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Date:   Fri, 23 Sep 2022 17:31:08 +0100
From:   Ionela Voinescu <ionela.voinescu@arm.com>
To:     Jeremy Linton <jeremy.linton@arm.com>
Cc:     linux-acpi@vger.kernel.org, rafael@kernel.org, lenb@kernel.org,
        viresh.kumar@linaro.org, robert.moore@intel.com,
        punit.agrawal@bytedance.com, lukasz.luba@arm.com,
        pierre.gondois@arm.com, linux-kernel@vger.kernel.org,
        devel@acpica.org, linux-pm@vger.kernel.org
Subject: Re: [PATCH v5 1/1] ACPI: CPPC: Disable FIE if registers in PCC
 regions
Message-ID: <Yy3fTFVyAEx9R/qQ@arm.com>
References: <20220912203722.205185-1-jeremy.linton@arm.com>
 <20220912203722.205185-2-jeremy.linton@arm.com>
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Hi Jeremy,

On Monday 12 Sep 2022 at 15:37:22 (-0500), Jeremy Linton wrote:
> PCC regions utilize a mailbox to set/retrieve register values used by
> the CPPC code. This is fine as long as the operations are
> infrequent. With the FIE code enabled though the overhead can range
> from 2-11% of system CPU overhead (ex: as measured by top) on Arm
> based machines.
> 
> So, before enabling FIE assure none of the registers used by
> cppc_get_perf_ctrs() are in the PCC region. Finally, add a module
> parameter which can override the PCC region detection at boot or
> module reload.
> 
> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
> Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
>  drivers/acpi/cppc_acpi.c       | 42 ++++++++++++++++++++++++++++++++++
>  drivers/cpufreq/cppc_cpufreq.c | 25 ++++++++++++++++----
>  include/acpi/cppc_acpi.h       |  5 ++++
>  3 files changed, 68 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index 1e15a9f25ae9..55693e6f7153 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -1240,6 +1240,48 @@ int cppc_get_perf_caps(int cpunum, struct cppc_perf_caps *perf_caps)
>  }
>  EXPORT_SYMBOL_GPL(cppc_get_perf_caps);
>  
> +/**
> + * cppc_perf_ctrs_in_pcc - Check if any perf counters are in a PCC region.
> + *
> + * CPPC has flexibility about how CPU performance counters are accessed.
> + * One of the choices is PCC regions, which can have a high access latency. This
> + * routine allows callers of cppc_get_perf_ctrs() to know this ahead of time.
> + *
> + * Return: true if any of the counters are in PCC regions, false otherwise
> + */
> +bool cppc_perf_ctrs_in_pcc(void)
> +{
> +	int cpu;
> +
> +	for_each_present_cpu(cpu) {
> +		struct cpc_register_resource *ref_perf_reg;
> +		struct cpc_desc *cpc_desc;
> +
> +		cpc_desc = per_cpu(cpc_desc_ptr, cpu);
> +
> +		if (CPC_IN_PCC(&cpc_desc->cpc_regs[DELIVERED_CTR]) ||
> +		    CPC_IN_PCC(&cpc_desc->cpc_regs[REFERENCE_CTR]) ||
> +		    CPC_IN_PCC(&cpc_desc->cpc_regs[CTR_WRAP_TIME]))
> +			return true;
> +
> +
> +		ref_perf_reg = &cpc_desc->cpc_regs[REFERENCE_PERF];
> +
> +		/*
> +		 * If reference perf register is not supported then we should
> +		 * use the nominal perf value
> +		 */
> +		if (!CPC_SUPPORTED(ref_perf_reg))
> +			ref_perf_reg = &cpc_desc->cpc_regs[NOMINAL_PERF];
> +
> +		if (CPC_IN_PCC(ref_perf_reg))
> +			return true;
> +	}
> +
> +	return false;
> +}
> +EXPORT_SYMBOL_GPL(cppc_perf_ctrs_in_pcc);
> +
>  /**
>   * cppc_get_perf_ctrs - Read a CPU's performance feedback counters.
>   * @cpunum: CPU from which to read counters.
> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> index 24eaf0ec344d..9e2a48ac5830 100644
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c
> @@ -63,7 +63,15 @@ static struct cppc_workaround_oem_info wa_info[] = {
>  
>  static struct cpufreq_driver cppc_cpufreq_driver;
>  
> +static enum {
> +	FIE_UNSET = -1,
> +	FIE_ENABLED,
> +	FIE_DISABLED
> +} fie_disabled = FIE_UNSET;
> +
>  #ifdef CONFIG_ACPI_CPPC_CPUFREQ_FIE
> +module_param(fie_disabled, int, 0444);

Why 'int' and not 'bool' here?

