linux-kernel - Re: [PATCH v2 3/3] swiotlb: Honour dma_alloc_coherent() alignment in swiotlb

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Date: Thu, 1 Feb 2024 12:52:20 +0000
From: Will Deacon <will@...nel.org>
To: Robin Murphy <robin.murphy@....com>
Cc: linux-kernel@...r.kernel.org, kernel-team@...roid.com,
	iommu@...ts.linux.dev, Christoph Hellwig <hch@....de>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Petr Tesarik <petr.tesarik1@...wei-partners.com>,
	Dexuan Cui <decui@...rosoft.com>
Subject: Re: [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