linux-kernel - Re: [PATCH v2 2/2] RISC-V: Add common csr_read_num() and csr_write

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Message-ID: <CAK9=C2XmDGOz9_euC4vtchOxr+8f+m+9zZYVewCc2s7GZhd4Pw@mail.gmail.com>
Date: Tue, 19 Aug 2025 09:00:03 +0530
From: Anup Patel <apatel@...tanamicro.com>
To: Yao Zi <ziyao@...root.org>
Cc: Sunil V L <sunilvl@...tanamicro.com>, "Rafael J . Wysocki" <rafael@...nel.org>, 
	Palmer Dabbelt <palmer@...belt.com>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Alexandre Ghiti <alex@...ti.fr>, Len Brown <lenb@...nel.org>, Atish Patra <atish.patra@...ux.dev>, 
	Andrew Jones <ajones@...tanamicro.com>, Anup Patel <anup@...infault.org>, 
	Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>, linux-acpi@...r.kernel.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] RISC-V: Add common csr_read_num() and
 csr_write_num() functions

On Tue, Aug 19, 2025 at 8:56 AM Yao Zi <ziyao@...root.org> wrote:
>
> On Mon, Aug 18, 2025 at 08:06:00PM +0530, Anup Patel wrote:
> > In RISC-V, there is no CSR read/write instruction which takes CSR
> > number via register so add common csr_read_num() and csr_write_num()
> > functions which allow accessing certain CSRs by passing CSR number
> > as parameter. These common functions will be first used by the
> > ACPI CPPC driver and RISC-V PMU driver.
> >
> > Signed-off-by: Anup Patel <apatel@...tanamicro.com>
> > Reviewed-by: Sunil V L <sunilvl@...tanamicro.com>
> > ---
> >  arch/riscv/include/asm/csr.h |   3 +
> >  arch/riscv/kernel/Makefile   |   1 +
> >  arch/riscv/kernel/csr.c      | 165 +++++++++++++++++++++++++++++++++++
> >  drivers/acpi/riscv/cppc.c    |  17 ++--
> >  drivers/perf/riscv_pmu.c     |  54 ++----------
> >  5 files changed, 184 insertions(+), 56 deletions(-)
> >  create mode 100644 arch/riscv/kernel/csr.c
> >
> > diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
> > index 6fed42e37705..1540626b3540 100644
> > --- a/arch/riscv/include/asm/csr.h
> > +++ b/arch/riscv/include/asm/csr.h
> > @@ -575,6 +575,9 @@
> >                             : "memory");                      \
> >  })
> >
> > +extern unsigned long csr_read_num(unsigned long csr_num, int *out_err);
> > +extern void csr_write_num(unsigned long csr_num, unsigned long val, int *out_err);
>
> I think it's more consistent to directly return the error code, and for
> csr_read_num, we could pass out the read value by a pointer. e.g.
>
>         int csr_read_num(unsigned long csr_num, unsigned long *val);
>         int csr_write_num(unsigned long csr_num, unsigned long val);
>
> This allows the caller to eliminate a variable for temporarily storing
> the error code if they use it just after the invokation, and fits the
> common convention of Linux better.

Drew had similar comments so see my response in the previous
patch revision. (Refer, https://www.spinics.net/lists/kernel/msg5808113.html)

Regards,
Anup