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

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Date:	Mon, 2 Feb 2015 17:36:10 -0500
From:	Rob Clark <robdclark@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	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-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.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 Mon, Feb 2, 2015 at 4:46 PM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Mon, Feb 02, 2015 at 03:30:21PM -0500, Rob Clark wrote:
>> On Mon, Feb 2, 2015 at 11:54 AM, Daniel Vetter <daniel@...ll.ch> wrote:
>> >> My initial thought is for dma-buf to not try to prevent something than
>> >> an exporter can actually do.. I think the scenario you describe could
>> >> be handled by two sg-lists, if the exporter was clever enough.
>> >
>> > That's already needed, each attachment has it's own sg-list. After all
>> > there's no array of dma_addr_t in the sg tables, so you can't use one sg
>> > for more than one mapping. And due to different iommu different devices
>> > can easily end up with different addresses.
>>
>>
>> Well, to be fair it may not be explicitly stated, but currently one
>> should assume the dma_addr_t's in the dmabuf sglist are bogus.  With
>> gpu's that implement per-process/context page tables, I'm not really
>> sure that there is a sane way to actually do anything else..
>
> That's incorrect - and goes dead against the design of scatterlists.

yeah, a bit of an abuse, although I'm not sure I see a much better way
when a device vaddr depends on user context..

> Not only that, but it is entirely possible that you may get handed
> memory via dmabufs for which there are no struct page's associated
> with that memory - think about display systems which have their own
> video memory which is accessible to the GPU, but it isn't system
> memory.

well, I guess anyways when it comes to sharing buffers, it won't be
the vram placement of the bo that gets shared ;-)

BR,
-R

> In those circumstances, you have to use the dma_addr_t's and not the
> pages.
>
> --
> FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up
> according to speedtest.net.
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