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Re: [Xen-devel] [PATCH 01/23] dma-mapping: provide a generic DMA_MAPPING_ERROR

On 30/11/2018 13:22, Christoph Hellwig wrote:
Error handling of the dma_map_single and dma_map_page APIs is a little
problematic at the moment, in that we use different encodings in the
returned dma_addr_t to indicate an error.  That means we require an
additional indirect call to figure out if a dma mapping call returned
an error, and a lot of boilerplate code to implement these semantics.

Instead return the maximum addressable value as the error.  As long
as we don't allow mapping single-byte ranges with single-byte alignment
this value can never be a valid return.  Additionaly if drivers do
not check the return value from the dma_map* routines this values means
they will generally not be pointed to actual memory.

Once the default value is added here we can start removing the
various mapping_error methods and just rely on this generic check.

Signed-off-by: Christoph Hellwig <hch@xxxxxx>
  include/linux/dma-mapping.h | 6 ++++++
  1 file changed, 6 insertions(+)

diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index 0f81c713f6e9..46bd612d929e 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -133,6 +133,8 @@ struct dma_map_ops {
        u64 (*get_required_mask)(struct device *dev);
+#define DMA_MAPPING_ERROR (~(dma_addr_t)0)
  extern const struct dma_map_ops dma_direct_ops;
  extern const struct dma_map_ops dma_virt_ops;
@@ -576,6 +578,10 @@ static inline int dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
        const struct dma_map_ops *ops = get_dma_ops(dev);
debug_dma_mapping_error(dev, dma_addr);
+       if (dma_addr == DMA_MAPPING_ERROR)
+               return 1;
        if (ops->mapping_error)
                return ops->mapping_error(dev, dma_addr);
        return 0;

I'd have been inclined to put the default check here, i.e.

-       return 0
+       return dma_addr == DMA_MAPPING_ERROR

such that the callback retains full precedence and we don't have to deal with the non-trivial removals immediately if it comes to it. Not that it makes a vast difference though, so either way,

Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>

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