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

Anup Patel
2025-08-14T13:37:48Z

On Thu, Aug 14, 2025 at 11:49 AM yunhui cui <cuiyunhui@bytedance.com> wrote:
>
> Hi Anup,
>
> On Thu, Aug 14, 2025 at 1:48 PM Anup Patel <apatel@ventanamicro.com> wrote:
> >
> > On Thu, Aug 14, 2025 at 9:08 AM yunhui cui <cuiyunhui@bytedance.com> wrote:
> > >
> > > Hi Anup,
> > >
> > > On Wed, Aug 13, 2025 at 7:12 PM Anup Patel <apatel@ventanamicro.com> wrote:
> > > >
> > > > 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.
> > >
> > > Yes, FFH can map any CSR, and this is not the point of contention.
> > >
> > > If that's the case, the CSR_TIME used in the current kernel code is
> > > inappropriate. Some vendors may design a counter that does not count
> > > during WFI, making CSR_TIME irrelevant. Even if counting continues
> > > during WFI, are you planning to have one counter operate in S-mode
> > > while the other traps to M-mode?
> > >
> > > In that case, the code would need to be modified as proposed above. Do
> > > you agree?
> >
> > I disagree.
> >
> > Like Sunil already explained, if an implementation has reference counter
> > which does not count during WFI state then for such implementation the
> > delivered performance counter should also not increment during WFI
> > to maintain the relative delta of increments. This means if an implementation
> > uses TIME CSR as reference counter then for such implementation
> > the delivered performance counter should increment accordingly. Ultimately,
> > what matters is OS being able to correctly compute the performance level
> > using reference and delivered performance counters.
>
>
> For calculating the actual CPU frequency, both implementations are
> acceptable where either both counters continue counting during WFI or
> both stop counting.
> In the current code, how do you read the other counter?
> Shouldn't it be modified like this to support it? This way, all
> counters can be read directly in S-mode without trapping to M-mode:
> + data->ret.value = csr_read_num(data->reg);
> + data->ret.error = 0;

Yes, the current switch-case needs to replaced by common
csr_read_num() and csr_write_num() implemented in arch/riscv/

The RISC-V CSR space is limited so with it is straightforward
to implement csr_read_num() and csr_write_num() using
macros where each CSR access will turn-out to be roughly
3-4 instructions.

>
>
> >
> > >
> > > Without a specification defining these two counters, the above code
> > > would lack robustness.
> >
> > Can you elaborate the "robustness" part ?
> > Do you have data to back your claims ?
>
> Because there is no specification defining address access rules for
> data->reg across different vendors' implementations, you cannot write
> a validation logic like this:
> if (data->reg < CSR_CYCLE || data->reg > CSR_HPMCOUNTER31H) {
> ...
> return -EINVAL;
> }
> Reading data->reg directly lacks robustness, because data->reg may
> potentially hold an invalid value.
>

I don't think there is any specification gap here. Platforms can either use
SBI CPPC extension or time CSR or other implementation specific CSRs.
All this is abstracted away by RISC-V ACPI FFH.

If data->reg points to invalid / unimplemented CSR then csr_read_num()
and csr_write_num() will cause an illegal instruction trap which is expected
behaviour for a buggy ACPI table.

Regards,
Anup