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

-----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:

quoted

So I'd like to ask for other DRM maintainers. How do you think about it?

it

quoted

seems like that Intel DRM (maintained by Daniel), OMAP DRM (maintained

by

quoted

Rob) and GEM CMA helper also have same issue Russell pointed out. I

think

quoted

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.

I guess Intel i915 is only used on x86, which is a coherent platform and
requires no cache maintanence for DMA.

OMAP DRM does not support importing non-DRM buffers buffers back into

Correct. TODO yet.

DRM.  Moreover, it will suffer from the problems I described if any
attempt is made to write to the buffer after it has been re-imported.

Lastly, I should point out that the dma_buf stuff is really only useful
when you need to export a dma buffer from one driver and import it into
another driver - for example to pass data from a camera device driver to

Most people know that.

a display device driver.  It shouldn't be used within a single driver
as a means of passing buffers between userspace and kernel space.

What I try to do is not really such ugly thing. What I try to do is to
notify that, when CPU tries to access a buffer , to kernel side through
dmabuf interface. So it's not really to send the buffer to kernel.

Thanks,
Inki Dae