Re: [Linaro-mm-sig] [RFC 1/4] dma-buf: Add constraints sharing information

[Laura Abbott]

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@xxxxxxxxxx>
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Cc: linux-media@xxxxxxxxxxxxxxx
> > Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > Cc: linaro-mm-sig@xxxxxxxxxxxxxxxx
>
> 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


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