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

Anup Patel
2025-08-13T11:12:07Z

On Wed, Aug 13, 2025 at 12:14 PM yunhui cui <cuiyunhui@bytedance.com> wrote:
>
> Hi Sunil,
>
> On Wed, Aug 13, 2025 at 1:28 PM Sunil V L <sunilvl@ventanamicro.com> wrote:
> >
> > Hi Yunhui,
> >
> > On Wed, Aug 13, 2025 at 11:23:39AM +0800, yunhui cui wrote:
> > > Hi Sunil,
> > >
> > > On Tue, Aug 12, 2025 at 10:06 PM Sunil V L <sunilvl@ventanamicro.com> wrote:
> > > >
> > [...]
> > > > > > >
> > > > > > > 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.
> > > > > > >
> > > > > > Hi Yunhui,
> > > > > >
> > > > > > Yes, but we anticipated that vendors might define their own custom CSRs.
> > > > > > So, we introduced FFH encoding to accommodate such cases.
> > > > > >
> > > > > > Thanks,
> > > > > > Sunil
> > > > >
> > > > > As mentioned earlier, it is best to directly read CSR_XXX (a reference
> > > > > clock that does not count during WFI) and CSR_CYCLE in S-mode, rather
> > > > > than trapping to SBI.
> > > > >
> > > > No. I meant direct CSR access itself not SBI. Please take a look at
> > > > Table 6 of RISC-V FFH spec.
> > > >
> > > > > drivers/acpi/riscv/cppc.c is a generic driver that is not specific to
> > > > > any vendor. Currently, the upstream code already uses CSR_TIME, and
> > > > > the logic of CSR_TIME is incorrect.
> > > > >
> > ACPI spec for "Reference Performance Register" says,
> >
> > "The Reference Performance Counter Register counts at a fixed rate any
> > time the processor is active. It is not affected by changes to Desired
> > Performance, processor throttling, etc."
> >
> > > > CSR_TIME is just an example. It is upto the vendor how _CPC objects are
> > > > encoded using FFH. The linux code doesn't mean one should use CSR_TIME
> > > > always.
> > >
> > > First, the example of CSR_TIME is incorrect. What is needed is a
> > > CSR_XXX (a reference clock that does not count during WFI).
> > >
> > > Second, you mentioned that each vendor can customize their own
> > > implementations. But should all vendors' CSR_XXX/YYY/... be added to
> > > drivers/acpi/riscv/cppc.c? Shouldn’t drivers/acpi/riscv/cppc.c fall
> > > under the scope defined by the RISC-V architecture?
> > >
> > No. One can implement similar to csr_read_num() in opensbi. We didn't
> > add it since there was no HW implementing such thing. What I am
> > saying is we have FFH encoding to support such case.
> >
> > > >
> > > > > It would be best to promote a specification to support CSR_XXX, just
> > > > > like what has been done for x86 and arm64. What do you think?
> > > > >
> > > > Wouldn't above work? For a standard extension, you may have to provide
> > > > more data with actual HW.
> > >
> > > This won’t work. May I ask how the current upstream code can calculate
> > > the actual CPU frequency using CSR_TIME without trapping to SBI?
> > > This is a theoretical logical issue. Why is data needed here?
> > >
> > As I mentioned above, one can implement a generic CSR read without
> > trapping to SBI.
> >
> > > Could you take a look at the "AMU events and event numbers" chapter in
> > > the ARM64 manual?
> > >
> > As-per ACPI spec reference performance counter is not affected by CPU
> > state. The RISC-V FFH encoding is sufficiently generic to support this
> > requirement, even if the standard CSR_TIME cannot be used. In such
> > cases, an alternative CSR can be encodeded, accessed via an OS-level
> > abstraction such as csr_read_num().
>
> So what you're saying is that we should submit a patch like this, right?
>
> diff --git a/drivers/acpi/riscv/cppc.c b/drivers/acpi/riscv/cppc.c
> index 440cf9fb91aab..953c259d46c69 100644
> --- a/drivers/acpi/riscv/cppc.c
> +++ b/drivers/acpi/riscv/cppc.c
> @@ -66,16 +66,8 @@ 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;
> - default:
> - data->ret.error = -EINVAL;
> - break;
> - }
> + data->ret.value = csr_read_num(data->reg);
> + data->ret.error = 0;
> }
>
> If that's the case, the robustness of the code cannot be guaranteed,
> because the range of CSRs from different vendors is unknown.

ACPI FFH is allows mapping to any CSR.

>
> Since each vendor will define their own CSRs, why not formalize them
> into a specification?

The _CPC objects in the ACPI table point to platform specific mechanisms
of accessing CPPC CSR so it can point to a vendor specific CSR.

Regards,
Anup