Re: [RFC PATCH v2] dmabuf-sync: Introduce buffer synchronization framework

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Tue, Jun 18, 2013 at 06:04:44PM +0900, Inki Dae wrote:
> 
> 
> > -----Original Message-----
> > From: Russell King - ARM Linux [mailto:linux@arm.linux.org.uk]
> > Sent: Tuesday, June 18, 2013 5:43 PM
> > To: Inki Dae
> > Cc: 'Maarten Lankhorst'; 'linux-fbdev'; 'Kyungmin Park'; 'DRI mailing
> > list'; 'Rob Clark'; 'myungjoo.ham'; 'YoungJun Cho'; 'Daniel Vetter';
> > linux-arm-kernel@lists.infradead.org; linux-media@vger.kernel.org
> > Subject: Re: [RFC PATCH v2] dmabuf-sync: Introduce buffer synchronization
> > framework
> > 
> > On Tue, Jun 18, 2013 at 02:27:40PM +0900, Inki Dae wrote:
> > > So I'd like to ask for other DRM maintainers. How do you think about it?
> > it
> > > seems like that Intel DRM (maintained by Daniel), OMAP DRM (maintained
> > by
> > > Rob) and GEM CMA helper also have same issue Russell pointed out. I
> > think
> > > not only the above approach but also the performance is very important.
> > 
> > CMA uses coherent memory to back their buffers, though that might not be
> > true of memory obtained from other drivers via dma_buf.  Plus, there is
> > no support in the CMA helper for exporting or importng these buffers.
> > 
> 
> It's not so. Please see Dave's drm next. recently dmabuf support for the CMA
> helper has been merged to there.

The point stands: CMA is DMA coherent memory.  It doesn't need and must
never be dma-map-sg'd or dma-sync'd or dma-unmap'd.