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). -Daniel