On 10/11/2014 11:55 AM, Daniel Vetter wrote:
On Sat, Oct 11, 2014 at 01:37:55AM +0530, Sumit Semwal wrote:
At present, struct device lacks a mechanism of exposing memory access constraints for the device.
Consequently, there is also no mechanism to share these constraints while sharing buffers using dma-buf.
If we add support for sharing such constraints, we could use that to try to collect requirements of different buffer-sharing devices to allocate buffers from a pool that satisfies requirements of all such devices.
This is an attempt to add this support; at the moment, only a bitmask is added, but if post discussion, we realise we need more information, we could always extend the definition of constraint.
A new dma-buf op is also added, to allow exporters to interpret or decide on constraint-masks on their own. A default implementation is provided to just AND (&) all the constraint-masks.
What constitutes a constraint-mask could be left for interpretation on a per-platform basis, while defining some common masks.
Signed-off-by: Sumit Semwal sumit.semwal@linaro.org Cc: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: linux-media@vger.kernel.org Cc: dri-devel@lists.freedesktop.org Cc: linaro-mm-sig@lists.linaro.org
Just a few high-level comments, I'm between conference travel but hopefully I can discuss this a bit at plumbers next week.
I agree that for the insane specific cases we need something opaque like the access constraints mask you propose here. But for the normal case I think the existing dma constraints in dma_params would go a long way, and I think we should look at Rob's RFC from aeons ago to solve those:
https://lkml.org/lkml/2012/7/19/285
With this we should be able to cover the allocation constraints of 90% of all cases hopefully.
I'm not sure whether an opaque bitmask is good enough really, I suspect that we also need various priorities between different allocators. With the option that some allocators are flat-out incompatible.
From my experience with Ion, the bitmask is okay if you have only a few types but as soon as there are multiple regions it gets complicated and when you start adding in priority via id it really gets unwieldy.
Thanks, Laura