On Wed, Oct 26, 2016 at 10:22:44PM +0300, Alexey Brodkin wrote: > ------------------------>8----------------------- > arc_dma_alloc() > ioremap_nocache() AKA ioremap() > ioremap_prot() > get_vm_area() + ioremap_page_range() on obtained vaddr > ------------------------>8----------------------- > > As a result we get TLB entry of the following kind: > ------------------------>8----------------------- > vaddr = 0x7200_0000 > paddr = 0x8200_0000 > flags = _uncached_ > ------------------------>8----------------------- > > Kerenl thinks frame buffer is located @ 0x7200_0000 and uses it > perfectly fine. > > But here comes a time for user-space application to request frame buffer > to be mapped for it. That happens easily with the following call path: > ------------------------>8----------------------- > fb_mmap() > drm_fb_cma_mmap() > dma_mmap_writecombine() AKA dma_mmap_wc() > dma_mmap_attrs() > dma_common_mmap() since we don't [yet] have dma_map_ops.mmap() > for ARC > ------------------------>8----------------------- > > And in dma_common_mmap() we first calculate pfn of what we think is > "physical page" and then do remap_pfn_range() to that "physical page". > > Here we're getting to the interesting thing - how pfn is calculated. > As of now this is done as simple as: > ------------------------>8----------------------- > pfn = page_to_pfn(virt_to_page(cpu_addr)); > ------------------------>8----------------------- The virt_to_page() function here only works for addresses in the kernel linear map. In your case, the DMA buffer is mapped out of the ioremap space, so the cpu_addr you pass in here would return the incorrect pfn (as you've already noticed). > Simplest fix for ARC is to use dma_addr instead because it matches > real physical memory address and so mapping for user-space we're > getting then is this: > ------------------------>8----------------------- > vaddr = 0x0200_0000 > paddr = 0x8200_0000 > flags = _uncached_ > ------------------------>8----------------------- > And it works perfectly fine. But it breaks the other architectures where dma_addr is actually closer to the phys_addr than the kernel linear map. > diff --git a/drivers/base/dma-mapping.c b/drivers/base/dma-mapping.c > index 8f8b68c80986..16307eed453f 100644 > --- a/drivers/base/dma-mapping.c > +++ b/drivers/base/dma-mapping.c > @@ -252,7 +252,7 @@ int dma_common_mmap(struct device *dev, struct vm_area_struct *vma, > #if defined(CONFIG_MMU) && !defined(CONFIG_ARCH_NO_COHERENT_DMA_MMAP) > unsigned long user_count = vma_pages(vma); > unsigned long count = PAGE_ALIGN(size) >> PAGE_SHIFT; > - unsigned long pfn = page_to_pfn(virt_to_page(cpu_addr)); > + unsigned long pfn = page_to_pfn(virt_to_page(dma_addr)); As I said above, this is incorrect. I would suggest that you implement an arc specific mmap operation. We do this for arm64 using remap_pfn_range; see __swiotlb_mmap under arch/arm64/mm/dma-mapping.c where the pfn is calculated using an arm64-specific dma_to_phys() function. -- Catalin
