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Re: SMTC support status in latest git head.

To: Anoop P A <>
Subject: Re: SMTC support status in latest git head.
From: "Kevin D. Kissell" <>
Date: Fri, 24 Dec 2010 06:53:46 -0800
Cc: STUART VENTERS <>, "Anoop P.A." <>,
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Excellent! Now, does the attached patch (relative to also fix things, while preserving the other fixes and performance enhancements?


On 12/24/10 6:39 AM, Anoop P A wrote:
Hi Kevin, Stuart ,

Woohooo You guys spotted !.;a=commit;h=d5ec6e3c seems to be
the culprit

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


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.


              Kevin K.

On 12/23/10 1:09 PM, STUART VENTERS wrote:

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 


    SAVE_SOME SAVE_TEMP get/set_saved_sp




Enclosed are the two subsets of files for a more expert look.

I'll try to look in more detail after Christmas.



Attachment: smtc_stackframe.h.patch
Description: Text document

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