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From: Michael Kelley <mhklinux@outlook.com>
To: Will Deacon <will@kernel.org>, "linux-kernel@vger.kernel.org"
	<linux-kernel@vger.kernel.org>
CC: "kernel-team@android.com" <kernel-team@android.com>,
	"iommu@lists.linux.dev" <iommu@lists.linux.dev>, Christoph Hellwig
	<hch@lst.de>, Marek Szyprowski <m.szyprowski@samsung.com>, Robin Murphy
	<robin.murphy@arm.com>, Petr Tesarik <petr.tesarik1@huawei-partners.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
Thread-Topic: [PATCH v5 6/6] swiotlb: Remove pointless stride adjustment for
 allocations >= PAGE_SIZE
Thread-Index: AQHaakuep9s0mIf3eE+om+sQlmyZtbEg06/Q
Date: Thu, 29 Feb 2024 06:07:32 +0000
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References: <20240228133930.15400-1-will@kernel.org>
 <20240228133930.15400-7-will@kernel.org>
In-Reply-To: <20240228133930.15400-7-will@kernel.org>
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From: Will Deacon <will@kernel.org> Sent: Wednesday, February 28, 2024 5:40=
 AM
>=20
> For swiotlb allocations >=3D PAGE_SIZE, the slab search historically
> adjusted the stride to avoid checking unaligned slots. However, this is
> no longer needed now that the code around it has evolved and the
> stride is calculated from the required alignment.
>=20
> Either 'alloc_align_mask' is used to specify the allocation alignment or
> the DMA 'min_align_mask' is used to align the allocation with 'orig_addr'=
.
> At least one of these masks is always non-zero.

I think the patch is correct, but this justification is not.  alloc_align_m=
ask
and the DMA min_align_mask are often both zero.  While the NVMe
PCI driver sets min_align_mask, SCSI disk drivers do not (except for the
Hyper-V synthetic SCSI driver).   When both are zero, presumably
there are no alignment requirements, so a stride of 1 is appropriate.

Michael

>=20
> In light of that, remove the redundant (and slightly confusing) check.
>=20
> Link: https://lore.kernel.org/all/SN6PR02MB4157089980E6FC58D5557BCED4572@=
SN6PR02MB4157.namprd02.prod.outlook.com/
> Reported-by: Michael Kelley <mhklinux@outlook.com>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  kernel/dma/swiotlb.c | 7 -------
>  1 file changed, 7 deletions(-)
>=20
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index c381a7ed718f..0d8805569f5e 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -1006,13 +1006,6 @@ static int swiotlb_search_pool_area(struct device
> *dev, struct io_tlb_pool *pool
>  	 */
>  	stride =3D get_max_slots(max(alloc_align_mask, iotlb_align_mask));
>=20
> -	/*
> -	 * For allocations of PAGE_SIZE or larger only look for page aligned
> -	 * allocations.
> -	 */
> -	if (alloc_size >=3D PAGE_SIZE)
> -		stride =3D umax(stride, PAGE_SHIFT - IO_TLB_SHIFT + 1);
> -
>  	spin_lock_irqsave(&area->lock, flags);
>  	if (unlikely(nslots > pool->area_nslabs - area->used))
>  		goto not_found;
> --
> 2.44.0.rc1.240.g4c46232300-goog