Pushing image transport logic down the stack

[Michel Dänzer]

On Tue, 2006-09-05 at 06:11 +0200, Soeren Sandmann wrote:
> Keith Packard <keithp at keithp.com> writes:
> 
> > I'm interested in learning how we could create a shared object that
> > referred to existing address space. We already do that from user space
> > to kernel; why not between two processes?
> 
> One thing that should be kept in mind is the interaction with the
> graphics memory manager. In a world where the memory manager is the
> only entity knowing the exact location of graphics buffers, this is
> also the only entity that can actually initiate DMA.
> 
> For some time my view has been that the memory manager should be part
> of a generic kernel graphics driver with these features:
> 
> - Has API for applications to allocate and deallocate buffers. When a
>   buffer is allocated, a globally unique handle is returned to
>   userspace applications who are allowed to share them with
>   eachother. A buffer can be initialized with data from the
>   application'smemory.
> 
> - Applications can submit command buffers. If the commands need to
>   refer to buffers, they pass handles instead along with information
>   about where in the command stream such handles are located. The
>   driver is then responsible for fixing them up.
> 
> - The driver will validate the command buffers to make sure they don't
>   DMA to random places in memory.
> 
> - Individual command buffers are guaranteed to be executed
>   atomically. 
> 
> Given this API, an image transfer between application and X server
> becomes a matter of simply sending a buffer handle. If the application
> is willing to commit to never changing the data, it would never even
> have to be DMA'ed; it could just be mapped into AGP space as needed.

Incidentally, the DRM memory manager that Thomas Hellström is in the
process (on drm-ttm-0-2-branch) of merging to git master addresses a lot
if not all of the above. Now would be a good time to make any comments
you may have about it. :) I'm not sure Thomas is currently reading this
list, so I'm adding him to the CC list.


-- 
Earthling Michel Dänzer           |          http://tungstengraphics.com
Libre software enthusiast         |          Debian, X and DRI developer