linux-kernel - Re: [PATCH] ACPI: RISC-V: CPPC: Add CSR

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Message-ID: <CAEEQ3w=XqoKmVu1kvc5XUbGbQJsHVkRx=T65tXvYEYo0HCTcnQ@mail.gmail.com>
Date: Tue, 12 Aug 2025 19:25:44 +0800
From: yunhui cui <cuiyunhui@...edance.com>
To: sunilvl@...tanamicro.com, rafael@...nel.org, lenb@...nel.org, 
	paul.walmsley@...ive.com, palmer@...belt.com, aou@...s.berkeley.edu, 
	alex@...ti.fr, linux-acpi@...r.kernel.org, linux-riscv@...ts.infradead.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ACPI: RISC-V: CPPC: Add CSR_CYCLE for CPPC FFH

Hi Sunil,

On Thu, May 15, 2025 at 5:44 PM Yunhui Cui <cuiyunhui@...edance.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@...edance.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