Re: [PATCHv2 2/3] ACPI: CPPC: Skip desired_perf read in cppc_get_perf()

[Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>]

On 7/29/2026 6:02 PM, Christian Loehle wrote:
> ACPI 6.6 changed the Optional Attribute of Desired Performance from
> Read/Write to Write. cppc_get_perf() nevertheless reads the register when
> initializing performance controls, even though cppc-cpufreq overwrites the
> value before using it.
> 
> Use the readability check from cppc_get_desired_perf() and leave
> desired_perf zero instead of reading it on ACPI 6.6 or later. Also exclude
> the register from PCC read-command detection so it cannot trigger an
> otherwise unnecessary read command.
> 
> Fixes: 658fa7b1c47a ("ACPI: CPPC: Add cppc_get_perf() API to read performance controls")
> Cc: stable@vger.kernel.org
> Suggested-by: Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>
> Signed-off-by: Christian Loehle <christian.loehle@arm.com>

Looks good to me.

Reviewed-by: Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>

> ---
>   drivers/acpi/cppc_acpi.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index 6e5381f8de38..210988d57b71 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -1843,6 +1843,7 @@ int cppc_get_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls)
>   	u64 desired_perf = 0, min = 0, max = 0, energy_perf = 0, auto_sel = 0;
>   	int pcc_ss_id = per_cpu(cpu_pcc_subspace_idx, cpu);
>   	struct cppc_pcc_data *pcc_ss_data = NULL;
> +	bool read_desired_perf = cppc_desired_perf_readable();
>   	int ret = 0, regs_in_pcc = 0;
>   
>   	if (!cpc_desc) {
> @@ -1862,7 +1863,8 @@ int cppc_get_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls)
>   	auto_sel_reg = &cpc_desc->cpc_regs[AUTO_SEL_ENABLE];
>   
>   	/* Are any of the regs PCC ?*/
> -	if (CPC_IN_PCC(desired_perf_reg) || CPC_IN_PCC(min_perf_reg) ||
> +	if ((read_desired_perf && CPC_IN_PCC(desired_perf_reg)) ||
> +	    CPC_IN_PCC(min_perf_reg) ||
>   	    CPC_IN_PCC(max_perf_reg) || CPC_IN_PCC(energy_perf_reg) ||
>   	    CPC_IN_PCC(auto_sel_reg)) {
>   		if (pcc_ss_id < 0) {
> @@ -1894,7 +1896,7 @@ int cppc_get_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls)
>   	}
>   	perf_ctrls->min_perf = min;
>   
> -	if (CPC_SUPPORTED(desired_perf_reg)) {
> +	if (read_desired_perf && CPC_SUPPORTED(desired_perf_reg)) {
>   		ret = cpc_read(cpu, desired_perf_reg, &desired_perf);
>   		if (ret)
>   			goto out_err;


-- 
Thx and BRs,
Zhongqiu Han