Re: [PATCH v6 03/17] riscv: vector: Use vlenb from DT

From: Andy Chiu <andy.chiu@sifive.com>
To: Charlie Jenkins <charlie@rivosinc.com>
Cc: "Conor Dooley" <conor@kernel.org>,
	"Rob Herring" <robh@kernel.org>,
	"Krzysztof Kozlowski" <krzysztof.kozlowski+dt@linaro.org>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Guo Ren" <guoren@kernel.org>,
	"Conor Dooley" <conor+dt@kernel.org>,
	"Chen-Yu Tsai" <wens@csie.org>,
	"Jernej Skrabec" <jernej.skrabec@gmail.com>,
	"Samuel Holland" <samuel@sholland.org>,
	"Conor Dooley" <conor.dooley@microchip.com>,
	"Evan Green" <evan@rivosinc.com>,
	"Clément Léger" <cleger@rivosinc.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Shuah Khan" <shuah@kernel.org>,
	linux-riscv@lists.infradead.org, devicetree@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	"Palmer Dabbelt" <palmer@rivosinc.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-sunxi@lists.linux.dev, linux-doc@vger.kernel.org,
	linux-kselftest@vger.kernel.org
Subject: Re: [PATCH v6 03/17] riscv: vector: Use vlenb from DT
Date: Thu, 16 May 2024 22:00:12 +0800	[thread overview]
Message-ID: <CABgGipXg68VEGt=oZZSENmbqs4-g3PB=CBobNwgqQjLHfxo+VQ@mail.gmail.com> (raw)
In-Reply-To: <20240503-dev-charlie-support_thead_vector_6_9-v6-3-cb7624e65d82@rivosinc.com>

Sorry Charlie, I forgot to include the mailing list. Here is the same as
what I sent in the private message.

On Sat, May 4, 2024 at 2:21 AM Charlie Jenkins <charlie@rivosinc.com> wrote:
>
> If vlenb is provided in the device tree, prefer that over reading the
> vlenb csr.
>
> Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> ---
>  arch/riscv/include/asm/cpufeature.h |  2 ++
>  arch/riscv/kernel/cpufeature.c      | 47 +++++++++++++++++++++++++++++++++++++
>  arch/riscv/kernel/vector.c          | 12 +++++++++-
>  3 files changed, 60 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
> index 347805446151..0c4f08577015 100644
> --- a/arch/riscv/include/asm/cpufeature.h
> +++ b/arch/riscv/include/asm/cpufeature.h
> @@ -31,6 +31,8 @@ DECLARE_PER_CPU(struct riscv_cpuinfo, riscv_cpuinfo);
>  /* Per-cpu ISA extensions. */
>  extern struct riscv_isainfo hart_isa[NR_CPUS];
>
> +extern u32 riscv_vlenb_of;
> +
>  void riscv_user_isa_enable(void);
>
>  #if defined(CONFIG_RISCV_MISALIGNED)
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 3ed2359eae35..6c143ea9592b 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -35,6 +35,8 @@ static DECLARE_BITMAP(riscv_isa, RISCV_ISA_EXT_MAX) __read_mostly;
>  /* Per-cpu ISA extensions. */
>  struct riscv_isainfo hart_isa[NR_CPUS];
>
> +u32 riscv_vlenb_of;
> +
>  /**
>   * riscv_isa_extension_base() - Get base extension word
>   *
> @@ -648,6 +650,46 @@ static int __init riscv_isa_fallback_setup(char *__unused)
>  early_param("riscv_isa_fallback", riscv_isa_fallback_setup);
>  #endif
>
> +static int has_riscv_homogeneous_vlenb(void)
> +{
> +       int cpu;
> +       u32 prev_vlenb = 0;
> +       u32 vlenb;
> +
> +       /* Ignore vlenb if vector is not enabled in the kernel */
> +       if (!IS_ENABLED(CONFIG_RISCV_ISA_V))
> +               return 0;
> +
> +       for_each_possible_cpu(cpu) {
> +               struct device_node *cpu_node;
> +
> +               cpu_node = of_cpu_device_node_get(cpu);
> +               if (!cpu_node) {
> +                       pr_warn("Unable to find cpu node\n");
> +                       return -ENOENT;
> +               }
> +
> +               if (of_property_read_u32(cpu_node, "riscv,vlenb", &vlenb)) {
> +                       of_node_put(cpu_node);
> +
> +                       if (prev_vlenb)
> +                               return -ENOENT;
> +                       continue;
> +               }
> +
> +               if (prev_vlenb && vlenb != prev_vlenb) {
> +                       of_node_put(cpu_node);
> +                       return -ENOENT;
> +               }
> +
> +               prev_vlenb = vlenb;
> +               of_node_put(cpu_node);
> +       }
> +
> +       riscv_vlenb_of = vlenb;
> +       return 0;
> +}
> +
>  void __init riscv_fill_hwcap(void)
>  {
>         char print_str[NUM_ALPHA_EXTS + 1];
> @@ -671,6 +713,11 @@ void __init riscv_fill_hwcap(void)
>                         pr_info("Falling back to deprecated \"riscv,isa\"\n");
>                         riscv_fill_hwcap_from_isa_string(isa2hwcap);
>                 }
> +
> +               if (elf_hwcap & COMPAT_HWCAP_ISA_V && has_riscv_homogeneous_vlenb() < 0) {
> +                       pr_warn("Unsupported heterogeneous vlen detected, vector extension disabled.\> +                       elf_hwcap &= ~COMPAT_HWCAP_ISA_V;
> +               }

We only touch COMPAT_HWCAP_ISA_V and the failed case only turns off the
rectified V. So here we have nothing to do with the Xtheadvector.

However, I am still confused because I think Xtheadvector would also
need to call into this check, so as to setup vlenb.

Apart from that, it seems like some vendor stating Xtheadvector is
actually vector-0.7. Please correct me if I speak anything wrong. One
thing I noticed is that Xtheadvector wouldn't trap on reading
th.vlenb but vector-0.7 would. If that is the case, should we require
Xtheadvector to specify `riscv,vlenb` on the device tree?

>         }
>
>         /*
> diff --git a/arch/riscv/kernel/vector.c b/arch/riscv/kernel/vector.c
> index 6727d1d3b8f2..e04586cdb7f0 100644
> --- a/arch/riscv/kernel/vector.c
> +++ b/arch/riscv/kernel/vector.c
> @@ -33,7 +33,17 @@ int riscv_v_setup_vsize(void)
>  {
>         unsigned long this_vsize;
>
> -       /* There are 32 vector registers with vlenb length. */
> +       /*
> +        * There are 32 vector registers with vlenb length.
> +        *
> +        * If the riscv,vlenb property was provided by the firmware, use that
> +        * instead of probing the CSRs.
> +        */
> +       if (riscv_vlenb_of) {
> +               this_vsize = riscv_vlenb_of * 32;
> +               return 0;
> +       }
> +
>         riscv_v_enable();
>         this_vsize = csr_read(CSR_VLENB) * 32;
>         riscv_v_disable();
>
> --
> 2.44.0
>
>
> _______________________________________________
> linux-riscv mailing list
> linux-riscv@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-riscv

Thanks,
Andy