On Wed, Apr 16, 2014 at 12:03 PM, Laura Abbott <lauraa@xxxxxxxxxxxxxx> wrote: > 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. Thanks for the explanation. > > What's the reason you can't use dma_alloc_coherent? I'm actually testing WIFI RX performance on a ARM based AP. WIFI to Ethernet traffic, that is WIFI driver RX packets and then Ethernet driver TX packets. I used dma_alloc_coherent() to allocate uncached buffer in WIFI driver to receive packets. But then Ethernet driver can't send packets successfully. If I used kmalloc() to allocate buffers in WIFI driver, then everything is OK. I know this is too platform/drivers specific problem, but any suggestion would be appreciated. Thanks. > > 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>