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

Vanshidhar Konda
2024-02-29T18:01:07Z

On Thu, Feb 29, 2024 at 06:32:59PM +0100, Rafael J. Wysocki wrote:
>On Thu, Feb 29, 2024 at 5:25 PM Vanshidhar Konda
><vanshikonda@os.amperecomputing.com> 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@os.amperecomputing.com/
>>
>> Signed-off-by: Vanshidhar Konda <vanshikonda@os.amperecomputing.com>
>> ---
>> 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?
>
I kept this similar to cpc_read_ffh(). I didn't see any usage of the error
codes. The return value of cpc_read_ffh() is returned only from cpc_read(),
but I didn't see anyone consuming the return value of cpc_read().
Additionally, checkpatch complains about using -ENOTSUPP and suggests
replacing it with -EOPNOTSUPP. Does it make sense to update
cpc_read/write_ffh() as well to switch to return type bool?

>> +
>> /*
>> * 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.
>
Thanks for pointing that out. I'll do that for the next version.

Thanks,
Vanshi

>>
>> 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);
>> --