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

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Message-ID: <aJwiXKWXik8BmpL8@sunil-laptop>
Date: Wed, 13 Aug 2025 10:57:56 +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>, juwenlong@...edance.com
Subject: Re: [External] Re: [PATCH] ACPI: RISC-V: CPPC: Add CSR_CYCLE for
 CPPC FFH

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@...tanamicro.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().

Thanks,
Sunil