linux-kernel - Re: [PATCH v5 6/6] swiotlb: Remove pointless stride adjustment for allocations >= PAGE

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Date: Thu, 29 Feb 2024 16:47:56 +0100
From: Christoph Hellwig <hch@....de>
To: Michael Kelley <mhklinux@...look.com>
Cc: Christoph Hellwig <hch@....de>, Will Deacon <will@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Petr Tesarik <petr.tesarik1@...wei-partners.com>,
	"kernel-team@...roid.com" <kernel-team@...roid.com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Dexuan Cui <decui@...rosoft.com>,
	Nicolin Chen <nicolinc@...dia.com>
Subject: Re: [PATCH v5 6/6] swiotlb: Remove pointless stride adjustment for
 allocations >= PAGE_SIZE

On Thu, Feb 29, 2024 at 03:44:11PM +0000, Michael Kelley wrote:
> Any thoughts on how that historical behavior should apply if
> the DMA min_align_mask is non-zero, or the alloc_align_mask
> parameter to swiotbl_tbl_map_single() is non-zero? As currently
> used, alloc_align_mask is page aligned if the IOMMU granule is
> >= PAGE_SIZE. But a non-zero min_align_mask could mandate
> returning a buffer that is not page aligned. Perhaps do the
> historical behavior only if alloc_align_mask and min_align_mask
> are both zero?

I think the driver setting min_align_mask is a clear indicator
that the driver requested a specific alignment and the defaults
don't apply.  For swiotbl_tbl_map_single as used by dma-iommu
I'd have to tak a closer look at how it is used.