linux-kernel - Re: [RFCv3 2/2] dma-buf: add helpers for sharing attacher constraints with dma-parms

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Date:	Tue, 3 Feb 2015 10:19:32 -0500
From:	Rob Clark <robdclark@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Arnd Bergmann <arnd@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
	DRI mailing list <dri-devel@...ts.freedesktop.org>,
	Linaro MM SIG Mailman List <linaro-mm-sig@...ts.linaro.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	Tomasz Stanislawski <stanislawski.tomasz@...glemail.com>,
	Robin Murphy <robin.murphy@....com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Daniel Vetter <daniel@...ll.ch>
Subject: Re: [RFCv3 2/2] dma-buf: add helpers for sharing attacher constraints
 with dma-parms

On Tue, Feb 3, 2015 at 9:41 AM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Tue, Feb 03, 2015 at 03:17:27PM +0100, Arnd Bergmann wrote:
>> On Tuesday 03 February 2015 09:04:03 Rob Clark wrote:
>> > Since I'm stuck w/ an iommu, instead of built in mmu, my plan was to
>> > drop use of dma-mapping entirely (incl the current call to dma_map_sg,
>> > which I just need until we can use drm_cflush on arm), and
>> > attach/detach iommu domains directly to implement context switches.
>> > At that point, dma_addr_t really has no sensible meaning for me.
>>
>> I think what you see here is a quite common hardware setup and we really
>> lack the right abstraction for it at the moment. Everybody seems to
>> work around it with a mix of the dma-mapping API and the iommu API.
>> These are doing different things, and even though the dma-mapping API
>> can be implemented on top of the iommu API, they are not really compatible.
>
> I'd go as far as saying that the "DMA API on top of IOMMU" is more
> intended to be for a system IOMMU for the bus in question, rather
> than a device-level IOMMU.
>
> If an IOMMU is part of a device, then the device should handle it
> (maybe via an abstraction) and not via the DMA API.  The DMA API should
> be handing the bus addresses to the device driver which the device's
> IOMMU would need to generate.  (In other words, in this circumstance,
> the DMA API shouldn't give you the device internal address.)

if the dma_addr_t becomes the address upstream of the iommu (in
practice, the phys addr), that would, I think, address my concerns
about dma_addr_t

BR,
-R

> --
> FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up
> according to speedtest.net.
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