Re: [PATCH 1/4] ARM64: dma: Add support for NO_KERNEL_MAPPING attribute

[Robin Murphy]

On 08/03/16 09:22, Stephen Boyd wrote:
Both the IOMMU and non-IOMMU allocations don't respect the
NO_KERNEL_MAPPING attribute, therefore drivers can't save virtual
address space and time spent mapping large buffers that are
intended only for userspace. Plumb this attribute into the code
for both types of DMA ops.

I have to say I'm a little dubious about how much time we save by not 
creating one mapping once, and instead going on to repeatedly create and 
tear down lots of mappings - my initial hunch would place that number 
somewhere below 0. Similarly for the degree of pressure on our hundreds 
of gigabytes of vmalloc space, especially on "memory constrained" systems.

Some real data would be a lot more convincing ;)

Robin.

Cc: Robin Murphy <robin.mur...@arm.com>
Cc: Laura Abbott <lau...@codeaurora.org>
Cc: Arnd Bergmann <a...@arndb.de>
Cc: Marek Szyprowski <m.szyprow...@samsung.com>
Cc: Catalin Marinas <catalin.mari...@arm.com>
Cc: Will Deacon <will.dea...@arm.com>
Signed-off-by: Stephen Boyd <stephen.b...@linaro.org>
---
  arch/arm64/mm/dma-mapping.c | 39 ++++++++++++++++++++++++++++++---------
  1 file changed, 30 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c
index 331c4ca6205c..06a593653f23 100644
--- a/arch/arm64/mm/dma-mapping.c
+++ b/arch/arm64/mm/dma-mapping.c
@@ -169,6 +169,9 @@ static void *__dma_alloc(struct device *dev, size_t size,

        /* create a coherent mapping */
        page = virt_to_page(ptr);
+       if (dma_get_attr(DMA_ATTR_NO_KERNEL_MAPPING, attrs))
+               return page;
+
        coherent_ptr = dma_common_contiguous_remap(page, size, VM_USERMAP,
                                                   prot, NULL);
        if (!coherent_ptr)
@@ -194,7 +197,8 @@ static void __dma_free(struct device *dev, size_t size,
        if (!is_device_dma_coherent(dev)) {
                if (__free_from_pool(vaddr, size))
                        return;
-               vunmap(vaddr);
+               if (!dma_get_attr(DMA_ATTR_NO_KERNEL_MAPPING, attrs))
+                       vunmap(vaddr);
        }
        __dma_free_coherent(dev, size, swiotlb_addr, dma_handle, attrs);
  }
@@ -567,6 +571,9 @@ static void *__iommu_alloc_attrs(struct device *dev, size_t 
size,
                if (!pages)
                        return NULL;

+               if (dma_get_attr(DMA_ATTR_NO_KERNEL_MAPPING, attrs))
+                       return pages;
+
                addr = dma_common_pages_remap(pages, size, VM_USERMAP, prot,
                                              __builtin_return_address(0));
                if (!addr)
@@ -624,18 +631,32 @@ static void __iommu_free_attrs(struct device *dev, size_t 
size, void *cpu_addr,
                if (WARN_ON(!area || !area->pages))
                        return;
                iommu_dma_free(dev, area->pages, iosize, &handle);
-               dma_common_free_remap(cpu_addr, size, VM_USERMAP);
+               if (!dma_get_attr(DMA_ATTR_NO_KERNEL_MAPPING, attrs))
+                       dma_common_free_remap(cpu_addr, size, VM_USERMAP);
        } else {
                iommu_dma_unmap_page(dev, handle, iosize, 0, NULL);
                __free_pages(virt_to_page(cpu_addr), get_order(size));
        }
  }

+static struct page **__iommu_get_pages(void *cpu_addr, struct dma_attrs *attrs)
+{
+       struct vm_struct *area;
+
+       if (dma_get_attr(DMA_ATTR_NO_KERNEL_MAPPING, attrs))
+               return cpu_addr;
+
+       area = find_vm_area(cpu_addr);
+       if (area)
+               return area->pages;
+       return NULL;
+}
+
  static int __iommu_mmap_attrs(struct device *dev, struct vm_area_struct *vma,
                              void *cpu_addr, dma_addr_t dma_addr, size_t size,
                              struct dma_attrs *attrs)
  {
-       struct vm_struct *area;
+       struct page **pages;
        int ret;

        vma->vm_page_prot = __get_dma_pgprot(attrs, vma->vm_page_prot,
@@ -644,11 +665,11 @@ static int __iommu_mmap_attrs(struct device *dev, struct 
vm_area_struct *vma,
        if (dma_mmap_from_coherent(dev, vma, cpu_addr, size, &ret))
                return ret;

-       area = find_vm_area(cpu_addr);
-       if (WARN_ON(!area || !area->pages))
+       pages = __iommu_get_pages(cpu_addr, attrs);
+       if (WARN_ON(!pages))
                return -ENXIO;

-       return iommu_dma_mmap(area->pages, size, vma);
+       return iommu_dma_mmap(pages, size, vma);
  }

  static int __iommu_get_sgtable(struct device *dev, struct sg_table *sgt,
@@ -656,12 +677,12 @@ static int __iommu_get_sgtable(struct device *dev, struct 
sg_table *sgt,
                               size_t size, struct dma_attrs *attrs)
  {
        unsigned int count = PAGE_ALIGN(size) >> PAGE_SHIFT;
-       struct vm_struct *area = find_vm_area(cpu_addr);
+       struct page **pages = __iommu_get_pages(cpu_addr, attrs);

-       if (WARN_ON(!area || !area->pages))
+       if (WARN_ON(!pages))
                return -ENXIO;

-       return sg_alloc_table_from_pages(sgt, area->pages, count, 0, size,
+       return sg_alloc_table_from_pages(sgt, pages, count, 0, size,
                                         GFP_KERNEL);
  }