Re: [PATCH v6 1/3] iommu: Implement common IOMMU ops for DMA mapping

From: Tomasz Figa <tfiga@chromium.org>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: Robin Murphy <robin.murphy@arm.com>,
	Daniel Kurtz <djkurtz@chromium.org>,
	Lin PoChun <pochun.lin@mediatek.com>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	Yingjoe Chen <yingjoe.chen@mediatek.com>,
	Will Deacon <will.deacon@arm.com>,
	linux-media@vger.kernel.org, Thierry Reding <treding@nvidia.com>,
	"open list:IOMMU DRIVERS" <iommu@lists.linux-foundation.org>,
	"Bobby Batacharia (via Google Docs)" <Bobby.Batacharia@arm.com>,
	Kyungmin Park <kyungmin.park@samsung.com>,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	Yong Wu <yong.wu@mediatek.com>, Pawel Osciak <pawel@osciak.com>,
	Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>,
	Joerg Roedel <joro@8bytes.org>,
	thunder.leizhen@huawei.com,
	Catalin Marinas <catalin.marinas@arm.com>,
	linux-mediatek@lists.infradead.org
Subject: Re: [PATCH v6 1/3] iommu: Implement common IOMMU ops for DMA mapping
Date: Wed, 4 Nov 2015 14:15:41 +0900	[thread overview]
Message-ID: <CAAFQd5COY-dvBE73R=sUWoGfXR9CvgurGchYgXB6y9eqQ=BBUQ@mail.gmail.com> (raw)
In-Reply-To: <20151103184019.GD8644@n2100.arm.linux.org.uk>

On Wed, Nov 4, 2015 at 3:40 AM, Russell King - ARM Linux
<linux@arm.linux.org.uk> wrote:
> On Tue, Nov 03, 2015 at 05:41:24PM +0000, Robin Murphy wrote:
>> Hi Tomasz,
>>
>> On 02/11/15 13:43, Tomasz Figa wrote:
>> >Agreed. The dma_map_*() API is not guaranteed to return a single
>> >contiguous part of virtual address space for any given SG list.
>> >However it was understood to be able to map buffers contiguously
>> >mappable by the CPU into a single segment and users,
>> >videobuf2-dma-contig in particular, relied on this.
>>
>> I don't follow that - _any_ buffer made of page-sized chunks is going to be
>> mappable contiguously by the CPU; it's clearly impossible for the streaming
>> DMA API itself to offer such a guarantee, because it's entirely orthogonal
>> to the presence or otherwise of an IOMMU.
>
> Tomasz's use of "virtual address space" above in combination with the
> DMA API is really confusing.

I suppose I must have mistakenly use "virtual address space" somewhere
instead of "IO virtual address space". I'm sorry for causing
confusion.

The thing being discussed here is mapping of buffers described by
scatterlists into IO virtual address space, i.e. the operation
happening when dma_map_sg() is called for an IOMMU-enabled device.

Best regards,
Tomasz