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Date: Thu, 29 Feb 2024 16:47:56 +0100
From: Christoph Hellwig <hch@lst.de>
To: Michael Kelley <mhklinux@outlook.com>
Cc: Christoph Hellwig <hch@lst.de>, Will Deacon <will@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Petr Tesarik <petr.tesarik1@huawei-partners.com>,
	"kernel-team@android.com" <kernel-team@android.com>,
	"iommu@lists.linux.dev" <iommu@lists.linux.dev>,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	Robin Murphy <robin.murphy@arm.com>,
	Dexuan Cui <decui@microsoft.com>,
	Nicolin Chen <nicolinc@nvidia.com>
Subject: Re: [PATCH v5 6/6] swiotlb: Remove pointless stride adjustment for
 allocations >= PAGE_SIZE
Message-ID: <20240229154756.GA10137@lst.de>
References: <20240228133930.15400-1-will@kernel.org> <20240228133930.15400-7-will@kernel.org> <SN6PR02MB4157A62353559DA8DB8BC4ADD45F2@SN6PR02MB4157.namprd02.prod.outlook.com> <SN6PR02MB41577D09E97B1D9645369D58D45F2@SN6PR02MB4157.namprd02.prod.outlook.com> <20240229133346.GA7177@lst.de> <SN6PR02MB4157314F142D05E279B7991ED45F2@SN6PR02MB4157.namprd02.prod.outlook.com>
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User-Agent: Mutt/1.5.17 (2007-11-01)

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.