Re: [PATCH v1 3/3] ACPI: CPPC: Read CPC FFH counters in a single IPI

[Rafael J. Wysocki]

On Thu, Feb 29, 2024 at 5:25 PM Vanshidhar Konda
<vanshikonda@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> The CPPC driver reads delivered and reference counters using cpc_read
> one at a time. This leads to inaccurate measurement of CPU frequency
> discussed in [1]. If the firmware indicates that both the registers are
> in the FFH interface the kernel can read the registers together in a
> single IPI. This has two benefits:
> 1. Reduces the number of IPIs needed to read the two registers
> 2. The two registers will be read in close proximity resulting in more
>    accurate CPU frequency measurement
>
> [1]: https://lore.kernel.org/all/20230328193846.8757-1-yang@xxxxxxxxxxxxxxxxxxxxxx/
>
> Signed-off-by: Vanshidhar Konda <vanshikonda@xxxxxxxxxxxxxxxxxxxxxx>
> ---
>  arch/arm64/kernel/topology.c | 37 ++++++++++++++++++++++++++++++++++++
>  drivers/acpi/cppc_acpi.c     | 32 +++++++++++++++++++++++++++----
>  include/acpi/cppc_acpi.h     | 13 +++++++++++++
>  3 files changed, 78 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index 8905eb0c681f..8207565f43ee 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -421,6 +421,43 @@ int cpc_read_ffh(int cpu, struct cpc_reg *reg, u64 *val)
>         return ret;
>  }
>
> +static void cpc_update_freq_counters(void *info)
> +{
> +       update_freq_counters_refs();
> +}
> +
> +int cpc_read_regs_ffh(int cpu, struct ffh_cpc_reg_values *ffh_regs)
> +{
> +       struct amu_counters *ctrs = per_cpu_ptr(&cpu_samples, cpu);
> +       int idx;
> +
> +       if (!cpc_ffh_supported() || !freq_counters_valid(cpu))
> +               return -EOPNOTSUPP;
> +
> +       if (WARN_ON_ONCE(irqs_disabled()))
> +               return -EPERM;
> +
> +       if (!idle_cpu(cpu))
> +               smp_call_function_single(cpu, cpc_update_freq_counters, NULL, 1);
> +
> +       for (idx = 0; idx < MAX_NUM_CPC_REGS_FFH; idx++) {
> +
> +               if (!ffh_regs->regs[idx].reg)
> +                       continue;
> +
> +               switch ((u64)(ffh_regs->regs[idx].reg->address)) {
> +               case 0x0:
> +                       ffh_regs->regs[idx].value = ctrs->core_cnt;
> +                       break;
> +               case 0x1:
> +                       ffh_regs->regs[idx].value = ctrs->const_cnt;
> +                       break;
> +               }
> +       }
> +
> +       return 0;
> +}
> +
>  int cpc_write_ffh(int cpunum, struct cpc_reg *reg, u64 val)
>  {
>         return -EOPNOTSUPP;
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index d155a86a8614..55ffb1915e4f 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -113,6 +113,10 @@ static DEFINE_PER_CPU(struct cpc_desc *, cpc_desc_ptr);
>                                 (cpc)->cpc_entry.reg.space_id ==        \
>                                 ACPI_ADR_SPACE_SYSTEM_IO)
>
> +#define CPC_IN_FFH(cpc) ((cpc)->type == ACPI_TYPE_BUFFER &&            \
> +                               (cpc)->cpc_entry.reg.space_id ==        \
> +                               ACPI_ADR_SPACE_FIXED_HARDWARE)
> +
>  /* Evaluates to True if reg is a NULL register descriptor */
>  #define IS_NULL_REG(reg) ((reg)->space_id ==  ACPI_ADR_SPACE_SYSTEM_MEMORY && \
>                                 (reg)->address == 0 &&                  \
> @@ -974,6 +978,11 @@ int __weak cpc_write_ffh(int cpunum, struct cpc_reg *reg, u64 val)
>         return -ENOTSUPP;
>  }
>
> +int __weak cpc_read_regs_ffh(int cpu, struct ffh_cpc_reg_values *regs)
> +{
> +       return -ENOTSUPP;
> +}

This might return a bool value.

Is there any case in which the caller would handle different error
codes differently?

