linux-kernel - Re: [PATCH v2 12/15] powerpc/85xx: add time base sync support for e6500

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Date:	Thu, 25 Apr 2013 19:07:24 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	Zhao Chenhui <chenhui.zhao@...escale.com>
CC:	<linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>,
	<r58472@...escale.com>
Subject: Re: [PATCH v2 12/15] powerpc/85xx: add time base sync support for
 e6500

On 04/24/2013 07:28:18 PM, Zhao Chenhui wrote:
> On Wed, Apr 24, 2013 at 05:38:16PM -0500, Scott Wood wrote:
> > On 04/24/2013 06:29:29 AM, Zhao Chenhui wrote:
> > >On Tue, Apr 23, 2013 at 07:04:06PM -0500, Scott Wood wrote:
> > >> On 04/19/2013 05:47:45 AM, Zhao Chenhui wrote:
> > >> >From: Chen-Hui Zhao <chenhui.zhao@...escale.com>
> > >> >
> > >> >For e6500, two threads in one core share one time base. Just  
> need
> > >> >to do time base sync on first thread of one core, and skip it on
> > >> >the other thread.
> > >> >
> > >> >Signed-off-by: Zhao Chenhui <chenhui.zhao@...escale.com>
> > >> >Signed-off-by: Li Yang <leoli@...escale.com>
> > >> >Signed-off-by: Andy Fleming <afleming@...escale.com>
> > >> >---
> > >> > arch/powerpc/platforms/85xx/smp.c |   52
> > >> >+++++++++++++++++++++++++++++++-----
> > >> > 1 files changed, 44 insertions(+), 8 deletions(-)
> > >> >
> > >> >diff --git a/arch/powerpc/platforms/85xx/smp.c
> > >> >b/arch/powerpc/platforms/85xx/smp.c
> > >> >index 74d8cde..5f3eee3 100644
> > >> >--- a/arch/powerpc/platforms/85xx/smp.c
> > >> >+++ b/arch/powerpc/platforms/85xx/smp.c
> > >> >@@ -53,26 +55,40 @@ static inline u32 get_phy_cpu_mask(void)
> > >> > 	u32 mask;
> > >> > 	int cpu;
> > >> >
> > >> >-	mask = 1 << cur_booting_core;
> > >> >-	for_each_online_cpu(cpu)
> > >> >-		mask |= 1 << get_hard_smp_processor_id(cpu);
> > >> >+	if (smt_capable()) {
> > >> >+		/* two threads in one core share one time base  
> */
> > >> >+		mask = 1 <<  
> cpu_core_index_of_thread(cur_booting_core);
> > >> >+		for_each_online_cpu(cpu)
> > >> >+			mask |= 1 << cpu_core_index_of_thread(
> > >> >+					 
> get_hard_smp_processor_id(cpu));
> > >> >+	} else {
> > >> >+		mask = 1 << cur_booting_core;
> > >> >+		for_each_online_cpu(cpu)
> > >> >+			mask |= 1 <<  
> get_hard_smp_processor_id(cpu);
> > >> >+	}
> > >>
> > >> Where is smt_capable defined()?  I assume somewhere in the  
> patchset
> > >> but it's a pain to search 12 patches...
> > >>
> > >
> > >It is defined in arch/powerpc/include/asm/topology.h.
> > >	#define smt_capable()           (cpu_has_feature(CPU_FTR_SMT))
> > >
> > >Thanks for your review again.
> >
> > We shouldn't base it on CPU_FTR_SMT.  For example, e6500 doesn't
> > claim that feature yet, except in our SDK kernel.  That doesn't
> > change the topology of CPU numbering.
> >
> 
> Then, where can I get the thread information? dts?
> Or, wait for upstream of the thread suppport of e6500.

It's an inherent property of e6500 (outside of some virtualization  
scenarios, but you wouldn't run this code under a hypervisor) that you  
have two threads per core (whether Linux uses them or not).  Or you  
could read TMCFG0[NTHRD] if you know you're on a chip that has TMRs but  
aren't positive it's an e6500, but I wouldn't bother.  If we do ever  
have such a chip, there are probably other things that will need  
updating.

> > >static inline u32 get_phy_cpu_mask(void)
> > >{
> > >	u32 mask;
> > >	int cpu;
> > >
> > >	mask = 1 << cpu_core_index_of_thread(cur_booting_core);
> > >	for_each_online_cpu(cpu)
> > >		mask |= 1 << cpu_core_index_of_thread(
> > >				get_hard_smp_processor_id(cpu));
> > >
> > >	return mask;
> > >}
> >
> > Likewise, this will get it wrong if SMT is disabled or not yet
> > implemented on a core.
> >
> > -Scott
> 
> Let's look into cpu_core_index_of_thread() in  
> arch/powerpc/kernel/smp.c.
> 
>   int cpu_core_index_of_thread(int cpu)
>   {
>       return cpu >> threads_shift;
>   }
> 
> If no thread, the threads_shift is equal to 0. It can work with no
> thread.

My point is that if threads are disabled, threads_shift will be 0, but  
e6500 cores will still be numbered 0, 2, 4, etc.

> Perhaps, I should submit this patch after the thread patches for  
> e6500.

Why?

-Scott
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