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Message-ID: <1442954285.29498.79.camel@transmode.se>
Date: Tue, 22 Sep 2015 20:38:05 +0000
From: Joakim Tjernlund <joakim.tjernlund@...nsmode.se>
To: "scottwood@...escale.com" <scottwood@...escale.com>
CC: "christophe.leroy@....fr" <christophe.leroy@....fr>,
"paulus@...ba.org" <paulus@...ba.org>,
"mpe@...erman.id.au" <mpe@...erman.id.au>,
"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v2 22/25] powerpc32: move xxxxx_dcache_range() functions
inline
On Tue, 2015-09-22 at 15:35 -0500, Scott Wood wrote:
> On Tue, 2015-09-22 at 20:32 +0000, Joakim Tjernlund wrote:
> > On Tue, 2015-09-22 at 15:14 -0500, Scott Wood wrote:
> > > On Tue, 2015-09-22 at 19:55 +0000, Joakim Tjernlund wrote:
> > > > On Tue, 2015-09-22 at 14:42 -0500, Scott Wood wrote:
> > > > > On Tue, 2015-09-22 at 19:34 +0000, Joakim Tjernlund wrote:
> > > > > > On Tue, 2015-09-22 at 13:58 -0500, Scott Wood wrote:
> > > > > > > On Tue, 2015-09-22 at 18:12 +0000, Joakim Tjernlund wrote:
> > > > > > > > On Tue, 2015-09-22 at 18:51 +0200, Christophe Leroy wrote:
> > > > > > > > > flush/clean/invalidate _dcache_range() functions are all very
> > > > > > > > > similar and are quite short. They are mainly used in
> > > > > > > > > __dma_sync()
> > > > > > > > > perf_event locate them in the top 3 consumming functions
> > > > > > > > > during
> > > > > > > > > heavy ethernet activity
> > > > > > > > >
> > > > > > > > > They are good candidate for inlining, as __dma_sync() does
> > > > > > > > > almost nothing but calling them
> > > > > > > > >
> > > > > > > > > Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> > > > > > > > > ---
> > > > > > > > > New in v2
> > > > > > > > >
> > > > > > > > > arch/powerpc/include/asm/cacheflush.h | 55
> > > > > > > > > +++++++++++++++++++++++++++--
> > > > > > > > > arch/powerpc/kernel/misc_32.S | 65 ------------------
> > > > > > > > > ----
> > > > > > > > > ----
> > > > > > > > > ----
> > > > > > > > > -----
> > > > > > > > > arch/powerpc/kernel/ppc_ksyms.c | 2 ++
> > > > > > > > > 3 files changed, 54 insertions(+), 68 deletions(-)
> > > > > > > > >
> > > > > > > > > diff --git a/arch/powerpc/include/asm/cacheflush.h
> > > > > > > > > b/arch/powerpc/include/asm/cacheflush.h
> > > > > > > > > index 6229e6b..6169604 100644
> > > > > > > > > --- a/arch/powerpc/include/asm/cacheflush.h
> > > > > > > > > +++ b/arch/powerpc/include/asm/cacheflush.h
> > > > > > > > > @@ -47,12 +47,61 @@ static inline void
> > > > > > > > > __flush_dcache_icache_phys(unsigned long physaddr)
> > > > > > > > > }
> > > > > > > > > #endif
> > > > > > > > >
> > > > > > > > > -extern void flush_dcache_range(unsigned long start, unsigned
> > > > > > > > > long
> > > > > > > > > stop);
> > > > > > > > > #ifdef CONFIG_PPC32
> > > > > > > > > -extern void clean_dcache_range(unsigned long start, unsigned
> > > > > > > > > long
> > > > > > > > > stop);
> > > > > > > > > -extern void invalidate_dcache_range(unsigned long start,
> > > > > > > > > unsigned
> > > > > > > > > long
> > > > > > > > > stop);
> > > > > > > > > +/*
> > > > > > > > > + * Write any modified data cache blocks out to memory and
> > > > > > > > > invalidate
> > > > > > > > > them.
> > > > > > > > > + * Does not invalidate the corresponding instruction cache
> > > > > > > > > blocks.
> > > > > > > > > + */
> > > > > > > > > +static inline void flush_dcache_range(unsigned long start,
> > > > > > > > > unsigned
> > > > > > > > > long
> > > > > > > > > stop)
> > > > > > > > > +{
> > > > > > > > > + void *addr = (void *)(start & ~(L1_CACHE_BYTES - 1));
> > > > > > > > > + unsigned int size = stop - (unsigned long)addr +
> > > > > > > > > (L1_CACHE_BYTES -
> > > > > > > > > 1);
> > > > > > > > > + unsigned int i;
> > > > > > > > > +
> > > > > > > > > + for (i = 0; i < size >> L1_CACHE_SHIFT; i++, addr +=
> > > > > > > > > L1_CACHE_BYTES)
> > > > > > > > > + dcbf(addr);
> > > > > > > > > + if (i)
> > > > > > > > > + mb(); /* sync */
> > > > > > > > > +}
> > > > > > > >
> > > > > > > > This feels optimized for the uncommon case when there is no
> > > > > > > > invalidation.
> > > > > > >
> > > > > > > If you mean the "if (i)", yes, that looks odd.
> > > > > >
> > > > > > Yes.
> > > > > >
> > > > > > >
> > > > > > > > I THINK it would be better to bail early
> > > > > > >
> > > > > > > Bail under what conditions?
> > > > > >
> > > > > > test for "i = 0" and return.
> > > > >
> > > > > Why bother?
> > > >
> > > > I usally find it better to dela with special cases upfront så the rest
> > > > doesn't need to
> > > > bother. i=0 is a NOP and it is clearer to show that upfront.
> > >
> > > No, I mean why bother special casing this at all?
> >
> > I just said why?
> > You to found the if(i) mb() a bit odd and it took a little time to see why
> > it was there.
> > Ahh, you mean just skip the if(i) and have mb() unconditionally?
>
> Yes.
>
> > That changes the semantics a little from the ASM version but perhaps that
> > doesn't matter?
>
> Adding more barriers than strictly necessary is a performance concern, not a
> semantic change.
Semantics :)
> How often are we really calling this function over an empty
> range?
Never hopefully, it does not make much sense.
>
> Not that it matters much one way or another...
probably not.
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