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

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Date:	Wed, 11 Feb 2015 13:56:46 +0100
From:	Daniel Vetter <daniel@...ll.ch>
To:	Rob Clark <robdclark@...il.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	linux-mm <linux-mm@...ck.org>,
	Robin Murphy <robin.murphy@....com>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	Tomasz Stanislawski <stanislawski.tomasz@...glemail.com>,
	Daniel Vetter <daniel@...ll.ch>
Subject: Re: [RFCv3 2/2] dma-buf: add helpers for sharing attacher
 constraints with dma-parms

On Wed, Feb 11, 2015 at 06:23:52AM -0500, Rob Clark wrote:
> On Wed, Feb 11, 2015 at 6:12 AM, Russell King - ARM Linux
> <linux@....linux.org.uk> wrote:
> > As I've already pointed out, there's a major problem if you have already
> > had a less restrictive attachment which has an active mapping, and a new
> > more restrictive attachment comes along later.
> >
> > It seems from Rob's descriptions that we also need another flag in the
> > importer to indicate whether it wants to have a valid struct page in the
> > scatter list, or whether it (correctly) uses the DMA accessors on the
> > scatter list - so that exporters can reject importers which are buggy.
> 
> to be completely generic, we would really need a way that the device
> could take over only just the last iommu (in case there were multiple
> levels of address translation)..

I still hold that if the dma api steals the iommu your gpu needs for
context switching then that's a bug in the platform setup code. dma api
really doesn't have any concept of switchable hw contexts. So trying to
work around this brokeness by mandating it as a valid dma-buf use-case is
totally backwards.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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