Re: [Xenomai-help] ARM cached,non-cached memory

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Sinisa Denic wrote:
> On Saturday 06 March 2010 11:42:07 you wrote:
>> Sinisa Denic wrote:
>>> Hi, last year working with xenomai 2.4.2-3 we met problem (BUG) with
>>> memory caching on ARM AT91sam9260 arch.
>>> Then we were advised using non-cached memory and we've used it till
>>> now,but in our next application it has big performace slow down effect
>>> and I'm interested is this BUG solved in latest 2.4.10 - xenomai
>>> Regards,Sinisa
>> Actually, there may be something we can do. In heap.c, could you try
>> replacing calls to pgprot_noncached with calls to pgprot_writecombine ?
>>
>> Here is a patch doing this for 2.4 head. I do not know whether it will
>> apply to the version you are using. But I guess you will get the idea.
>>
>> diff --git a/ksrc/nucleus/heap.c b/ksrc/nucleus/heap.c
>> index 1c02183..d40844b 100644
>> --- a/ksrc/nucleus/heap.c
>> +++ b/ksrc/nucleus/heap.c
>> @@ -962,7 +962,7 @@ static inline void *__alloc_and_reserve_heap(size_t
>> size, in t kmflags)
>>                 else
>>                         ptr = __vmalloc(size,
>>                                         GFP_KERNEL | __GFP_HIGHMEM,
>> -                                       pgprot_noncached(PAGE_KERNEL));
>> +                                       pgprot_writecombine(PAGE_KERNEL));
>>                 if (ptr == NULL)
>>                         return NULL;
>>
>> @@ -1126,7 +1126,7 @@ static int xnheap_mmap(struct file *file, struct
>> vm_area_struct *vma) unsigned long maddr = vma->vm_start;
>>
>>                 if (heap->archdep.kmflags == XNHEAP_GFP_NONCACHED)
>> -                       vma->vm_page_prot =
>> pgprot_noncached(vma->vm_page_prot); +                      
>> vma->vm_page_prot = pgprot_writecombine(vma->vm_page_pro t);
>>
>>                 while (size > 0) {
>>                         if (xnarch_remap_vm_page(vma, maddr, vaddr))
>> @@ -1147,7 +1147,7 @@ static int xnheap_mmap(struct file *file, struct
>> vm_area_s truct *vma)
>>         (void)vaddr;
>>         if ((heap->archdep.kmflags & ~XNHEAP_GFP_NONCACHED) != 0 ||
>>             heap->archdep.kmflags == XNHEAP_GFP_NONCACHED)
>> -               vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>> +               vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
>>  #endif /* !CONFIG_MMU */
>>
>>         atomic_inc(&heap->archdep.numaps);
> 
> Ah, thank you Gilles we've already reconciled with destiny not to having 
> cached memory. 
> This is good news,but as I've read "write-combining does not guarantee that 
> the combination of writes and reads is done in the correct order" and we 
> could meet inconsistency in data - maybe.. 
> Does Xeno treat this problem properly?

The ARM kernel routinely uses this mode for shared mapping mapped at
several places in the same address space. So, I think it works. Are you
sure the sentence you quote applies to ARMv5?

-- 
					    Gilles.