kmalloc and uncached memory

[Lin Ming]

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)?

Thanks.

>
> Laura
>
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