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

Punit Agrawal
2022-07-29T12:59:52Z

Hi Jeremy,

One comment / query below.

Jeremy Linton <jeremy.linton@arm.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@arm.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? …