linux-kernel - Re: [Linaro-mm-sig] [RFC v2 1/2] dma-buf: Introduce dma buffer sharing mechanism

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Message-ID: <CAB2ybb_dU7BzJmPo6vA92pe1YCNerCLc+bv7Qi_EfkfGaik6bQ@mail.gmail.com>
Date:	Mon, 19 Dec 2011 11:46:40 +0530
From:	"Semwal, Sumit" <sumit.semwal@...com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Daniel Vetter <daniel@...ll.ch>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, linux@....linux.org.uk,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, linux-mm@...ck.org,
	linux-media@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [Linaro-mm-sig] [RFC v2 1/2] dma-buf: Introduce dma buffer
 sharing mechanism

Hi Arnd, Daniel,

On Mon, Dec 12, 2011 at 10:18 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Saturday 10 December 2011, Daniel Vetter wrote:
>> If userspace (through some driver calls)
>> tries to do stupid things, it'll just get garbage. See
>> Message-ID: <CAKMK7uHeXYn-v_8cmpLNWsFY14KtmuRZy8YRKR5Xst2-2WdFSQ@...l.gmail.com>
>> for my reasons why it think this is the right way to go forward. So in
>> essence I'm really interested in the reasons why you want the kernel
>> to enforce this (or I'm completely missing what's the contentious
>> issue here).
>
> This has nothing to do with user space mappings. Whatever user space does,
> you get garbage if you don't invalidate cache lines that were introduced
> through speculative prefetching before you access cache lines that were
> DMA'd from a device.
I didn't see a consensus on whether dma_buf should enforce some form
of serialization within the API - so atleast for v1 of dma-buf, I
propose to 'not' impose a restriction, and we can tackle it (add new
ops or enforce as design?) whenever we see the first need of it - will
that be ok? [I am bending towards the thought that it is a problem to
solve at a bigger platform than dma_buf.]
>
>        Arnd

Best regards,
~Sumit.
>
> --
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