Re: [Xen-devel] [RFC v2] dma-mapping: Use unsigned long for dma_attrs

[Christoph Hellwig <hch@xxxxxxxxxxxxx>]

On Mon, May 30, 2016 at 01:54:06PM +0200, Krzysztof Kozlowski wrote:
> The dma-mapping core and the implementations do not change the
> DMA attributes passed by pointer.  Thus the pointer can point to const
> data.  However the attributes do not have to be a bitfield. Instead
> unsigned long will do fine:
> 
> 1. This is just simpler.  Both in terms of reading the code and setting
>    attributes.  Instead of initializing local attributes on the stack and
>    passing pointer to it to dma_set_attr(), just set the bits.
> 
> 2. It brings safeness and checking for const correctness because the
>    attributes are passed by value.
> 
> Please have in mind that this is RFC, not finished yet.  Only ARM and
> ARM64 are fixed (and not everywhere).
> However other API users also have to be converted which is quite
> intrusive.  I would rather avoid it until the overall approach is
> accepted.

This looks great!  Please continue doing the full conversion.

> +/**
> + * List of possible attributes associated with a DMA mapping. The semantics
> + * of each attribute should be defined in Documentation/DMA-attributes.txt.
> + */
> +#define DMA_ATTR_WRITE_BARRIER               BIT(1)
> +#define DMA_ATTR_WEAK_ORDERING               BIT(2)
> +#define DMA_ATTR_WRITE_COMBINE               BIT(3)
> +#define DMA_ATTR_NON_CONSISTENT              BIT(4)
> +#define DMA_ATTR_NO_KERNEL_MAPPING   BIT(5)
> +#define DMA_ATTR_SKIP_CPU_SYNC               BIT(6)
> +#define DMA_ATTR_FORCE_CONTIGUOUS    BIT(7)
> +#define DMA_ATTR_ALLOC_SINGLE_PAGES  BIT(8)

No really for this patch, but I would much prefer to document them next
to the code in the long run.  Also I really think these BIT() macros
are a distraction compared to the (1 << N) notation.

> +/**
> + * dma_get_attr - check for a specific attribute
> + * @attr: attribute to look for
> + * @attrs: attributes to check within
> + */
> +static inline bool dma_get_attr(unsigned long attr, unsigned long attrs)
> +{
> +     return !!(attr & attrs);
> +}

I'd just kill this helper, much easier to simply open code it in the
caller.

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