[FFmpeg-devel] [PATCH] lavu/riscv: do not fallback to AT_HWCAP auxillary vector

[Brad Smith]

On 2024-07-19 11:46 a.m., Rémi Denis-Courmont wrote:
> If __riscv_hwprobe() fails, then the kernel version is presumably too
> old. There is not much point falling back to the auxillary vector.
>
> - The Linux kernel requires I, so the flag is always set on Linux, and
>    run-time detection is unnecessary. Our RISC-V assembler does anyway not
>    support targets without I.
>
> - Linux can compile with or without F and D, but it cannot perform
>    run-time detection for them (a kernel with F support will not boot a
>    processor without F). The run-time detection is thus useless in that
>    case. Besides F and D extensions are used throughout the C code, so
>    their run-time detection would not be practical.
>
> - Support for V was added in a later kernel version than riscv_hwprobe(),
>    so the system call will always be available if the kernel supports V.
>    The only exception would be vendor kernel forks, but those are known to
>    haphasardly pretend to support V on systems without actual V support, or
>    with only pre-ratification binary-incompatible version. Furthermore, a
>    large chunk of our optimisations require Zba and/or Zbb which cannot be
>    detected with HWCAP in those kernels.
>
> For what it is worth, OpenJDK already took a similar action. Note that this
> keeps AT_HWCAP usage for platforms with neither C run-time <sys/hwprobe.h>
> nor kernel <asm/hwprobe.h>, notably kernels other than Linux.

Thanks for the last part. The bits for OpenBSD have just landed for 
supporting
elf_aux_info() so I'll look at adding support for FreeBSD and OpenBSD.

> ---
>   libavutil/riscv/cpu.c | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/libavutil/riscv/cpu.c b/libavutil/riscv/cpu.c
> index 73abd289a6..04ac404bbf 100644
> --- a/libavutil/riscv/cpu.c
> +++ b/libavutil/riscv/cpu.c
> @@ -83,9 +83,8 @@ int ff_get_cpu_flags_riscv(void)
>                   break;
>               default:
>           }
> -    } else
> -#endif
> -#if HAVE_GETAUXVAL
> +    }
> +#elif HAVE_GETAUXVAL
>       {
>           const unsigned long hwcap = getauxval(AT_HWCAP);
>