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

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Message-ID: <CAF6AEGuzex2Y7+=hP73Sp9CtOnn1t0veHRdzJrs3w8p6b8_rEg@mail.gmail.com>
Date:	Wed, 11 Feb 2015 08:30:52 -0500
From:	Rob Clark <robdclark@...il.com>
To:	Rob Clark <robdclark@...il.com>,
	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>
Cc:	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 7:56 AM, Daniel Vetter <daniel@...ll.ch> wrote:
> 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.

sure, my only point is that if I'm the odd man out, I can live with a
hack (ie. requiring drm/msm to be aware enough of the platform to know
if there is >1 level of address translation and frob'ing my 'struct
device' accordingly)... no point in a generic solution for one user.
I like to be practical.

BR,
-R

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