Re: [PATCH 16/16] dma-mapping: use exact allocation in dma_alloc_contiguous

[Robin Murphy]

On 14/06/2019 15:50, 'Christoph Hellwig' wrote:
> On Fri, Jun 14, 2019 at 02:15:44PM +0000, David Laight wrote:
> > Does this still guarantee that requests for 16k will not cross a 16k boundary?
> > It looks like you are losing the alignment parameter.
>
> The DMA API never gave you alignment guarantees to start with,
> and you can get not naturally aligned memory from many of our
> current implementations.

Well, apart from the bit in DMA-API-HOWTO which has said this since 
forever (well, before Git history, at least):

"The CPU virtual address and the DMA address are both
guaranteed to be aligned to the smallest PAGE_SIZE order which
is greater than or equal to the requested size.  This invariant
exists (for example) to guarantee that if you allocate a chunk
which is smaller than or equal to 64 kilobytes, the extent of the
buffer you receive will not cross a 64K boundary."

That said, I don't believe this particular patch should make any 
appreciable difference - alloc_pages_exact() is still going to give back 
the same base address as the rounded up over-allocation would, and 
PAGE_ALIGN()ing the size passed to get_order() already seemed to be 
pointless.

Robin.