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

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Date:	Wed, 21 Dec 2011 20:04:33 +0100
From:	Daniel Vetter <daniel@...ll.ch>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Daniel Vetter <daniel@...ll.ch>, linux@....linux.org.uk,
	"Semwal, Sumit" <sumit.semwal@...com>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, linux-mm@...ck.org,
	Rob Clark <robdclark@...il.com>,
	linux-arm-kernel@...ts.infradead.org,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-media@...r.kernel.org
Subject: Re: [Linaro-mm-sig] [RFC v2 1/2] dma-buf: Introduce dma buffer
 sharing mechanism

On Wed, Dec 21, 2011 at 05:27:16PM +0000, Arnd Bergmann wrote:
> On Tuesday 20 December 2011, Daniel Vetter wrote:
> > It also sounds like that at least for proper userspace mmap support we'd
> > need some dma api extensions on at least arm, and that might take a while
> > ...
> 
> I think it's actually the opposite -- you'd need dma api extensions on
> everything else other than arm, which already has dma_mmap_coherent()
> and dma_mmap_writecombine() for this purpose.

Yeah, that's actually what I wanted to say, but failed at ... Another
thing is that at least for i915, _writecombine isn't what we want actually
because:
- It won't work anyway cause i915 maps stuff cached and does the flushing
  itself and x86 PAT doesn't support mixed mappings (kinda like arm).
- It isn't actually enough, there's another hidden buffer between the
  memory controller interface and the gpu that i915 manually flushes
  (because even a readback on a wc mapping doesn't flush things in there).

So I assume we'll have plenty of funny beating out a good api for cpu
access ;-)

Cheers, Daniel
-- 
Daniel Vetter
Mail: daniel@...ll.ch
Mobile: +41 (0)79 365 57 48
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