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Re: [PATCH 1/3] fixed mtc0_tlbw_hazard

To: Yoichi Yuasa <yoichi_yuasa@tripeaks.co.jp>
Subject: Re: [PATCH 1/3] fixed mtc0_tlbw_hazard
From: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Date: Thu, 21 Sep 2006 21:01:14 +0400
Cc: ralf@linux-mips.org, linux-mips@linux-mips.org
In-reply-to: <20060922014142.2a1985c1.yoichi_yuasa@tripeaks.co.jp>
Organization: MontaVista Software Inc.
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References: <20060922010713.657f2861.yoichi_yuasa@tripeaks.co.jp> <4512BC2A.6040003@dev.rtsoft.ru> <20060922014142.2a1985c1.yoichi_yuasa@tripeaks.co.jp>
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Hello.

Yoichi Yuasa wrote:

Some mtc0_tlbw_hazard() were broken by "[MIPS] Cleanup hazard handling" patch.
Please apply this patch.

tlb-r4k.o disassemble:

8009018c <local_flush_tlb_all>:
8009018c:       40066000        mfc0    a2,$12
80090190:       34c1001f        ori     at,a2,0x1f
80090194:       3821001f        xori    at,at,0x1f
80090198:       40816000        mtc0    at,$12
8009019c:       00000040        ssnop
800901a0:       00000040        ssnop
800901a4:       00000040        ssnop

   Hm, why there are ssnop's here...

ssnop is a part of dvpe().

   Yep, this is irq_disable_hazard, looking sane.

800901a8:       40075000        mfc0    a3,$10
800901ac:       40801000        mtc0    zero,$2
800901b0:       40801800        mtc0    zero,$3
800901b4:       40043000        mfc0    a0,$6
800901b8:       3c028035        lui     v0,0x8035
800901bc:       8c457ac0        lw      a1,31424(v0)
800901c0:       0085182a        slt     v1,a0,a1
800901c4:       1060000b        beqz    v1,800901f4 <local_flush_tlb_all+0x68>
800901c8:       00044340        sll     t0,a0,0xd
800901cc:       3c098000        lui     t1,0x8000
800901d0:       01091821        addu    v1,t0,t1
800901d4:       40835000        mtc0    v1,$10
800901d8:       10000002        b       800901e4 <local_flush_tlb_all+0x58> <-- 
mtc0_tlbw_hazard()
800901dc:       40840000        mtc0    a0,$0
800901e0:       42000002        tlbwi

Yoichi

Signed-off-by: Yoichi Yuasa <yoichi_yuasa@tripeaks.co.jp>

diff -pruN -X mips/Documentation/dontdiff mips-orig/include/asm-mips/hazards.h 
mips/include/asm-mips/hazards.h
--- mips-orig/include/asm-mips/hazards.h        2006-09-21 18:21:11.793973750 
+0900
+++ mips/include/asm-mips/hazards.h     2006-09-21 18:55:07.569201750 +0900
@@ -138,7 +138,7 @@ ASMMACRO(back_to_back_c0_hazard,

   This is under #elif defined(CONFIG_CPU_SB1), right?

 * Mostly like R4000 for historic reasons
 */
ASMMACRO(mtc0_tlbw_hazard,
-        b      . + 8
+        nop; nop; nop; nop; nop; nop

   ... and nop's there? This looks inconsistent.

previous mtc0_tlbw_hazard() for C used nop.
"b . + 8" is trick for R4000/R4400, see comment in old hazard.h .

I fail to see what was changed WRT SB1 CPUs by the suspected patch. Though wait... the previous version was inconsistent, using the different barrier definitions for C and assembly (nops in the former, and branch in the latter). But since the assembly version was not really used, it couldn't break anything... :-/

Anyway, shouldn't ssnop's be used for SB1 instead? CPU has quad-issue pipeline, hasn't it?

Yoichi

WBR, Sergei

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