linux-kernel - Re: [PATCH v2 1/1] ACPI: CPPC: Disable FIE if registers in PCC regions

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Date:   Fri, 29 Jul 2022 13:59:43 +0100
From:   Punit Agrawal <punit.agrawal@...edance.com>
To:     Jeremy Linton <jeremy.linton@....com>
Cc:     linux-pm@...r.kernel.org, rafael@...nel.org, lenb@...nel.org,
        viresh.kumar@...aro.org, robert.moore@...el.com, devel@...ica.org,
        linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] ACPI: CPPC: Disable FIE if registers in PCC regions

Hi Jeremy,

One comment / query below.

Jeremy Linton <jeremy.linton@....com> writes:

> 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. Furthermore lets also
> enable a module parameter which can also disable it at boot or module
> reload.
>
> Signed-off-by: Jeremy Linton <jeremy.linton@....com>
> ---
>  drivers/acpi/cppc_acpi.c       | 41 ++++++++++++++++++++++++++++++++++
>  drivers/cpufreq/cppc_cpufreq.c | 19 ++++++++++++----
>  include/acpi/cppc_acpi.h       |  5 +++++
>  3 files changed, 61 insertions(+), 4 deletions(-)
>

[...]

> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> index 24eaf0ec344d..ed607e27d6bb 100644
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c

[...]

> @@ -229,7 +233,12 @@ static void __init cppc_freq_invariance_init(void)
>  	};
>  	int ret;
>  
> -	if (cppc_cpufreq_driver.get == hisi_cppc_cpufreq_get_rate)
> +	if (cppc_perf_ctrs_in_pcc()) {
> +		pr_debug("FIE not enabled on systems with registers in PCC\n");

The message should probably be promoted to a pr_info() and exposed as
part of the kernel logs. It is a change in the default behaviour we've
had until now. The message will provide some hint about why it was
disabled.

Thoughts?

[...]