Re: cache policy of arm mmu

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Hello Jacky,

On Tue, May 29, 2012 at 10:09 AM, h.t.jacky@xxxxxxxxx
<h.t.jacky@xxxxxxxxx> wrote:
> Hi Cunsuo,
>
> Thanks for you prompt reply.
> But I'm just wondering why the other policies(such as writethrough) are
> abandoned, does anyone know the architecture difference?

They are not abandoned from architecture.

If you do a git log -p on arch/arm/mm/mmu.c, this is what we get.

commit 4b46d6416548fb6a0940dfd9911fd895eb6247b3
Author: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx>
Date:   Sun Nov 1 17:44:24 2009 +0000

    ARM: ensure initial page tables are setup for SMP systems

    Mapping the same memory using two different attributes (memory
    type, shareability, cacheability) is unpredictable.  During boot,
    we encounter a situation when we're updating the kernel's page
    tables which can lead to dirty cache lines existing in the cache
    which are subsequently missed.  This causes stack corruption,
    and therefore a crash.

    Therefore, ensure that the shared and cacheability settings
    matches the configuration that will be used later; this together
    with the restriction in early_cachepolicy() ensures that we won't
    create a mismatch during boot.

    Acked-by: Catalin Marinas <catalin.marinas@xxxxxxx>
    Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx>

diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
index 02243ee..ea67be0 100644
--- a/arch/arm/mm/mmu.c
+++ b/arch/arm/mm/mmu.c
@@ -117,6 +117,13 @@ static void __init early_cachepolicy(char **p)
        }
        if (i == ARRAY_SIZE(cache_policies))
                printk(KERN_ERR "ERROR: unknown or unsupported cache policy\n");
+       /*
+        * This restriction is partly to do with the way we boot; it is
+        * unpredictable to have memory mapped using two different sets of
+        * memory attributes (shared, type, and cache attribs).  We can not
+        * change these attributes once the initial assembly has setup the
+        * page tables.
+        */
        if (cpu_architecture() >= CPU_ARCH_ARMv6) {
                printk(KERN_WARNING "Only cachepolicy=writeback
supported on ARMv6 and later\n");
                cachepolicy = CPOLICY_WRITEBACK;

Thanks,
Arun
>
> Cheers,
> Jacky
> Sent from my HTC
>
>
> ----- Reply message -----
> From: "Cunsuo Guo" <cunsuo.guo@xxxxxxxxx>
> To: "Jacky lin" <h.t.jacky@xxxxxxxxx>
> Cc: <kernelnewbies@xxxxxxxxxxxxxxxxx>
> Subject: cache policy of arm mmu
> Date: Tue, May 29, 2012 09:10
>
>
> Hi Jacky,
> I think the information of printk has already tell us the reason.
>
> 2012/5/29 Jacky lin <h.t.jacky@xxxxxxxxx>:
>> Hi all,
>>
>> I found that the cache policy in after ARMv6 is always set as writeback
>> mode
>> in mmu.c.
>>
>> static int __init early_cachepolicy(char *p)
>> {
>>    ...
>>     if (cpu_architecture() >= CPU_ARCH_ARMv6) {
>>         printk(KERN_WARNING "Only cachepolicy=writeback supported on ARMv6
>> and later\n");
>>         cachepolicy = CPOLICY_WRITEBACK;
>>     }
>>    ...
>> }
>>
>> Does anyone know why and provide some documents or links for that?
>>
>> Thanks so much!
>>
>> Cheers,
>> Jacky
>>
>> _______________________________________________
>> Kernelnewbies mailing list
>> Kernelnewbies@xxxxxxxxxxxxxxxxx
>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>
>
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