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 08:48:56 +0100
From:	Daniel Vetter <daniel@...ll.ch>
To:	Rob Clark <robdclark@...il.com>
Cc:	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>,
	Daniel Vetter <daniel@...ll.ch>
Subject: Re: [RFCv3 2/2] dma-buf: add helpers for sharing attacher
 constraints with dma-parms

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