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

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Date:	Tue, 20 Dec 2011 15:41:17 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	"Semwal, Sumit" <sumit.semwal@...com>
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

On Monday 19 December 2011, Semwal, Sumit wrote:
> 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.]

The problem is generally understood for streaming mappings with a 
single device using it: if you have a long-running mapping, you have
to use dma_sync_*. This obviously falls apart if you have multiple
devices and no serialization between the accesses.

If you don't want serialization, that implies that we cannot have
use the  dma_sync_* API on the buffer, which in turn implies that
we cannot have streaming mappings. I think that's ok, but then
you have to bring back the mmap API on the buffer if you want to
allow any driver to provide an mmap function for a shared buffer.

	Arnd
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