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Re: SMTC support status in latest git head.

To: "Kevin D. Kissell" <kevink@paralogos.com>
Subject: Re: SMTC support status in latest git head.
From: Anoop P A <anoop.pa@gmail.com>
Date: Fri, 24 Dec 2010 20:09:05 +0530
Cc: STUART VENTERS <stuart.venters@adtran.com>, "Anoop P.A." <Anoop_P.A@pmc-sierra.com>, linux-mips@linux-mips.org
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Hi Kevin, Stuart ,

Woohooo You guys spotted !.

 http://git.linux-mips.org/?p=linux.git;a=commit;h=d5ec6e3c seems to be
the culprit

Once I restored previous version of stackframe.h 2.6.33-stable started
booting !.

Thanks,
Anoop

On Fri, 2010-12-24 at 04:32 -0800, Kevin D. Kissell wrote:
> Thank you, Stuart!  I've spotted some definite breakage to SMTC between 
> those versions.  In arch/mips/include/asm/stackframe.h, someone moved 
> the store of the Status register value in SAVE_SOME (line 169 or 204, 
> depending on the version) from two instructions after the mfc0 to a 
> point after the #ifdef for SMTC, presumably to get better pipelining of 
> the register access.  Unfortunately, the v1 register is also used in the 
> SMTC-specific fragment to save TCStatus, so the Status value gets 
> clobbered before it gets stored.  This will eventually result in the 
> Status register getting a TCStatus value, which has some bits on common, 
> but isn't identical and sooner or later Bad Things will happen.
> 
> I'm a little surprised this wasn't caught by visual inspection of the patch.
> 
> Possible solutions would include reverting the store of the CP0_STATUS 
> value to the block above the #ifdef, or, to retain whatever performance 
> advantage was obtained by moving the store downward, to use v0/$2 
> instead of v1/$3, as the staging register for the TCStatus value.  I'd 
> lean toward the second option, but I'm not in a position to test and 
> submit a patch just now.
> 
>              Regards,
> 
>              Kevin K.
> 
> On 12/23/10 1:09 PM, STUART VENTERS wrote:
> > Kevin,
> >
> > I'm not sure if it's useful,
> >     but finally I got the time to look at the two kernel versions Anoop 
> > pointed out.
> >      works   2.6.32-stable with patch 804
> >      works_not 2.6.33-stable
> >
> > greping for files with CONFIG_MIPS_MT_SMTC
> >     and looking for timer interrupt related stuff found the following 
> > differences:
> >
> >
> > arch/mips/include/asm/irq.h
> > arch/mips/kernel/irq.c
> >    do_IRQ
> >
> > arch/mips/include/asm/stackframe.h
> >    SAVE_SOME SAVE_TEMP get/set_saved_sp
> >
> > arch/mips/include/asm/time.h
> >    clocksource_set_clock
> >
> > arch/mips/kernel/process.c
> >    cpu_idle
> >
> > arch/mips/kernel/smtc.c
> >    __irq_entry
> >    ipi_decode
> >        SMTC_CLOCK_TICK
> >
> >
> > Enclosed are the two subsets of files for a more expert look.
> >
> > I'll try to look in more detail after Christmas.
> >
> >
> > Cheers,
> >
> > Stuart
> >
> >
> >
> >
> 



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