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

[Christophe LEROY]

Le 14/09/2015 17:20, Scott Wood a Ãcrit :
> 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 <christophe.leroy@xxxxxx>
> > ---
> > 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"?

Yes I could but it means adding a global symbol there at the "bne 2f". I 
thought it was what Michael didn't like in my v1 of memcpy().
What name could I give to that symbol ? Something like 
memcpy_nocache_patch: ?
What would be the best ? Having b 2f, and replacing it with bne 2f ? Or 
have b 2f just above and replace it by nop once cache is up ?

Christophe

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