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

Robin Murphy
2018-12-04T16:41:34Z

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@lst.de>
> ---
> 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@arm.com>