Re: [PATCH] ACPI: RISC-V: CPPC: Add CSR_CYCLE for CPPC FFH

yunhui cui
2025-08-12T11:25:56Z

Hi Sunil,

On Thu, May 15, 2025 at 5:44 PM Yunhui Cui <cuiyunhui@bytedance.com> wrote:
>
> Add the read of CSR_CYCLE to cppc_ffh_csr_read() to fix the
> warning message: "CPPC Cpufreq: cppc_scale_freq_wokrfn: failed
> to read perf counters".
>
> Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
> ---
> drivers/acpi/riscv/cppc.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/acpi/riscv/cppc.c b/drivers/acpi/riscv/cppc.c
> index 4cdff387deff6..c1acaeb18eac3 100644
> --- a/drivers/acpi/riscv/cppc.c
> +++ b/drivers/acpi/riscv/cppc.c
> @@ -69,11 +69,14 @@ static void cppc_ffh_csr_read(void *read_data)
> struct sbi_cppc_data *data = (struct sbi_cppc_data *)read_data;
>
> switch (data->reg) {
> - /* Support only TIME CSR for now */
> case CSR_TIME:
> data->ret.value = csr_read(CSR_TIME);
> data->ret.error = 0;
> break;
> + case CSR_CYCLE:
> + data->ret.value = csr_read(CSR_CYCLE);
> + data->ret.error = 0;
> + break;
> default:
> data->ret.error = -EINVAL;
> break;
> --
> 2.39.2
>

The purpose of cppc_ffh_csr_read() is to calculate the actual
frequency of the CPU, which is delta_CSR_CYCLE/delta_CSR_XXX.

CSR_XXX should be a reference clock and does not count during WFI
(Wait For Interrupt).

Similar solutions include: x86's aperf/mperf, and ARM64's AMU with
registers SYS_AMEVCNTR0_CORE_EL0/SYS_AMEVCNTR0_CONST_EL0.

However, we know that CSR_TIME in the current code does count during
WFI. So, is this design unreasonable?

Should we consider proposing an extension to support such a dedicated
counter (a reference clock that does not count during WFI)? This way,
the value can be obtained directly in S-mode without trapping to
M-mode, especially since reading this counter is very frequent.

Thanks,
Yunhui