On 4/16/2014 11:50 AM, Lin Ming wrote: > On Wed, Apr 16, 2014 at 11:33 AM, Laura Abbott <lauraa@xxxxxxxxxxxxxx> 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 -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>