Re: [PATCH] cpufreq: CPPC: Set CPPC Enable register in cpu_init

[Christian Loehle <christian.loehle@arm.com>]

On 11/5/26 12:32, Sumit Gupta wrote:
> As per ACPI 6.x s8.4.6.1.4 (CPPC Enable register):
>   "If supported by the platform, OSPM writes a one to this register
>    to enable CPPC on this processor. If not implemented, OSPM assumes
>    the platform always has CPPC enabled."
> 
> The driver never writes this register. Call cppc_set_enable() at the
> start of cppc_cpufreq_cpu_init(). Errors are logged but not fatal, as
> the register is optional.
> 
> Fixes: 5477fb3bd1e8 ("ACPI / CPPC: Add a CPUFreq driver for use with CPPC")
> Cc: stable@vger.kernel.org # v6.16+
> Signed-off-by: Sumit Gupta <sumitg@nvidia.com>

So apparently Nvidia GB10 has and needs this
(I guess it doesn't hurt putting this into the
commit message)
amd-pstate sets it so all good.

> ---
> Sending this separately from the autonomous mode boot parameter series
> where it was patch 1/2, as it is an independent bug fix.
> v5 of the series:
>  https://lore.kernel.org/lkml/20260623080652.3353386-1-sumitg@nvidia.com/
> 
>  drivers/cpufreq/cppc_cpufreq.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> index 957fa7775260..b8f4399b6e0b 100644
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c
> @@ -679,6 +679,14 @@ static int cppc_cpufreq_cpu_init(struct cpufreq_policy *policy)
>  	caps = &cpu_data->perf_caps;
>  	policy->driver_data = cpu_data;
>  
> +	/*
> +	 * Enable CPPC. The Enable register is optional - some platforms
> +	 * may not support it.
> +	 */
> +	ret = cppc_set_enable(cpu, true);
> +	if (ret && ret != -EOPNOTSUPP)
> +		pr_warn("Failed to enable CPPC for CPU%u (%d)\n", cpu, ret);
> +

Any reason to not fail here?

>  	/*
>  	 * Set min to lowest nonlinear perf to avoid any efficiency penalty (see
>  	 * Section 8.4.7.1.1.5 of ACPI 6.1 spec)


... also I guess we need to set_enable(false) on
other failures?

It should be okay to do this only for policy->cpu AFAICS
but I guess we do need to also reenable on resume.