> +
>  /*
>   * Since cpc_read and cpc_write are called while holding pcc_lock, it should be
>   * as fast as possible. We have already mapped the PCC subspace during init, so
> @@ -1317,7 +1326,7 @@ int cppc_get_perf_ctrs(int cpunum, struct cppc_perf_fb_ctrs *perf_fb_ctrs)
>         int pcc_ss_id = per_cpu(cpu_pcc_subspace_idx, cpunum);
>         struct cppc_pcc_data *pcc_ss_data = NULL;
>         u64 delivered, reference, ref_perf, ctr_wrap_time;
> -       int ret = 0, regs_in_pcc = 0;
> +       int ret = 0, regs_in_pcc = 0, regs_read_in_ffh = 0;

Please use bool as the type for boolean variables.

>
>         if (!cpc_desc) {
>                 pr_debug("No CPC descriptor for CPU:%d\n", cpunum);
> @@ -1353,8 +1362,23 @@ int cppc_get_perf_ctrs(int cpunum, struct cppc_perf_fb_ctrs *perf_fb_ctrs)
>                 }
>         }
>
> -       cpc_read(cpunum, delivered_reg, &delivered);
> -       cpc_read(cpunum, reference_reg, &reference);
> +       if (CPC_IN_FFH(delivered_reg) && CPC_IN_FFH(reference_reg)) {
> +               struct ffh_cpc_reg_values ffh_regs;
> +
> +               ffh_regs.regs[0].reg = &(delivered_reg->cpc_entry.reg);
> +               ffh_regs.regs[1].reg = &(reference_reg->cpc_entry.reg);
> +               ret = cpc_read_regs_ffh(cpunum, &ffh_regs);
> +               if (!ret) {

If cpc_read_regs_ffh() returned 'true' on success, the above could be written as

if (cpc_read_regs_ffh(cpunum, &ffh_regs)) {

> +                       delivered = ffh_regs.regs[0].value;
> +                       reference = ffh_regs.regs[1].value;
> +                       regs_read_in_ffh = 1;
> +               }
> +       }
> +
> +       if (!regs_read_in_ffh) {
> +               cpc_read(cpunum, delivered_reg, &delivered);
> +               cpc_read(cpunum, reference_reg, &reference);
> +       }
>         cpc_read(cpunum, ref_perf_reg, &ref_perf);
>
>         /*
> @@ -1366,7 +1390,7 @@ int cppc_get_perf_ctrs(int cpunum, struct cppc_perf_fb_ctrs *perf_fb_ctrs)
>         if (CPC_SUPPORTED(ctr_wrap_reg))
>                 cpc_read(cpunum, ctr_wrap_reg, &ctr_wrap_time);
>
> -       if (!delivered || !reference || !ref_perf) {
> +       if (!delivered || !reference || !ref_perf) {
>                 ret = -EFAULT;
>                 goto out_err;
>         }
> diff --git a/include/acpi/cppc_acpi.h b/include/acpi/cppc_acpi.h
> index 3a0995f8bce8..0da614a50edd 100644
> --- a/include/acpi/cppc_acpi.h
> +++ b/include/acpi/cppc_acpi.h
> @@ -137,6 +137,18 @@ struct cppc_cpudata {
>  };
>
>  #ifdef CONFIG_ACPI_CPPC_LIB
> +
> +#define MAX_NUM_CPC_REGS_FFH 2
> +
> +struct ffh_cpc_reg {
> +       struct cpc_reg *reg;
> +       u64 value;
> +};
> +
> +struct ffh_cpc_reg_values {
> +       struct ffh_cpc_reg regs[MAX_NUM_CPC_REGS_FFH];
> +};
> +
>  extern int cppc_get_desired_perf(int cpunum, u64 *desired_perf);
>  extern int cppc_get_nominal_perf(int cpunum, u64 *nominal_perf);
>  extern int cppc_get_perf_ctrs(int cpu, struct cppc_perf_fb_ctrs *perf_fb_ctrs);
> @@ -153,6 +165,7 @@ extern unsigned int cppc_get_transition_latency(int cpu);
>  extern bool cpc_ffh_supported(void);
>  extern bool cpc_supported_by_cpu(void);
>  extern int cpc_read_ffh(int cpunum, struct cpc_reg *reg, u64 *val);
> +extern int cpc_read_regs_ffh(int cpu, struct ffh_cpc_reg_values *regs);
>  extern int cpc_write_ffh(int cpunum, struct cpc_reg *reg, u64 val);
>  extern int cppc_get_epp_perf(int cpunum, u64 *epp_perf);
>  extern int cppc_set_epp_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls, bool enable);
> --