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

From: Sergei Shtylylov <[email-masked]>
Date: Thu, 29 Dec 2005 01:25:57 +0300

Hello

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 <[email-masked]>
>> Acked-by: Jordan Crouse <[email-masked]>

>    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 <[email-masked]>
Signed-off-by: Sergei Shtylyov <[email-masked]>

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();
+#endif
+
 	/*
 	 * c0_status.cu=0 specifies that updates by the sc instruction use
 	 * the coherency mode specified by the TLB; 1 means cachable