linux-kernel - Re: [RFC patch 04/18] get_cycles() : powerpc64 HAVE_GET

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Date:	Fri, 7 Nov 2008 13:14:12 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Josh Boyer <jwboyer@...ux.vnet.ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, benh@...nel.crashing.org,
	paulus@...ba.org, David Miller <davem@...emloft.net>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-arch@...r.kernel.org
Subject: Re: [RFC patch 04/18] get_cycles() : powerpc64 HAVE_GET_CYCLES

* Josh Boyer (jwboyer@...ux.vnet.ibm.com) wrote:
> On Fri, Nov 07, 2008 at 12:23:40AM -0500, Mathieu Desnoyers wrote:
> >This patch selects HAVE_GET_CYCLES and makes sure get_cycles_barrier() and
> >get_cycles_rate() are implemented.
> >
> >Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
> >CC: benh@...nel.crashing.org
> >CC: paulus@...ba.org
> >CC: David Miller <davem@...emloft.net>
> >CC: Linus Torvalds <torvalds@...ux-foundation.org>
> >CC: Andrew Morton <akpm@...ux-foundation.org>
> >CC: Ingo Molnar <mingo@...hat.com>
> >CC: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> >CC: Thomas Gleixner <tglx@...utronix.de>
> >CC: Steven Rostedt <rostedt@...dmis.org>
> >CC: linux-arch@...r.kernel.org
> >---
> > arch/powerpc/Kconfig             |    1 +
> > arch/powerpc/include/asm/timex.h |   11 +++++++++++
> > 2 files changed, 12 insertions(+)
> >
> >Index: linux.trees.git/arch/powerpc/Kconfig
> >===================================================================
> >--- linux.trees.git.orig/arch/powerpc/Kconfig	2008-11-07 00:09:44.000000000 -0500
> >+++ linux.trees.git/arch/powerpc/Kconfig	2008-11-07 00:09:46.000000000 -0500
> >@@ -121,6 +121,7 @@ config PPC
> > 	select HAVE_DMA_ATTRS if PPC64
> > 	select USE_GENERIC_SMP_HELPERS if SMP
> > 	select HAVE_OPROFILE
> >+	select HAVE_GET_CYCLES if PPC64
> 
> So maybe it's just me because it's Friday and I'm on vacation, but I don't
> see anything overly specific to ppc64 here.  In fact, you use get_cycles_rate
> for all of powerpc in a later patch in the series.
> 
> Is there something special about HAVE_GET_CYCLES that I'm missing that would
> make it only apply to ppc64 and not ppc32?
> 
> josh

Hi Josh,

powerpc32 only uses the 32 LSBs for the TSC in the current get_cycles()
implementation. We could either define HAVE_GET_CYCLES_32 like I did on
mips32, or change get_cycles so it also reads the 32 MSBs in a loop like
this (it does not take care of the CPU_FTR_CELL_TB_BUG though) :

typedef unsigned long long cycles_t;

cycles_t get_cycles_ppc32(void)
{
  union {
    cycles_t v;
    struct {
      u32 ms32, ls32; /* powerpc is big endian */
    } s;
  } cycles;

  do {
    cycles.ls32 = mftbu();
    cycles.ms32 = mftbl();
  } while (cycles.ls32 != mftbu());
  return cycles.v;
}

I'd prefer this second solution. If one needs a specific get_cycles() to
be only 32-bits (but really really fast) for the scheduler, then this
could be a get_cycles_sched() or something like this which does not
guarantee that it returns full 64-bits...

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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