Re: [PATCH] cpufreq/cppc: fix perf_to_khz conversion exception

[Pierre Gondois <pierre.gondois@arm.com>]

Hello Liwei,

Thanks for the fix. I think there might be a similar issue
in cppc_khz_to_perf(),

Regards,
Pierre

On 4/30/24 12:12, liwei wrote:
> When the nominal_freq recorded by the kernel is equal to lowest_freq and
> the frequency reduction operation is triggered externally, there is a
> conversion logic error in cppc_perf_to_khz(), causing the kernel to be
> unable to feedback the true frequency.
> 
> Fix this by adding the branch processing logic when nominal_freq is equal
> to lowest_freq.
> 
> Fixes: ec1c7ad47664 ("cpufreq: CPPC: Fix performance/frequency conversion")
> Signed-off-by: liwei <liwei728@huawei.com>
> ---
>   drivers/acpi/cppc_acpi.c | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index a40b6f3946ef..92aac6974e0e 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -1869,9 +1869,15 @@ unsigned int cppc_perf_to_khz(struct cppc_perf_caps *caps, unsigned int perf)
>   	u64 mul, div;
>   
>   	if (caps->lowest_freq && caps->nominal_freq) {
> -		mul = caps->nominal_freq - caps->lowest_freq;
> +		/* Avoid the special case when nominal_freq is equal to lowest_freq */
> +		if (caps->lowest_freq == caps->nominal_freq) {
> +			mul = caps->nominal_freq;
> +			div = caps->nominal_perf;
> +		} else {
> +			mul = caps->nominal_freq - caps->lowest_freq;
> +			div = caps->nominal_perf - caps->lowest_perf;
> +		}
>   		mul *= KHZ_PER_MHZ;
> -		div = caps->nominal_perf - caps->lowest_perf;
>   		offset = caps->nominal_freq * KHZ_PER_MHZ -
>   			 div64_u64(caps->nominal_perf * mul, div);
>   	} else {