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SubjectRe: [PATCH v2 3/3] swiotlb: Honour dma_alloc_coherent() alignment in swiotlb_alloc()
On Wed, Jan 31, 2024 at 04:03:38PM +0000, Robin Murphy wrote:
> On 31/01/2024 12:25 pm, Will Deacon wrote:
> > core-api/dma-api-howto.rst states the following properties of
> > dma_alloc_coherent():
> >
> > | 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.
> >
> > However, swiotlb_alloc() passes zero for the 'alloc_align_mask'
> > parameter of swiotlb_find_slots() and so this property is not upheld.
> > Instead, allocations larger than a page are aligned to PAGE_SIZE,
> >
> > Calculate the mask corresponding to the page order suitable for holding
> > the allocation and pass that to swiotlb_find_slots().
>
> I guess this goes back to at least e81e99bacc9f ("swiotlb: Support aligned
> swiotlb buffers") when the explicit argument was added - not sure what we do
> about 5.15 LTS though (unless the answer is to not care...)

Thanks. I'll add the Fixes: tag but, to be honest, if we backport the first
patch then I'm not hugely fussed about this one in -stable kernels simply
because I spotted it my inspection rather than an real failure.

> As before, though, how much of patch #1 is needed if this comes first?

See my reply over there, but I think we need all of this.

Will

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