On 2 September 2011 21:48, Daniel Vetter <daniel@ffwll.ch> wrote:

On Fri, Sep 02, 2011 at 05:51:37PM +0200, Arnd Bergmann wrote:
> On Friday 02 September 2011, Clark, Rob wrote:
> > > Imho the appeal of this is that there's no preferred party of a shared
> > > buffer (the one the buffer originated from and allocated it), but that is
> > > handled by the dma core (and some platform specific magic for really weird
> > > cases).
> > >
> > > We could even go so far and make the dma_buf creation a real syscall
> > > instead of shoving it into some random ioctls.
> >
> > hmm, I don't quite get why making it a syscall would help..
>
> It was indeed one of the main drivers for the current design to have no
> specific way to create a dma buffer but to let every subsystem handle
> it in its own way. That doesn't prevent you from adding a chardev, file
> system or syscall that only has the purpose of creating dma buffers,
> but it should not be essential to have that.

iirc we've converged on that design because it's simpler and requires
fewer changes in exisiting subsystems. But thinking more about this I'm
not sure anymore whether this is a good trade-off if we want to handle the
buffer negotiation problem. Imo that needs a priviledge/central party for
the buffer creation.

We certainly don't want to implement all that complexity right away, but
should keep it in mind when designing the userspace api. E.g. the central
allocator could easily (kernel-internally) fall back on the currently
discussed scheme by simply allocating the buffer on the first
attach_device (which whould happen through a subsystem specific ioctl).

Ok, so do you think the following as dma_buf_ops would be ok?

- create,

- attach_device,

- rename {get, put}_scatterlist to "buffer_{map, unmap}, adding struct device*

Obviously, the 'release' callback should then take care of doing all book-keeping related to 'de-attaching' struct device* as well.

Then we can have a 'central allocator' device which will use this framework to allow negotiation and sync'ing. Also in absence of this central allocator device, one of the subsystems can take up the role of allocator in the earlier-suggested way?

-Daniel

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