Subject: Re: [PATCH v3] powerpc32: memset: only use dcbz once cache is enabled

From: Scott Wood <[email-masked]>
Date: Mon, 14 Sep 2015 10:20:54 -0500

On Mon, 2015-09-14 at 08:21 +0200, Christophe Leroy wrote:
> memset() uses instruction dcbz to speed up clearing by not wasting time
> loading cache line with data that will be overwritten.
> Some platform like mpc52xx do no have cache active at startup and
> can therefore not use memset(). Allthough no part of the code
> explicitly uses memset(), GCC may makes calls to it.
> 
> This patch modifies memset() such that at startup, memset()
> unconditionally jumps to simple_memset() which doesn't use
> the dcbz instruction.
> 
> Once the initial MMU is set up, in machine_init() we patch memset()
> by replacing this inconditional jump by a NOP
> 
> Signed-off-by: Christophe Leroy <[email-masked]>
> ---
> This patch goes on to of [v3] powerpc32: memcpy: only use dcbz once cache 
> is enabled
> 
> Changes in v2:
>    was part of [v2] powerpc32: memcpy/memset: only use dcbz once cache is 
> enabled
> changes in v3:
>   Not using anymore feature-fixups
>   Handling of memcpy() and memset() split in two patches
>   
>  arch/powerpc/kernel/setup_32.c |  1 +
>  arch/powerpc/lib/copy_32.S     | 15 +++++++++++++++
>  2 files changed, 16 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/setup_32.c b/arch/powerpc/kernel/setup_32.c
> index 362495f..345ec3a 100644
> --- a/arch/powerpc/kernel/setup_32.c
> +++ b/arch/powerpc/kernel/setup_32.c
> @@ -124,6 +124,7 @@ notrace void __init machine_init(u64 dt_ptr)
>       udbg_early_init();
>  
>       patch_instruction((unsigned int *)&memcpy, PPC_INST_NOP);
> +     patch_instruction((unsigned int *)&memset, PPC_INST_NOP);
>  
>       /* Do some early initialization based on the flat device tree */
>       early_init_devtree(__va(dt_ptr));
> diff --git a/arch/powerpc/lib/copy_32.S b/arch/powerpc/lib/copy_32.S
> index da5847d..68a59d4 100644
> --- a/arch/powerpc/lib/copy_32.S
> +++ b/arch/powerpc/lib/copy_32.S
> @@ -73,8 +73,13 @@ CACHELINE_MASK = (L1_CACHE_BYTES-1)
>   * Use dcbz on the complete cache lines in the destination
>   * to set them to zero.  This requires that the destination
>   * area is cacheable.  -- paulus
> + *
> + * During early init, cache might not be active yet, so dcbz cannot be 
> used.
> + * We therefore jump to simple_memset which doesn't use dcbz. This jump is
> + * replaced by a nop once cache is active. This is done in machine_init()
>   */
>  _GLOBAL(memset)
> +     b       simple_memset
>       rlwimi  r4,r4,8,16,23
>       rlwimi  r4,r4,16,0,15
>  
> @@ -122,6 +127,16 @@ _GLOBAL(memset)
>       bdnz    8b
>       blr
>  
> +/* Simple version of memset used during early boot until cache is enabled 
> */
> +simple_memset:
> +     cmplwi  cr0,r5,0
> +     addi    r6,r3,-1
> +     beqlr
> +     mtctr   r5
> +1:   stbu    r4,1(r6)
> +     bdnz    1b
> +     blr

Instead couldn't you use the generic memset at label 2: and patch the "bne 
2f"?

-Scott