Re: [PATCH v3 04/17] riscv: vector: Use vlenb from DT

[Conor Dooley]

On Sat, Apr 20, 2024 at 06:04:36PM -0700, Charlie Jenkins wrote:
> If vlenb is provided in the device tree, prefer that over reading the
> vlenb csr.
> 
> Signed-off-by: Charlie Jenkins <charlie@xxxxxxxxxxxx>
> ---
>  arch/riscv/include/asm/cpufeature.h |  2 ++
>  arch/riscv/kernel/cpufeature.c      | 26 ++++++++++++++++++++++++++
>  arch/riscv/kernel/vector.c          | 13 +++++++++----
>  3 files changed, 37 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
> index 347805446151..809f61ffb667 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_dt[NR_CPUS];
> +
>  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 c6e27b45e192..48874aac4871 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_dt[NR_CPUS];
> +
>  /**
>   * riscv_isa_extension_base() - Get base extension word
>   *
> @@ -656,6 +658,28 @@ static int __init riscv_isa_fallback_setup(char *__unused)
>  early_param("riscv_isa_fallback", riscv_isa_fallback_setup);
>  #endif
>  
> +static void riscv_set_vlenb_from_dt(void)

I'd expect to see a name here that had "of" in it, not "dt".

> +{
> +	int cpu;
> +
> +	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");
> +			continue;
> +		}
> +
> +		if (!of_property_read_u32(cpu_node, "riscv,vlenb", &riscv_vlenb_dt[cpu])) {
> +			of_node_put(cpu_node);
> +			continue;
> +		}
> +
> +		of_node_put(cpu_node);
> +	}
> +}
> +
>  void __init riscv_fill_hwcap(void)
>  {
>  	char print_str[NUM_ALPHA_EXTS + 1];
> @@ -675,6 +699,8 @@ void __init riscv_fill_hwcap(void)
>  	} else {
>  		int ret = riscv_fill_hwcap_from_ext_list(isa2hwcap);
>  
> +		riscv_set_vlenb_from_dt();

Hmm, I think we can go a step further here. We know all of the CPUs
widths by the time we get to the first call to riscv_v_setup_vsize(), can
we examine the whole list and decide not to enable vector if they do
not match, rather than continuing and failing to online CPUs that having
the mismatched size?

I guess that can go into the `if (elf_hwcap & COMPAT_HWCAP_ISA_V)`
condition we already have, and would require clearing the bit from the
mask we have at the moment.

Cheers,
Conor.

> +
>  		if (ret && riscv_isa_fallback) {
>  			pr_info("Falling back to deprecated \"riscv,isa\"\n");
>  			riscv_fill_hwcap_from_isa_string(isa2hwcap);
> diff --git a/arch/riscv/kernel/vector.c b/arch/riscv/kernel/vector.c
> index 6727d1d3b8f2..fb7f3ca80d9e 100644
> --- a/arch/riscv/kernel/vector.c
> +++ b/arch/riscv/kernel/vector.c
> @@ -32,11 +32,16 @@ EXPORT_SYMBOL_GPL(riscv_v_vsize);
>  int riscv_v_setup_vsize(void)
>  {
>  	unsigned long this_vsize;
> +	int cpu = smp_processor_id();
>  
> -	/* There are 32 vector registers with vlenb length. */
> -	riscv_v_enable();
> -	this_vsize = csr_read(CSR_VLENB) * 32;
> -	riscv_v_disable();
> +	if (riscv_vlenb_dt[cpu]) {
> +		this_vsize = riscv_vlenb_dt[cpu];

> +	} else {
> +		/* There are 32 vector registers with vlenb length. */
> +		riscv_v_enable();
> +		this_vsize = csr_read(CSR_VLENB) * 32;
> +		riscv_v_disable();
> +	}
>  
>  	if (!riscv_v_vsize) {
>  		riscv_v_vsize = this_vsize;
> 
> -- 
> 2.44.0
>

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