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

Sunil V L
2025-08-13T12:10:08Z

On Wed, Aug 13, 2025 at 12:06:11AM -0700, 鞠文龙 wrote:
> Hi Sunil，
>
> > From: "Sunil V L"<sunilvl@ventanamicro.com>
> > Date:  Wed, Aug 13, 2025, 13:28
> > Subject:  Re: [External] Re: [PATCH] ACPI: RISC-V: CPPC: Add CSR_CYCLE for CPPC FFH
> > To: "yunhui cui"<cuiyunhui@bytedance.com>
> > Cc: <rafael@kernel.org>, <lenb@kernel.org>, <paul.walmsley@sifive.com>, <palmer@dabbelt.com>, <aou@eecs.berkeley.edu>, <alex@ghiti.fr>, <linux-acpi@vger.kernel.org>, <linux-riscv@lists.infradead.org>, <linux-kernel@vger.kernel.org>, "Anup Patel"<apatel@ventanamicro.com>, "Rahul Pathak"<rpathak@ventanamicro.com>, <juwenlong@bytedance.com>
> > 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().
>        As-per ACPI Spec，Both Reference performance counter register
> and Delivered Performance Counter are affected by CPU
> state。From ACPI Spec，“The Reference Performance Counter Register
> counts at a fixed rate any time the processor is active.”
>
> “The Delivered Performance Counter Register increments any time the
> processor is active, at a rate proportional to the current performance
> level, taking into account changes to Desired Performance”
> “ Processor power states include are designated C0, C1, C2, C3, . . .
> Cn. The C0 power state is an active power state where the CPU executes
> instructions. The C1 through Cn power states are processor sleeping
> states where the processor consumes less power and dissipates less
> heat than leaving the processor in the C0 state”. So the time csr can
> not meet this requirements.we need another csr, but not availiable for
> now.Either implement it as vendor-specific or create a community
> extension for all？
>
It is upto the interpretation. I am not sure what is "active" or
"etc" in the below statement.

"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."

Second, I don't see an issue if both reference and delivered counters
increment irrespective of idle state because ultimately the ratio
delta(delivered)/delta(reference) matters which will be same in either
case.

Thanks,
Sunil