Re: [PATCH] RISC-V: Use GCC __builtin_prefetch() to implement prefetch()

[Conor Dooley]

Hey,

On Wed, Jul 26, 2023 at 03:47:32PM +0800, wang_yang wrote:
>  GCC's __builtin_prefetch() was introduced a long time ago, all supported GCC
>  versions have it.So this patch is to use it for implementing the prefetch.
> 
>  RISC-V Cache Management Operation instructions has been supported by GCC last
>  year.you can refer to
>  https://github.com/gcc-mirror/gcc/commit/3df3ca9014f94fe4af07444fea19b4ab29ba8e73

This should be in a Link: tag.

>  It is worth noting that CPU based on RISC-V should support Zicbop extension.

How do you intend determining whether the CPU supports Zicbop?

>  This has been already done on other architectures (see the commit:
>  https://github.com/torvalds/linux/commit/0453fb3c528c5eb3483441a466b24a4cb409eec5).

That's not how to describe commits, please see how that is done
elsewhere in the git history.

> 
> Signed-off-by: Wang Yang <yangwang@xxxxxxxxxxxxxx>

WARNING: Possible unwrapped commit description (prefer a maximum 75 chars per line)
#6: 
 GCC's __builtin_prefetch() was introduced a long time ago, all supported GCC

ERROR: Missing Signed-off-by: line by nominal patch author 'wang_yang <wang_yang6662023@xxxxxxx>'

> ---
>  arch/riscv/include/asm/processor.h | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
> index c950a8d9edef..f16d4c85ca5b 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -70,6 +70,17 @@ extern void start_thread(struct pt_regs *regs,
>  
>  extern unsigned long __get_wchan(struct task_struct *p);
>  
> +#define ARCH_HAS_PREFETCH
> +static inline void prefetch(const void *ptr)
> +{
> +	__builtin_prefetch(ptr, 0, 3);

As far as I can tell, this does not do anything and will be removed by
the compiler. Please take a look at how other extensions are implemented.

Also, why 3? (Your answer should be in the commit message)

Thanks,
Conor.

> +}
> +
> +#define ARCH_HAS_PREFETCHW
> +static inline void prefetchw(const void *ptr)
> +{
> +	__builtin_prefetch(ptr, 1, 3);
> +}
>  
>  static inline void wait_for_interrupt(void)
>  {
> -- 
> 2.34.1
>

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