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Re: [PATCH] Retain the write-only OD from being clobbered

To: Linux MIPS <>
Subject: Re: [PATCH] Retain the write-only OD from being clobbered
From: Sergei Shtylylov <>
Date: Thu, 29 Dec 2005 01:25:57 +0300
Cc: Manish Lachwani <>, Jordan Crouse <>,
In-reply-to: <>
Organization: MostaVista Software Inc.
Original-recipient: rfc822;
References: <> <>
User-agent: Mozilla/5.0 (X11; U; Linux i686; rv:1.7.2) Gecko/20040803

Sergei Shtylylov wrote:

Jordan Crouse wrote:

First of several patches forwarded to me by Sergei Shtylyov.  Ralf,
these should be good to go for the tree.

Retain the write-only OD bit from being clobbered by coherency_setup()

Signed-off-by: Sergei Shtylyov <>
Acked-by: Jordan Crouse <>

   A long story of a long standing bug follows...
AMD Au1x00 board setup code in au1x00_setup() sets CONFIG.OD bit for the early steppings of the Au1x00 SOCs, consulting the PRID table in arch/mips/au1000/common/cputable.c. This bit disables the bus transaction overlapping which causes a number of errata in those early SOC steppings (including the one that I think we've run into with USB host--at least setting the bit back to 1 fixed it, although disabling USB host DMA coherency also seemd to help). The problem is that this bit is write-only and reads
as 0 on almost all SOCs that need it set. So, when the arch/mm/c-r4k.c code
does RMW to the CONFIG reg. in coherency_setup(), its value is lost, and the chip again becomes prone to all the nasty errata that it has just been saved
from... :-)
There's couple more places in the assembly code where the CP0 CONFIG reg. is written without care of OD bit: 1) In the cache parity error handler (arch/mips/mm/cex-gen.S) -- as the panic() call follows shortly, probably CACHE.OD=0 doesn't matter much at this point;
   2) In arch/mips/au1000/common/sleeper.S, when the CPU regs are being
restored on wakeup; Au1x00 PM code in 2.6 seem to have fallen out of
maintenance, and the stack frame set up there seemed just erratic (2.4
situation in this module is much better).
   I didn't touch both those places. And of course, this CONFIG.OD bug is
present in 2.4 kernels as well...

      Posting the reworked patch at last -- this variant doesn't include
machine-specific header into arch/mips/c-r4k.c...

WBR, Sergei

Signed-off-by: Konstantin Baidarov <>
Signed-off-by: Sergei Shtylyov <>

diff --git a/arch/mips/au1000/common/setup.c b/arch/mips/au1000/common/setup.c
index 08c8c85..e36289b 100644
--- a/arch/mips/au1000/common/setup.c
+++ b/arch/mips/au1000/common/setup.c
@@ -143,6 +143,17 @@ void __init plat_setup(void)
        au_writel(0, SYS_TOYTRIM);
+ * Fix up write-only Config[OD] bit after a write to that register. Since the
+ * bit always reads as 0 on those SOC revs that require it to be set to fight
+ * the various errata, we need to set it back to 1...
+ */
+void au1x00_fixup_config_od(void)
+       if (cur_cpu_spec[0]->cpu_od)
+               set_c0_config(1<<19);
 #if defined(CONFIG_64BIT_PHYS_ADDR)
 /* This routine should be valid for all Au1x based boards */
 phys_t __fixup_bigphys_addr(phys_t phys_addr, phys_t size)
diff --git a/arch/mips/mm/c-r4k.c b/arch/mips/mm/c-r4k.c
index 422b55f..8447699 100644
--- a/arch/mips/mm/c-r4k.c
+++ b/arch/mips/mm/c-r4k.c
@@ -1201,8 +1201,20 @@ static void __init setup_scache(void)
 static inline void coherency_setup(void)
+       extern void au1x00_fixup_config_od(void);
        change_c0_config(CONF_CM_CMASK, CONF_CM_DEFAULT);
+#ifdef CONFIG_SOC_AU1X00
+       /*
+        * c0_config.od (bit 19) is write only (and reads as 0) on many early
+        * revs of AMD Au1x00 SOCs. It disables the bus transaction overlapping
+        * and needs to be set to correct the various errata. So if it has been
+        * set by the board setup code we must leave it set...
+        */
+       au1x00_fixup_config_od();
         * specifies that updates by the sc instruction use
         * the coherency mode specified by the TLB; 1 means cachable

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