kmalloc and uncached memory

[Laura Abbott]

On 4/16/2014 11:50 AM, Lin Ming wrote:
> On Wed, Apr 16, 2014 at 11:33 AM, Laura Abbott <lauraa at codeaurora.org> wrote:
>> On 4/16/2014 11:11 AM, Lin Ming wrote:
>>> Hi Peter,
>>>
>>> I have a performance problem(on ARM board) that cpu is very bus at
>>> cache invalidation.
>>> So I'm trying to alloc an uncached memory to eliminate cache invalidation.
>>>
>>> But I also have problem with dma_alloc_coherent().
>>> If I don't use dma_alloc_coherent(), is it OK to use below code to
>>> alloc uncached memory?
>>>
>>> struct page *page;
>>> pgd_t *pgd;
>>> pud_t *pud;
>>> pmd_t *pmd;
>>> pte_t *pte;
>>> void *cpu_addr;
>>> dma_addr_t dma_addr;
>>> unsigned int vaddr;
>>>
>>> cpu_addr = kmalloc(PAGE_SIZE, GFP_KERNEL);
>>> dma_addr = pci_map_single(NULL, cpu_addr, PAGE_SIZE, (int)DMA_FROM_DEVICE);
>>> vaddr = (unsigned int)uncached->cpu_addr;
>>> pgd = pgd_offset_k(vaddr);
>>> pud = pud_offset(pgd, vaddr);
>>> pmd = pmd_offset(pud, vaddr);
>>> pte = pte_offset_kernel(pmd, vaddr);
>>> page = virt_to_page(vaddr);
>>> set_pte_ext(pte, mk_pte(page,  pgprot_dmacoherent(pgprot_kernel)), 0);
>>>
>>> /* This kmalloc memory won't be freed  */
>>>
>>
>> No, that will not work. lowmem pages are mapped with 1MB sections underneath
>> which cannot be (easily) changed at runtime. You really want to be using
>> dma_alloc_coherent here.
> 
> For "lowmem pages", do you mean the first 16M physical memory?
> How about that if I only use highmem pages(>16M)?
> 

By lowmem pages I am referring to the direct mapped kernel area. Highmem refers
to pages which do not have a permanent mapping in the kernel address space. If
you are calling kmalloc with GFP_KERNEL you will be getting a page from the lowmem
region.

What's the reason you can't use dma_alloc_coherent?

Thanks,
Laura

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