Hi Robin,

On Thu, Feb 2, 2017 at 2:15 PM, Robin Murphy [off-list ref] wrote:

On 31/01/17 11:12, Geert Uytterhoeven wrote:

quoted

Add support for allocating physically contiguous DMA buffers on arm64
systems with an IOMMU.  This can be useful when two or more devices
with different memory requirements are involved in buffer sharing.

Note that as this uses the CMA allocator, setting the
DMA_ATTR_FORCE_CONTIGUOUS attribute has a runtime-dependency on
CONFIG_DMA_CMA, just like on arm32.

quoted

@@ -625,7 +651,8 @@ static void __iommu_free_attrs(struct device *dev, size_t size, void *cpu_addr,

      size = PAGE_ALIGN(size);
      /*
-      * @cpu_addr will be one of 3 things depending on how it was allocated:
+      * @cpu_addr will be one of 4 things depending on how it was allocated:
+      * - A remapped array of pages for contiguous allocations.
       * - A remapped array of pages from iommu_dma_alloc(), for all
       *   non-atomic allocations.
       * - A non-cacheable alias from the atomic pool, for atomic

@@ -637,6 +664,12 @@ static void __iommu_free_attrs(struct device *dev, size_t size, void *cpu_addr,
      if (__in_atomic_pool(cpu_addr, size)) {
              iommu_dma_unmap_page(dev, handle, iosize, 0, 0);
              __free_from_pool(cpu_addr, size);
+     } else if (attrs & DMA_ATTR_FORCE_CONTIGUOUS) {
+             struct page *page = phys_to_page(dma_to_phys(dev, handle));

handle is the IOVA returned by the original iommu_dma_map_page() call,
so I can't see this working very well. I was about to naively suggest
virt_to_page(cpu_addr), except of course cpu_addr isn't going to be a
linear map address :(

You're right: upon closer look, this will free the wrong pages in the CMA
allocator. Funny, that the system didn't scream...

Thanks for noticing, I'll look into fixing that...

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds