Re: [PATCH v3 1/3] ACPI: CPPC: Reject desired_perf reads on ACPI 6.6+

[Rafael J. Wysocki (Intel)]

On Mon, Aug 3, 2026 at 12:03 PM Christian Loehle
<christian.loehle@xxxxxxx> 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. When checking
> the FADT minor revision, mask off its upper errata-generation bits and
> compare only the specification minor version.

I'm wondering if the _CPC revision can be used for this instead of the
spec revision?

The _CPC revision changed from 3 to 4 between ACPI 6.5 and ACPI 6.6,
so doing that should be straightforward.

> The FADT revision may not accurately describe the semantics implemented by
> _CPC. If a nominally pre-6.6 platform implements Desired Performance as
> write-only, a read may return zero and make cppc_cpufreq_get_rate() report
> 0 kHz. Treat a zero read as unusable and fall back to the cached OSPM
> request, just as for a failed read.
>
> Fixes: c47195631960 ("cppc_cpufreq: Use desired perf if feedback ctrs are 0 or unchanged")
> Cc: stable@xxxxxxxxxxxxxxx
> Suggested-by: Sumit Gupta <sumitg@xxxxxxxxxx>
> Signed-off-by: Christian Loehle <christian.loehle@xxxxxxx>
> ---
>  drivers/acpi/cppc_acpi.c       | 15 ++++++++++++++-
>  drivers/cpufreq/cppc_cpufreq.c |  2 +-
>  2 files changed, 15 insertions(+), 2 deletions(-)
>
> 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);
> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> index 6fe0e972952a..80893844353c 100644
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c
> @@ -836,7 +836,7 @@ static unsigned int cppc_cpufreq_get_rate(unsigned int cpu)
>          * value first as some platforms may update the actual delivered perf
>          * there; if failed, resort to the cached desired perf.
>          */
> -       if (cppc_get_desired_perf(cpu, &delivered_perf))
> +       if (cppc_get_desired_perf(cpu, &delivered_perf) || !delivered_perf)
>                 delivered_perf = cpu_data->perf_ctrls.desired_perf;
>
>         return cppc_perf_to_khz(&cpu_data->perf_caps, delivered_perf);
> --
> 2.34.1
>