Re: [PATCH] ACPI: RISC-V: CPPC: Implement cpc_read_ffh_fb_ctrs()

[Sunil V L]

On Wed, Sep 2, 2026 at 2:18 PM Yufan Dou <douyufan@xxxxxxxxxxxxx> wrote:
>
> On RISC-V, each cpc_read_ffh() for a CSR-type FFH register sends a
> separate IPI to the target hart via smp_call_function_single().
> cppc_get_perf_ctrs() therefore samples the delivered and reference
> counters in two separate IPIs, at two different instants. The skew
> between the two sampling points distorts the delivered/reference
> ratio and thus the frequency reported by cpufreq.
>
> Implement the cpc_read_ffh_fb_ctrs() hook to read both CSR counters
> back-to-back in a single IPI, which narrows the sampling window to a
> few instructions and halves the number of cross-CPU calls. This
> improves the accuracy of the reported frequency in the same way as
> the arm64 implementation of the hook, which reads both AMU counters
> in a single counters_read_on_cpu() call.
>
> SBI-type FFH registers still fall back to individual reads, since
> pairing them would not reduce the number of SBI calls on the target
> hart.
>
> Testing was performed while CPU1 was kept busy with:
>
>   # stress-ng --cpu 1 --taskset 1
>
> On a CPU with cpuinfo_max_freq of 3000000 kHz:
>
> Before:
>
>   Maximum observed cpuinfo_cur_freq: 3201369 kHz
>   Maximum observed error:            +201369 kHz (+6.71%)
>
> After:
>
>   Maximum observed cpuinfo_cur_freq: 3009646 kHz
>   Maximum observed error:              +9646 kHz (+0.32%)
>
> The maximum observed error is reduced by 95.2%.
>
> Signed-off-by: Yufan Dou <douyufan@xxxxxxxxxxxxx>
> ---
>  drivers/acpi/riscv/cppc.c | 47 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 47 insertions(+)
>
> diff --git a/drivers/acpi/riscv/cppc.c b/drivers/acpi/riscv/cppc.c
> index 42c1a9052470..2ca98fa69e9a 100644
> --- a/drivers/acpi/riscv/cppc.c
> +++ b/drivers/acpi/riscv/cppc.c
> @@ -85,6 +85,20 @@ static void cppc_ffh_csr_write(void *write_data)
>         data->ret.error = -EINVAL;
>  }
>
> +struct sbi_cppc_fb_ctrs_data {
> +       struct sbi_cppc_data first;
> +       struct sbi_cppc_data second;
> +};
> +
> +static void cppc_ffh_csr_read_fb_ctrs(void *read_data)
> +{
> +       struct sbi_cppc_fb_ctrs_data *data = read_data;
> +
> +       cppc_ffh_csr_read(&data->first);
> +       if (!data->first.ret.error)
> +               cppc_ffh_csr_read(&data->second);
> +}
> +
>  /*
>   * Refer to drivers/acpi/cppc_acpi.c for the description of the functions
>   * below.
> @@ -125,6 +139,39 @@ int cpc_read_ffh(int cpu, struct cpc_reg *reg, u64 *val)
>         return -EINVAL;
>  }
>
> +int cpc_read_ffh_fb_ctrs(int cpu, struct cpc_reg *reg1, u64 *val1,
> +                        struct cpc_reg *reg2, u64 *val2)
> +{
> +       struct sbi_cppc_fb_ctrs_data data;
> +       int ret;
> +
> +       if (WARN_ON_ONCE(irqs_disabled()))
> +               return -EPERM;
> +
> +       /* Only CSR counters can be paired within a single IPI. */
> +       if (FFH_CPPC_TYPE(reg1->address) != FFH_CPPC_CSR ||
> +           FFH_CPPC_TYPE(reg2->address) != FFH_CPPC_CSR)
> +               return -EOPNOTSUPP;
> +
>
 While I agree we can not reduce the SBI calls, can we reduce the IPI at least?

> +       data.first.reg = FFH_CPPC_CSR_NUM(reg1->address);
> +       data.second.reg = FFH_CPPC_CSR_NUM(reg2->address);
> +
> +       ret = smp_call_function_single(cpu, cppc_ffh_csr_read_fb_ctrs,
> +                                      &data, 1);
> +       if (ret)
> +               return ret;
> +
> +       if (data.first.ret.error)
> +               return data.first.ret.error;
> +       if (data.second.ret.error)
> +               return data.second.ret.error;
> +
> +       *val1 = data.first.ret.value;
> +       *val2 = data.second.ret.value;
> +
> +       return 0;
> +}
> +
>  int cpc_write_ffh(int cpu, struct cpc_reg *reg, u64 val)
>  {
>         struct sbi_cppc_data data;
>
> base-commit: 89a312991dc6e638a36adc43ccb91dbc25504c04
> --
> 2.34.1
>

Otherwise, LGTM. Thanks!

Reviewed-by: Sunil V L <sunilvl@xxxxxxxxxxxxxxxx>