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

[Benjamin Gaignard]

For me dmabuf and cenalloc offer two different features: one is buffer
sharing (dmabuf) and one is buffer allocation (cenalloc).

You may want to use dmabuf sharing feature whithout need of the buffer
allocation feature, that is what for drm, v4l2, ION and other use
dmabuf.

In addition of dmabuf we need something aware of hardware devices
constraints to allocate the buffer, that will be the role of cenalloc.
Like ION, cenalloc will also provide a usespace API to allocate
buffers but , unlike ION, the selection of the allocator will be based
on devices constraints not on heap ID.

In first step I think we can forget the bitmask approach and only use
the current fields of devices structure like coherent_dma_mask,
dma_parms->max_segment_size or dma_parms->segment_boundary_mask to
find matching allocator.

What I propose is to add in allocator ops a match(struct device*)
function which will be call at attache time.
In this way cenalloc could ask to each allocator if it is able to
allocate buffer for all attached devices.

Regards,
Benjamin

2014-12-10 14:47 GMT+01:00 Daniel Vetter <daniel at ffwll.ch>:
> On Wed, Dec 10, 2014 at 07:01:16PM +0530, Sumit Semwal wrote:
>> Hi Daniel,
>>
>> Thanks a bunch for your review comments! A few comments, post our
>> discussion at LPC;
>>
>> On 12 October 2014 at 00:25, Daniel Vetter <daniel at ffwll.ch> 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 at linaro.org>
>> >> Cc: linux-kernel at vger.kernel.org
>> >> Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
>> >> Cc: linux-media at vger.kernel.org
>> >> Cc: dri-devel at lists.freedesktop.org
>> >> Cc: linaro-mm-sig at 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.
>>
>> Your/Rob's idea to figure out the constraints wrt max number of
>> segments in the sg_list can provide, like you said, maybe 80-90% of
>> the allocation constraints hopefully. The opaque mask should help for
>> the remaining 'crazy' cases, so I'll be glad to merge Rob's and my
>> approach on defining the constraints.
>>
>> I should think a little bit more about the priority idea that you
>> propose here (and in another patch), but atm I am unable to see how
>> that could help solve the finding-out-constraints problem.
>> >
>> > - The big bummer imo with ION is that it fully side-steps, but this
>> >   proposal here also seems to add entirely new allocators. My rough idea
>>
>> This proposal does borrow this bit from ION, but once we have the
>> required changes done in the dma api itself, the allocators can just
>> become shims to the dma api allocators (eg dma_alloc_coherent etc) for
>> cases where they can be used directly, while leaving provision for any
>> crazy platform-specific allocators, without the userspace having to
>> worry about it.
>>
>> >   was that at allocate/attach time we iterate over all attached devices
>> >   like in Rob's patch and compute the most constrained allocation
>> >   requirements. Then we pick the underlying dma api allocator for these
>> >   constraints. That probably means that we need to open up the dma api a
>> >   bit. But I guess for a start we could simply try to allocate from the
>> >   most constrained device. Together with the opaque bits you propose here
>> >   we could even map additional crazy requirements like that an allocation
>> >   must come from a specific memory bank (provided by a special-purpose CMA
>> >   region). That might also mean that requirements are exclusive and no
>> >   allocation is possible.
>> >
>> My idea was a little variation on what you said here - rather than do
>> compute the most constraint allocation 'after' devices have attached
>> (and right now, we don't really have a way to know that - but that's
>> another point), I'd proposed to do the compute on each attach request,
>> so the requesting drivers can know immediately if the attachment will
>> not work for the other currently attached devices.
>
> Well I said allocate/attach ;-) But yeah if we check at attach and reject
> anything that doesn't work then there's no need to check again when
> allocating, it /should/ work. But perhaps good to be paranoid and check
> again.
>
>> > - I'm not sure we should allow drivers to override the access constraint
>> >   checks really - the dma_buf interfaces already provide this possibility
>> >   through the ->attach callback. In there exporters are allowed to reject
>> >   the attachment for any reason whatsover.
>> >
>> This override the access constraint check is again meant only as a
>> helper, but I could sure drop it.
>>
>> > - I think we should at least provide a helper implementation to allocate
>> >   dma-buffers for multiple devices using the dma constraints logic we
>> >   implement here. I think we should even go as far as providing a default
>> >   implementation for dma-bufs which uses dma_alloc_coherent and this new
>> >   dma contstraints computation code internally. This should be good enough
>>
>> Ok, my idea was to keep the allocation helpers separate from dma-buf
>> framework - hence the cenalloc idea; if it seems like an extremely
>> terrible approach to separate out helpers, I could try and do an RFC
>> based on your idea.
>
> Oh, I like helpers, it'd just put them into the dma-buf code and integrate
> it directly instead of creating something separate.
>
>> >   for almost all devices, except those that do crazy stuff like swap
>> >   support of buffer objects (gem/ttm), virtual hardware buffers (vmwgfx)
>> >   or have other special needs (e.g. non-coherent buffers as speed
>> >   optimization).
>> >
>> Cenalloc type of idea could allow for these special needs I think!
>
> Well imo we should aim for 90% first, fix out fallout and then reasses
> what's needed. Tends to leat to better design overall.
> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch
>
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-- 
Benjamin Gaignard

Graphic Working Group

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