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 09:04:03 -0500
From:	Rob Clark <robdclark@...il.com>
To:	Rob Clark <robdclark@...il.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	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>
Cc:	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 2:48 AM, Daniel Vetter <daniel@...ll.ch> 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..
>
> Hm, what does per-process/context page tables have to do here? At least on
> i915 we have a two levels of page tables:
> - first level for vm/device isolation, used through dma api
> - 2nd level for per-gpu-context isolation and context switching, handled
>   internally.
>
> Since atm the dma api doesn't have any context of contexts or different
> pagetables, I don't see who you could use that at all.

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.

BR,
-R

> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch
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