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From: Robin Murphy <robin.murphy@arm.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Joerg Roedel <joro@8bytes.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	iommu@lists.linux-foundation.org,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 03/19] dma-iommu: don't use a scatterlist in iommu_dma_alloc
Date: Wed, 6 Feb 2019 15:28:28 +0000	[thread overview]
Message-ID: <f59852aa-394c-3165-f7c0-fa6cdcea059f@arm.com> (raw)
In-Reply-To: <20190201161638.GJ6532@lst.de>

On 01/02/2019 16:16, Christoph Hellwig wrote:
> On Fri, Feb 01, 2019 at 03:24:45PM +0000, Robin Murphy wrote:
>> On 14/01/2019 09:41, Christoph Hellwig wrote:
>>> Directly iterating over the pages makes the code a bit simpler and
>>> prepares for the following changes.
>>
>> It also defeats the whole purpose of __iommu_dma_alloc_pages(), so I'm not
>> really buying the simplification angle - you've *seen* that code, right? ;)
> 
> How does it defeat the purpose of __iommu_dma_alloc_pages?

Because if iommu_map() only gets called at PAGE_SIZE granularity, then 
the IOMMU PTEs will be created at PAGE_SIZE (or smaller) granularity, so 
any effort to get higher-order allocations matching larger IOMMU block 
sizes is wasted, and we may as well have just done this:

	for (i = 0; i < count; i++) {
		struct page *page = alloc_page(gfp);
		...
		iommu_map(..., page_to_phys(page), PAGE_SIZE, ...);
	}

Really, it's a shame we have to split huge pages for the CPU remap, 
since in the common case the CPU MMU will have a matching block size, 
but IIRC there was something in vmap() or thereabouts that explicitly 
chokes on them.

Robin.

  reply	other threads:[~2019-02-06 15:28 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-14  9:41 implement generic dma_map_ops for IOMMUs Christoph Hellwig
2019-01-14  9:41 ` [PATCH 01/19] dma-mapping: add a Kconfig symbol to indicated arch_dma_prep_coherent presence Christoph Hellwig
2019-02-01 14:22   ` Robin Murphy
2019-02-01 16:12     ` Christoph Hellwig
2019-01-14  9:41 ` [PATCH 02/19] dma-iommu: cleanup dma-iommu.h Christoph Hellwig
2019-02-01 14:47   ` Robin Murphy
2019-02-01 16:13     ` Christoph Hellwig
2019-02-06 15:08       ` Robin Murphy
2019-02-11 15:59         ` Christoph Hellwig
2019-01-14  9:41 ` [PATCH 03/19] dma-iommu: don't use a scatterlist in iommu_dma_alloc Christoph Hellwig
2019-02-01 15:24   ` Robin Murphy
2019-02-01 16:16     ` Christoph Hellwig
2019-02-06 15:28       ` Robin Murphy [this message]
2019-02-11 16:00         ` Christoph Hellwig
2019-01-14  9:41 ` [PATCH 04/19] dma-iommu: remove the flush_page callback Christoph Hellwig
2019-02-01 15:28   ` Robin Murphy
2019-01-14  9:41 ` [PATCH 05/19] dma-iommu: move the arm64 wrappers to common code Christoph Hellwig
2019-01-14  9:41 ` [PATCH 06/19] dma-iommu: fix and refactor iommu_dma_mmap Christoph Hellwig
2019-02-05 15:02   ` Robin Murphy
2019-02-11 16:03     ` Christoph Hellwig
2019-01-14  9:41 ` [PATCH 07/19] dma-iommu: fix and refactor iommu_dma_get_sgtable Christoph Hellwig
2019-01-14  9:41 ` [PATCH 08/19] dma-iommu: move __iommu_dma_map Christoph Hellwig
2019-01-14  9:41 ` [PATCH 09/19] dma-iommu: refactor page array remap helpers Christoph Hellwig
2019-01-14  9:41 ` [PATCH 10/19] dma-iommu: factor atomic pool allocations into helpers Christoph Hellwig
2019-01-14  9:41 ` [PATCH 11/19] dma-iommu: factor contiguous " Christoph Hellwig
2019-01-14  9:41 ` [PATCH 12/19] dma-iommu: refactor iommu_dma_free Christoph Hellwig
2019-01-14  9:41 ` [PATCH 13/19] dma-iommu: don't remap contiguous allocations for coherent devices Christoph Hellwig
2019-01-14  9:41 ` [PATCH 14/19] dma-iommu: factor contiguous remapped allocations into helpers Christoph Hellwig
2019-01-14  9:41 ` [PATCH 15/19] dma-iommu: refactor iommu_dma_alloc Christoph Hellwig
2019-01-14  9:41 ` [PATCH 16/19] dma-iommu: don't depend on CONFIG_DMA_DIRECT_REMAP Christoph Hellwig
2019-02-06 11:55   ` Robin Murphy
2019-02-11 16:39     ` Christoph Hellwig
2019-01-14  9:41 ` [PATCH 17/19] dma-iommu: switch copyright boilerplace to SPDX Christoph Hellwig
2019-02-06 11:57   ` Robin Murphy
2019-01-14  9:41 ` [PATCH 18/19] arm64: switch copyright boilerplace to SPDX in dma-mapping.c Christoph Hellwig
2019-02-06 12:19   ` Robin Murphy
2019-01-14  9:41 ` [PATCH 19/19] arm64: trim includes " Christoph Hellwig
2019-01-28  7:53 ` implement generic dma_map_ops for IOMMUs Christoph Hellwig

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