Le 20 juillet 2024 00:28:23 GMT+03:00, Brad Smith
<brad-at-comstyle....@ffmpeg.org> a écrit :
>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.
Well if they're using the `misa` CSR layout for HWCAP as Linux did, then there
should be nothing to do, except maybe for compile fixes.
But if they use a custom bit-field layout then you probably need to rewrite the
code.
>
>> ---
>> 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);
>>
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