Re: [PATCH 1/2] dma-mapping: introduce dma_get_seg_boundary_nr_pages()

[Andy Shevchenko]

On Wed, Sep 2, 2020 at 1:20 AM Nicolin Chen <nicoleotsuka@xxxxxxxxx> wrote:
>
> We found that callers of dma_get_seg_boundary mostly do an ALIGN
> with page mask and then do a page shift to get number of pages:
>     ALIGN(boundary + 1, 1 << shift) >> shift
>
> However, the boundary might be as large as ULONG_MAX, which means
> that a device has no specific boundary limit. So either "+ 1" or
> passing it to ALIGN() would potentially overflow.
>
> According to kernel defines:
>     #define ALIGN_MASK(x, mask) (((x) + (mask)) & ~(mask))
>     #define ALIGN(x, a) ALIGN_MASK(x, (typeof(x))(a) - 1)
>
> We can simplify the logic here into a helper function doing:
>   ALIGN(boundary + 1, 1 << shift) >> shift
> = ALIGN_MASK(b + 1, (1 << s) - 1) >> s
> = {[b + 1 + (1 << s) - 1] & ~[(1 << s) - 1]} >> s
> = [b + 1 + (1 << s) - 1] >> s
> = [b + (1 << s)] >> s
> = (b >> s) + 1
>
> This patch introduces and applies dma_get_seg_boundary_nr_pages()
> as an overflow-free helper for the dma_get_seg_boundary() callers
> to get numbers of pages. It also takes care of the NULL dev case
> for non-DMA API callers.

...

> +static inline unsigned long dma_get_seg_boundary_nr_pages(struct device *dev,
> +               unsigned int page_shift)
> +{
> +       if (!dev)
> +               return (U32_MAX >> page_shift) + 1;
> +       return (dma_get_seg_boundary(dev) >> page_shift) + 1;

Can it be better to do something like
  unsigned long boundary = dev ? dma_get_seg_boundary(dev) : U32_MAX;

  return (boundary >> page_shift) + 1;

?

> +}

-- 
With Best Regards,
Andy Shevchenko