kmalloc and uncached memory

[Laura Abbott]

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.

Laura

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