Re: [PATCHv2 1/3] ACPI: CPPC: Reject desired_perf reads on ACPI 6.6+

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

On 7/31/26 11:45, Zhongqiu Han wrote:
> On 7/29/2026 6:02 PM, Christian Loehle wrote:
>> When CPPC feedback counters cannot provide a usable sample, cppc-cpufreq
>> calls cppc_get_desired_perf() because some platforms repurpose Desired
>> Performance to report actual delivered performance.
>>
>> The fallback was added for platforms on which Desired Performance reflects
>> delivered performance. ACPI 6.6 defines the register as write-only, so
>> invoking that workaround on an ACPI 6.6 or later platform would require an
>> invalid register read.
>>
>> Make cppc_get_desired_perf() return -EOPNOTSUPP in that case. Its caller
>> already handles an error by using the cached desired-performance value.
>> When checking the FADT minor revision, mask off its upper errata-generation
>> bits and compare only the specification minor version.
>>
>> Fixes: c47195631960 ("cppc_cpufreq: Use desired perf if feedback ctrs are 0 or unchanged")
> 
> Hi Christian,
> Please feel free to correct me if there is any misunderstanding.
> 
> ACPI 6.6 was released on 05/13/2025:
> https://uefi.org/sites/default/files/resources/ACPI_Spec_6.6.pdf
> 
> And the fixes tag commit c47195631960 was committed on 09/29/2024.
> 
> Would this be considered an adaptation rather than a regression fix?


That is all correct, but the fixes tag is supposed to mean "this patch makes
the commit in question work" (and therefore should be backported to all kernels
containing this), which is then correct, as older kernels may still boot on
newer (6.6) ACPI systems.
It doesn't mean that the mentioned commit is 'bad' or 'wrong'.


> 
> 
> With the Fixes tag confirmed/fixed:
> Reviewed-by: Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>
Thanks, I'll leave as-is and pick this up if you don't mind.

> 
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Christian Loehle <christian.loehle@arm.com>
>> ---
>>   drivers/acpi/cppc_acpi.c | 15 ++++++++++++++-
>>   1 file changed, 14 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
>> index 53d09ca98f06..6e5381f8de38 100644
>> --- a/drivers/acpi/cppc_acpi.c
>> +++ b/drivers/acpi/cppc_acpi.c
>> @@ -1316,15 +1316,28 @@ static int cppc_set_reg_val(int cpu, enum cppc_regs reg_idx, u64 val)
>>       return cpc_write(cpu, reg, val);
>>   }
>>   +static bool cppc_desired_perf_readable(void)
>> +{
>> +    u8 minor_revision = acpi_gbl_FADT.minor_revision & 0x0f;
>> +
>> +    return acpi_gbl_FADT.header.revision < 6 ||
>> +           (acpi_gbl_FADT.header.revision == 6 && minor_revision < 6);
>> +}
>> +
>>   /**
>>    * cppc_get_desired_perf - Get the desired performance register value.
>>    * @cpunum: CPU from which to get desired performance.
>>    * @desired_perf: Return address.
>>    *
>> - * Return: 0 for success, -EIO otherwise.
>> + * Return: 0 for success, -EOPNOTSUPP for ACPI 6.6 or later, and a negative
>> + * errno otherwise.
>>    */
>>   int cppc_get_desired_perf(int cpunum, u64 *desired_perf)
>>   {
>> +    /* ACPI 6.6 no longer specifies Desired Performance as readable. */
>> +    if (!cppc_desired_perf_readable())
>> +        return -EOPNOTSUPP;
>> +
>>       return cppc_get_reg_val(cpunum, DESIRED_PERF, desired_perf);
>>   }
>>   EXPORT_SYMBOL_GPL(cppc_get_desired_perf);
> 
>