IIUC, if you use 'bool' the user can pass any int/0/1/y/n/Y/N, which
will result in fie_disabled properly having either the value 0 or 1
(or default FIE_UNSET) if a parameter is not passed.

Then
'if (fie_disabled != FIE_ENABLED && fie_disabled != FIE_DISABLED)'
can become
'if (fie_disabled == FIE_UNSET)' or 'if (fie_disabled < 0)'.

I feel I'm missing something, otherwise you would have done this
already.

Otherwise FWIW, it looks good to me.
Reviewed-by: Ionela Voinescu <ionela.voinescu@arm.com>

Thanks,
Ionela.

> +MODULE_PARM_DESC(fie_disabled, "Disable Frequency Invariance Engine (FIE)");
>  
>  /* Frequency invariance support */
>  struct cppc_freq_invariance {
> @@ -158,7 +166,7 @@ static void cppc_cpufreq_cpu_fie_init(struct cpufreq_policy *policy)
>  	struct cppc_freq_invariance *cppc_fi;
>  	int cpu, ret;
>  
> -	if (cppc_cpufreq_driver.get == hisi_cppc_cpufreq_get_rate)
> +	if (fie_disabled)
>  		return;
>  
>  	for_each_cpu(cpu, policy->cpus) {
> @@ -199,7 +207,7 @@ static void cppc_cpufreq_cpu_fie_exit(struct cpufreq_policy *policy)
>  	struct cppc_freq_invariance *cppc_fi;
>  	int cpu;
>  
> -	if (cppc_cpufreq_driver.get == hisi_cppc_cpufreq_get_rate)
> +	if (fie_disabled)
>  		return;
>  
>  	/* policy->cpus will be empty here, use related_cpus instead */
> @@ -229,7 +237,15 @@ static void __init cppc_freq_invariance_init(void)
>  	};
>  	int ret;
>  
> -	if (cppc_cpufreq_driver.get == hisi_cppc_cpufreq_get_rate)
> +	if (fie_disabled != FIE_ENABLED && fie_disabled != FIE_DISABLED) {
> +		fie_disabled = FIE_ENABLED;
> +		if (cppc_perf_ctrs_in_pcc()) {
> +			pr_info("FIE not enabled on systems with registers in PCC\n");
> +			fie_disabled = FIE_DISABLED;
> +		}
> +	}
> +
> +	if (fie_disabled)
>  		return;
>  
>  	kworker_fie = kthread_create_worker(0, "cppc_fie");
> @@ -247,7 +263,7 @@ static void __init cppc_freq_invariance_init(void)
>  
>  static void cppc_freq_invariance_exit(void)
>  {
> -	if (cppc_cpufreq_driver.get == hisi_cppc_cpufreq_get_rate)
> +	if (fie_disabled)
>  		return;
>  
>  	kthread_destroy_worker(kworker_fie);
> @@ -936,6 +952,7 @@ static void cppc_check_hisi_workaround(void)
>  		    wa_info[i].oem_revision == tbl->oem_revision) {
>  			/* Overwrite the get() callback */
>  			cppc_cpufreq_driver.get = hisi_cppc_cpufreq_get_rate;
> +			fie_disabled = FIE_DISABLED;
>  			break;
>  		}
>  	}
> diff --git a/include/acpi/cppc_acpi.h b/include/acpi/cppc_acpi.h
> index f73d357ecdf5..c5614444031f 100644
> --- a/include/acpi/cppc_acpi.h
> +++ b/include/acpi/cppc_acpi.h
> @@ -140,6 +140,7 @@ extern int cppc_get_perf_ctrs(int cpu, struct cppc_perf_fb_ctrs *perf_fb_ctrs);
>  extern int cppc_set_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls);
>  extern int cppc_set_enable(int cpu, bool enable);
>  extern int cppc_get_perf_caps(int cpu, struct cppc_perf_caps *caps);
> +extern bool cppc_perf_ctrs_in_pcc(void);
>  extern bool acpi_cpc_valid(void);
>  extern bool cppc_allow_fast_switch(void);
>  extern int acpi_get_psd_map(unsigned int cpu, struct cppc_cpudata *cpu_data);
> @@ -173,6 +174,10 @@ static inline int cppc_get_perf_caps(int cpu, struct cppc_perf_caps *caps)
>  {
>  	return -ENOTSUPP;
>  }
> +static inline bool cppc_perf_ctrs_in_pcc(void)
> +{
> +	return false;
> +}
>  static inline bool acpi_cpc_valid(void)
>  {
>  	return false;
> -- 
> 2.37.1
>