> fail width validation and their return values are ignored, so the failed
>> access paths are repeated on every target request. An autonomous-only
>> platform can take the same path for DESIRED_PERF.
>>
>> Check that each performance control is supported before calling
>> cpc_write().
>>
>> Fixes: ea3db45ae476 ("cpufreq: cppc: Update MIN_PERF/MAX_PERF in target callbacks")
>> Reviewed-by: Sumit Gupta <sumitg@nvidia.com>
>> Signed-off-by: Christian Loehle <christian.loehle@arm.com>
>> ---
>> v2: Also added desired_perf check (Sumit)
>>
>>  drivers/acpi/cppc_acpi.c | 7 ++++---
>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
>> index 1d3a94100491..53d09ca98f06 100644
>> --- a/drivers/acpi/cppc_acpi.c
>> +++ b/drivers/acpi/cppc_acpi.c
>> @@ -1963,16 +1963,17 @@ int cppc_set_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls)
>>  		cpc_desc->write_cmd_status = 0;
>>  	}
>>  
>> -	cpc_write(cpu, desired_reg, perf_ctrls->desired_perf);
>> +	if (CPC_SUPPORTED(desired_reg))
>> +		cpc_write(cpu, desired_reg, perf_ctrls->desired_perf);
>>  
> 
> In general, can't be set perf_ctrls->desired_perf if and only if
> CPC_SUPPORTED(desired_reg) ?
> 
> I see cppc_get_perf() does that but I assume you are catering for some
> other access that may happen before ?
> 

cppc-cpufreq sets desired_perf regardless, I guess the check could also move up
there, but down here is fine as well IMO.