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From: Christoph Hellwig <hch@lst.de>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Christoph Hellwig <hch@lst.de>, 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 16/19] dma-iommu: don't depend on CONFIG_DMA_DIRECT_REMAP
Date: Mon, 11 Feb 2019 17:39:36 +0100	[thread overview]
Message-ID: <20190211163936.GA29978@lst.de> (raw)
In-Reply-To: <4698cdca-0e5b-c82f-bb80-3ba0f986c544@arm.com>

On Wed, Feb 06, 2019 at 11:55:49AM +0000, Robin Murphy wrote:
> On 14/01/2019 09:41, Christoph Hellwig wrote:
>> For entirely dma coherent architectures there is no good reason to ever
>> remap dma coherent allocation.
>
> Yes there is, namely assembling large buffers without the need for massive 
> CMA areas and compaction overhead under memory fragmentation. That has 
> always been a distinct concern from the DMA_DIRECT_REMAP cases; they've 
> just been able to share a fair few code paths.

Well, I guess I need to reword this - there is no _requirement_ to
remap.  And x86 has been happy to not remap so far and I see absolutely
no reason to force anyone to remap.

>>  Move all the remap and pool code under
>> CONFIG_DMA_DIRECT_REMAP ifdefs, and drop the Kconfig dependency.
>
> As far as I'm concerned that splits things the wrong way. Logically, 
> iommu_dma_alloc() should always have done its own vmap() instead of just 
> returning the bare pages array, but that was tricky to resolve with the 
> design of having the caller handle everything to do with coherency (forcing 
> the caller to unpick that mapping just to remap it yet again in the 
> noncoherent case didn't seem sensible).

I don't parse this.  In the old code base before this series
iommu_dma_alloc is a relatively low-level helper allocating and mapping
pages.  And that one should have done the remapping, and in fact does
so since ("dma-iommu: refactor page array remap helpers").  It just
happens that the function is now called iommu_dma_alloc_remap.

The new iommu_dma_alloc is the high level entry point that handles
every possible case of different allocations, including those where
we do not have a virtual mapping.

WARNING: multiple messages have this Message-ID (diff)
From: Christoph Hellwig <hch@lst.de>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Joerg Roedel <joro@8bytes.org>, Will Deacon <will.deacon@arm.com>,
	linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
	Christoph Hellwig <hch@lst.de>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 16/19] dma-iommu: don't depend on CONFIG_DMA_DIRECT_REMAP
Date: Mon, 11 Feb 2019 17:39:36 +0100	[thread overview]
Message-ID: <20190211163936.GA29978@lst.de> (raw)
In-Reply-To: <4698cdca-0e5b-c82f-bb80-3ba0f986c544@arm.com>

On Wed, Feb 06, 2019 at 11:55:49AM +0000, Robin Murphy wrote:
> On 14/01/2019 09:41, Christoph Hellwig wrote:
>> For entirely dma coherent architectures there is no good reason to ever
>> remap dma coherent allocation.
>
> Yes there is, namely assembling large buffers without the need for massive 
> CMA areas and compaction overhead under memory fragmentation. That has 
> always been a distinct concern from the DMA_DIRECT_REMAP cases; they've 
> just been able to share a fair few code paths.

Well, I guess I need to reword this - there is no _requirement_ to
remap.  And x86 has been happy to not remap so far and I see absolutely
no reason to force anyone to remap.

>>  Move all the remap and pool code under
>> CONFIG_DMA_DIRECT_REMAP ifdefs, and drop the Kconfig dependency.
>
> As far as I'm concerned that splits things the wrong way. Logically, 
> iommu_dma_alloc() should always have done its own vmap() instead of just 
> returning the bare pages array, but that was tricky to resolve with the 
> design of having the caller handle everything to do with coherency (forcing 
> the caller to unpick that mapping just to remap it yet again in the 
> noncoherent case didn't seem sensible).

I don't parse this.  In the old code base before this series
iommu_dma_alloc is a relatively low-level helper allocating and mapping
pages.  And that one should have done the remapping, and in fact does
so since ("dma-iommu: refactor page array remap helpers").  It just
happens that the function is now called iommu_dma_alloc_remap.

The new iommu_dma_alloc is the high level entry point that handles
every possible case of different allocations, including those where
we do not have a virtual mapping.

