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Message-ID: <89865f17-f405-445a-874d-9361fe247bfe@iscas.ac.cn>
Date: Sat, 16 Aug 2025 12:24:38 +0800
From: Vivian Wang <wangruikang@...as.ac.cn>
To: Anup Patel <apatel@...tanamicro.com>, Sunil V L
<sunilvl@...tanamicro.com>, "Rafael J . Wysocki" <rafael@...nel.org>
Cc: 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, Vivian Wang <wangruikang@...as.ac.cn>
Subject: Re: [PATCH 2/2] RISC-V: Add common csr_read_num() and csr_write_num()
functions
On 8/16/25 00:14, 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>
> ---
> arch/riscv/include/asm/csr.h | 3 +
> arch/riscv/kernel/Makefile | 1 +
> arch/riscv/kernel/csr.c | 177 +++++++++++++++++++++++++++++++++++
> drivers/acpi/riscv/cppc.c | 17 ++--
> drivers/perf/riscv_pmu.c | 43 +--------
> 5 files changed, 189 insertions(+), 52 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);
> +
> #endif /* __ASSEMBLY__ */
>
> #endif /* _ASM_RISCV_CSR_H */
> diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> index c7b542573407..0a75e20bde18 100644
> --- a/arch/riscv/kernel/Makefile
> +++ b/arch/riscv/kernel/Makefile
> @@ -50,6 +50,7 @@ obj-y += soc.o
> obj-$(CONFIG_RISCV_ALTERNATIVE) += alternative.o
> obj-y += cpu.o
> obj-y += cpufeature.o
> +obj-y += csr.o
> obj-y += entry.o
> obj-y += irq.o
> obj-y += process.o
> diff --git a/arch/riscv/kernel/csr.c b/arch/riscv/kernel/csr.c
> new file mode 100644
> index 000000000000..f7de45bb597c
> --- /dev/null
> +++ b/arch/riscv/kernel/csr.c
> @@ -0,0 +1,177 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (c) 2025 Ventana Micro Systems Inc.
> + */
> +
> +#define pr_fmt(fmt) "riscv: " fmt
> +#include <linux/err.h>
> +#include <linux/export.h>
> +#include <linux/printk.h>
> +#include <linux/types.h>
> +#include <asm/csr.h>
> +
> +#define CSR_CUSTOM0_U_RW_BASE 0x800
> +#define CSR_CUSTOM0_U_RW_COUNT 0x100
> +
> +#define CSR_CUSTOM1_U_RO_BASE 0xCC0
> +#define CSR_CUSTOM1_U_RO_COUNT 0x040
> +
> +#define CSR_CUSTOM2_S_RW_BASE 0x5C0
> +#define CSR_CUSTOM2_S_RW_COUNT 0x040
> +
> +#define CSR_CUSTOM3_S_RW_BASE 0x9C0
> +#define CSR_CUSTOM3_S_RW_COUNT 0x040
> +
> +#define CSR_CUSTOM4_S_RO_BASE 0xDC0
> +#define CSR_CUSTOM4_S_RO_COUNT 0x040
> +
> +#define CSR_CUSTOM5_HS_RW_BASE 0x6C0
> +#define CSR_CUSTOM5_HS_RW_COUNT 0x040
> +
> +#define CSR_CUSTOM6_HS_RW_BASE 0xAC0
> +#define CSR_CUSTOM6_HS_RW_COUNT 0x040
> +
> +#define CSR_CUSTOM7_HS_RO_BASE 0xEC0
> +#define CSR_CUSTOM7_HS_RO_COUNT 0x040
> +
> +#define CSR_CUSTOM8_M_RW_BASE 0x7C0
> +#define CSR_CUSTOM8_M_RW_COUNT 0x040
> +
> +#define CSR_CUSTOM9_M_RW_BASE 0xBC0
> +#define CSR_CUSTOM9_M_RW_COUNT 0x040
> +
> +#define CSR_CUSTOM10_M_RO_BASE 0xFC0
> +#define CSR_CUSTOM10_M_RO_COUNT 0x040
> +
> +unsigned long csr_read_num(unsigned long csr_num, int *out_err)
> +{
> +#define switchcase_csr_read(__csr_num) \
> + case (__csr_num): \
> + return csr_read(__csr_num)
> +#define switchcase_csr_read_2(__csr_num) \
> + switchcase_csr_read(__csr_num + 0); \
> + switchcase_csr_read(__csr_num + 1)
> +#define switchcase_csr_read_4(__csr_num) \
> + switchcase_csr_read_2(__csr_num + 0); \
> + switchcase_csr_read_2(__csr_num + 2)
> +#define switchcase_csr_read_8(__csr_num) \
> + switchcase_csr_read_4(__csr_num + 0); \
> + switchcase_csr_read_4(__csr_num + 4)
> +#define switchcase_csr_read_16(__csr_num) \
> + switchcase_csr_read_8(__csr_num + 0); \
> + switchcase_csr_read_8(__csr_num + 8)
> +#define switchcase_csr_read_32(__csr_num) \
> + switchcase_csr_read_16(__csr_num + 0); \
> + switchcase_csr_read_16(__csr_num + 16)
> +#define switchcase_csr_read_64(__csr_num) \
> + switchcase_csr_read_32(__csr_num + 0); \
> + switchcase_csr_read_32(__csr_num + 32)
> +#define switchcase_csr_read_128(__csr_num) \
> + switchcase_csr_read_64(__csr_num + 0); \
> + switchcase_csr_read_64(__csr_num + 64)
> +#define switchcase_csr_read_256(__csr_num) \
> + switchcase_csr_read_128(__csr_num + 0); \
> + switchcase_csr_read_128(__csr_num + 128)
> +
That's... a bit horrendous.
Since we know that each inner case is quite simple, can we just do our
own jump table for that? Each case can be one csrr/csrw and one jal
(possibly pad to 8 bytes), and we can just jump to
label + (csr_num - range_start) * 8. See bottom of this message for an
untested prototype.
Vivian "dramforever" Wang
UNTESTED prototype:
#define CSR_CYCLE 0xc00
#define CSR_CYCLEH 0xc80
/* Force expand argument if macro */
#define str(_x) #_x
unsigned long csr_read_num(long csr_num)
{
unsigned long res, tmp;
/*
* Generate a jump table for each range. Each (inner) case is 8 bytes of
* code, a csrr instruction and a jump, possibly padded, so we just jump
* to label_1f + (csr_num - _start) * 8
*/
#define csr_read_case_range(_start, _size) \
case (_start) ... ((_start) + (_size) - 1): \
asm volatile ( \
" lla %[tmp], 1f\n" \
" add %[tmp], %[tmp], %[offset]\n" \
" jr %[tmp]\n" \
"1:\n" \
" .rept (" str(_size) ")\n" \
" csrr %[res], (" str(_start) " + \\+)\n" \
" j 2f\n" \
" .balign 4\n" \
" .endr\n" \
"2:\n" \
: [tmp] "=&r"(tmp), [res] "=r"(res) \
: [offset] "r"((csr_num - _start) * 8) \
: ); \
break; \
switch (csr_num) {
csr_read_case_range(CSR_CYCLE, 4)
csr_read_case_range(CSR_CYCLEH, 4)
/* ... other cases */
default:
/* Error handling */
}
#undef csr_read_case_range
return res;
}
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