[PATCH v3 1/2] lib: sbi: hart: Detect existence of cycle and instret CSRs for Zicntr

[Anup Patel]

On Sat, May 3, 2025 at 4:28 PM Yao Zi <ziyao at disroot.org> wrote:
>
> Zicntr extension specifies three read-only CSRs, time, cycle and
> instret. It isn't sufficient to report Zicntr is fully supported with
> only time CSR detected.
>
> This patch introduces a bitmap to sbi_hart_features to record
> availability of these CSRs, which are detected using traps. Zicntr is
> reported as present if and only if three CSRs are all available on the
> HARTs.
>
> Suggested-by: Anup Patel <anup at brainfault.org>
> Signed-off-by: Yao Zi <ziyao at disroot.org>
> ---
>  include/sbi/sbi_hart.h | 10 ++++++++++
>  lib/sbi/sbi_hart.c     | 35 ++++++++++++++++++++++++++++-------
>  2 files changed, 38 insertions(+), 7 deletions(-)

Why are the changes in lib/sbi/sbi_timer.c dropped ?

Regards,
Anup

>
> diff --git a/include/sbi/sbi_hart.h b/include/sbi/sbi_hart.h
> index bce88c5..dfbe8e4 100644
> --- a/include/sbi/sbi_hart.h
> +++ b/include/sbi/sbi_hart.h
> @@ -93,6 +93,14 @@ struct sbi_hart_ext_data {
>
>  extern const struct sbi_hart_ext_data sbi_hart_ext[];
>
> +/** CSRs should be detected by access and trapping */
> +enum sbi_hart_csrs {
> +       SBI_HART_CSR_CYCLE = 0,
> +       SBI_HART_CSR_TIME,
> +       SBI_HART_CSR_INSTRET,
> +       SBI_HART_CSR_MAX,
> +};
> +
>  /*
>   * Smepmp enforces access boundaries between M-mode and
>   * S/U-mode. When it is enabled, the PMPs are programmed
> @@ -112,6 +120,7 @@ struct sbi_hart_features {
>         bool detected;
>         int priv_version;
>         unsigned long extensions[BITS_TO_LONGS(SBI_HART_EXT_MAX)];
> +       unsigned long csrs[BITS_TO_LONGS(SBI_HART_CSR_MAX)];
>         unsigned int pmp_count;
>         unsigned int pmp_addr_bits;
>         unsigned int pmp_log2gran;
> @@ -150,6 +159,7 @@ bool sbi_hart_has_extension(struct sbi_scratch *scratch,
>                             enum sbi_hart_extensions ext);
>  void sbi_hart_get_extensions_str(struct sbi_scratch *scratch,
>                                  char *extension_str, int nestr);
> +bool sbi_hart_has_csr(struct sbi_scratch *scratch, enum sbi_hart_csrs csr);
>
>  void __attribute__((noreturn)) sbi_hart_hang(void);
>
> diff --git a/lib/sbi/sbi_hart.c b/lib/sbi/sbi_hart.c
> index fc4925b..74ccdd8 100644
> --- a/lib/sbi/sbi_hart.c
> +++ b/lib/sbi/sbi_hart.c
> @@ -747,6 +747,20 @@ void sbi_hart_get_extensions_str(struct sbi_scratch *scratch,
>                 sbi_strncpy(extensions_str, "none", nestr);
>  }
>
> +/**
> + * Check whether a particular CSR is present on the HART
> + *
> + * @param scratch pointer to the HART scratch space
> + * @param csr the CSR number to check
> + */
> +bool sbi_hart_has_csr(struct sbi_scratch *scratch, enum sbi_hart_csrs csr)
> +{
> +       struct sbi_hart_features *hfeatures =
> +                       sbi_scratch_offset_ptr(scratch, hart_features_offset);
> +
> +       return __test_bit(csr, hfeatures->csrs);
> +}
> +
>  static unsigned long hart_pmp_get_allowed_addr(void)
>  {
>         unsigned long val = 0;
> @@ -803,7 +817,6 @@ static int hart_detect_features(struct sbi_scratch *scratch)
>         struct sbi_hart_features *hfeatures =
>                 sbi_scratch_offset_ptr(scratch, hart_features_offset);
>         unsigned long val, oldval;
> -       bool has_zicntr = false;
>         int rc;
>
>         /* If hart features already detected then do nothing */
> @@ -812,6 +825,7 @@ static int hart_detect_features(struct sbi_scratch *scratch)
>
>         /* Clear hart features */
>         sbi_memset(hfeatures->extensions, 0, sizeof(hfeatures->extensions));
> +       sbi_memset(hfeatures->csrs, 0, sizeof(hfeatures->csrs));
>         hfeatures->pmp_count = 0;
>         hfeatures->mhpm_mask = 0;
>         hfeatures->priv_version = SBI_HART_PRIV_VER_UNKNOWN;
> @@ -938,9 +952,6 @@ __pmp_skip:
>         /* Detect if hart supports sscofpmf */
>         __check_ext_csr(SBI_HART_PRIV_VER_1_11,
>                         CSR_SCOUNTOVF, SBI_HART_EXT_SSCOFPMF);
> -       /* Detect if hart supports time CSR */
> -       __check_ext_csr(SBI_HART_PRIV_VER_UNKNOWN,
> -                       CSR_TIME, SBI_HART_EXT_ZICNTR);
>         /* Detect if hart has AIA local interrupt CSRs */
>         __check_ext_csr(SBI_HART_PRIV_VER_UNKNOWN,
>                         CSR_MTOPI, SBI_HART_EXT_SMAIA);
> @@ -962,8 +973,16 @@ __pmp_skip:
>
>  #undef __check_ext_csr
>
> -       /* Save trap based detection of Zicntr */
> -       has_zicntr = sbi_hart_has_extension(scratch, SBI_HART_EXT_ZICNTR);
> +#define __check_csr_existence(__csr, __csr_id)                         \
> +       csr_read_allowed(__csr, &trap);                                 \
> +       if (!trap.cause)                                                \
> +               __set_bit(__csr_id, hfeatures->csrs);
> +
> +       __check_csr_existence(CSR_CYCLE, SBI_HART_CSR_CYCLE);
> +       __check_csr_existence(CSR_TIME, SBI_HART_CSR_TIME);
> +       __check_csr_existence(CSR_INSTRET, SBI_HART_CSR_INSTRET);
> +
> +#undef __check_csr_existence
>
>         /* Let platform populate extensions */
>         rc = sbi_platform_extensions_init(sbi_platform_thishart_ptr(),
> @@ -973,7 +992,9 @@ __pmp_skip:
>
>         /* Zicntr should only be detected using traps */
>         __sbi_hart_update_extension(hfeatures, SBI_HART_EXT_ZICNTR,
> -                                   has_zicntr);
> +                           sbi_hart_has_csr(scratch, SBI_HART_CSR_CYCLE) &&
> +                           sbi_hart_has_csr(scratch, SBI_HART_CSR_TIME)  &&
> +                           sbi_hart_has_csr(scratch, SBI_HART_CSR_INSTRET));
>
>         /* Extensions implied by other extensions and features */
>         if (hfeatures->mhpm_mask)
> --
> 2.49.0
>