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  reply	other threads:[~2019-02-11 16:39 UTC|newest]

Thread overview: 79+ 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 ` Christoph Hellwig
2019-01-14  9:41 ` 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
2019-01-14  9:41   ` Christoph Hellwig
2019-02-01 14:22   ` Robin Murphy
2019-02-01 14:22     ` Robin Murphy
2019-02-01 16:12     ` Christoph Hellwig
2019-02-01 16:12       ` Christoph Hellwig
2019-01-14  9:41 ` [PATCH 02/19] dma-iommu: cleanup dma-iommu.h Christoph Hellwig
2019-01-14  9:41   ` Christoph Hellwig
2019-02-01 14:47   ` Robin Murphy
2019-02-01 14:47     ` Robin Murphy
2019-02-01 16:13     ` Christoph Hellwig
2019-02-01 16:13       ` Christoph Hellwig
2019-02-06 15:08       ` Robin Murphy
2019-02-06 15:08         ` Robin Murphy
2019-02-06 15:08         ` Robin Murphy
2019-02-11 15:59         ` Christoph Hellwig
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
2019-01-14  9:41   ` Christoph Hellwig
2019-02-01 15:24   ` Robin Murphy
2019-02-01 15:24     ` Robin Murphy
2019-02-01 16:16     ` Christoph Hellwig
2019-02-01 16:16       ` Christoph Hellwig
2019-02-06 15:28       ` Robin Murphy
2019-02-06 15:28         ` Robin Murphy
2019-02-11 16:00         ` Christoph Hellwig
2019-02-11 16:00           ` Christoph Hellwig
2019-01-14  9:41 ` [PATCH 04/19] dma-iommu: remove the flush_page callback Christoph Hellwig
2019-01-14  9:41   ` Christoph Hellwig
2019-02-01 15:28   ` Robin Murphy
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   ` Christoph Hellwig
2019-01-14  9:41 ` [PATCH 06/19] dma-iommu: fix and refactor iommu_dma_mmap Christoph Hellwig
2019-01-14  9:41   ` Christoph Hellwig
2019-01-14  9:41   ` Christoph Hellwig
2019-02-05 15:02   ` Robin Murphy
2019-02-05 15:02     ` Robin Murphy
2019-02-11 16:03     ` Christoph Hellwig
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   ` Christoph Hellwig
2019-01-14  9:41 ` [PATCH 08/19] dma-iommu: move __iommu_dma_map Christoph Hellwig
2019-01-14  9:41   ` Christoph Hellwig
2019-01-14  9:41 ` [PATCH 09/19] dma-iommu: refactor page array remap helpers Christoph Hellwig
2019-01-14  9:41   ` 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   ` Christoph Hellwig
2019-01-14  9:41 ` [PATCH 11/19] dma-iommu: factor contiguous " Christoph Hellwig
2019-01-14  9:41   ` Christoph Hellwig
2019-01-14  9:41 ` [PATCH 12/19] dma-iommu: refactor iommu_dma_free Christoph Hellwig
2019-01-14  9:41   ` 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   ` 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   ` Christoph Hellwig
2019-01-14  9:41 ` [PATCH 15/19] dma-iommu: refactor iommu_dma_alloc Christoph Hellwig
2019-01-14  9:41   ` Christoph Hellwig
2019-01-14  9:41 ` [PATCH 16/19] dma-iommu: don't depend on CONFIG_DMA_DIRECT_REMAP Christoph Hellwig
2019-01-14  9:41   ` Christoph Hellwig
2019-02-06 11:55   ` Robin Murphy
2019-02-06 11:55     ` Robin Murphy
2019-02-11 16:39     ` Christoph Hellwig [this message]
2019-02-11 16:39       ` Christoph Hellwig
2019-01-14  9:41 ` [PATCH 17/19] dma-iommu: switch copyright boilerplace to SPDX Christoph Hellwig
2019-01-14  9:41   ` Christoph Hellwig
2019-02-06 11:57   ` Robin Murphy
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
2019-01-14  9:41   ` Christoph Hellwig
2019-02-06 12:19   ` Robin Murphy
2019-02-06 12:19     ` Robin Murphy
2019-01-14  9:41 ` [PATCH 19/19] arm64: trim includes " Christoph Hellwig
2019-01-14  9:41   ` Christoph Hellwig
2019-01-28  7:53 ` implement generic dma_map_ops for IOMMUs Christoph Hellwig
2019-01-28  7:53   ` Christoph Hellwig

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