[PATCHv2 3/9] v4l: add buffer exporting via dmabuf

[Rémi Denis-Courmont]

Le mardi 31 juillet 2012 14:56:14 Laurent Pinchart, vous avez écrit :
> > For that matter, wouldn't it be useful to support exporting a userptr
> > buffer at some point in the future?
> 
> Shouldn't USERPTR usage be discouraged once we get dma-buf support ?

USERPTR, where available, is currently the only way to perform zero-copy from 
kernel to userspace. READWRITE does not support zero-copy at all. MMAP only 
supports zero-copy if userspace knows a boundary on the number of concurrent 
buffers *and* the device can deal with that number of buffers; in general, 
MMAP requires memory copying.

I am not sure DMABUF even supports transmitting data efficiently to userspace. 
In my understanding, it's meant for transmitting data between DSP's bypassing 
userspace entirely, in other words the exact opposite of what USERBUF does.

-- 
Rémi Denis-Courmont
http://www.remlab.net/
http://fi.linkedin.com/in/remidenis