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

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Message-ID: <aJtKZhvNX0p3obFw@sunil-laptop>
Date: Tue, 12 Aug 2025 19:36:30 +0530
From: Sunil V L <sunilvl@...tanamicro.com>
To: yunhui cui <cuiyunhui@...edance.com>
Cc: 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, Anup Patel <apatel@...tanamicro.com>,
	Rahul Pathak <rpathak@...tanamicro.com>
Subject: Re: [External] Re: [PATCH] ACPI: RISC-V: CPPC: Add CSR_CYCLE for
 CPPC FFH

On Tue, Aug 12, 2025 at 09:32:10PM +0800, yunhui cui wrote:
> Hi Sunil,
> 
> 
> On Tue, Aug 12, 2025 at 9:15 PM Sunil V L <sunilvl@...tanamicro.com> wrote:
> >
> > On Tue, Aug 12, 2025 at 07:25:44PM +0800, yunhui cui wrote:
> > > 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.
> > >
> > 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.
>
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.
 
> 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.

Thanks,
Sunil