RFC: unpinned DMA-buf exporting

[Christian König]

Am 12.03.2018 um 18:24 schrieb Daniel Vetter:
> On Fri, Mar 09, 2018 at 08:11:40PM +0100, Christian K??nig wrote:
>> This set of patches adds an option invalidate_mappings callback to each
>> DMA-buf attachment which can be filled in by the importer.
>>
>> This callback allows the exporter to provided the DMA-buf content
>> without pinning it. The reservation objects lock acts as synchronization
>> point for buffer moves and creating mappings.
>>
>> This set includes an implementation for amdgpu which should be rather
>> easily portable to other DRM drivers.
> Bunch of higher level comments, and one I've forgotten in reply to patch
> 1:
>
> - What happens when a dma-buf is pinned (e.g. i915 loves to pin buffers
>    for scanout)?

When you need to pin an imported DMA-buf you need to detach and reattach 
without the invalidate_mappings callback.

> - pulling the dma-buf implementations into amdgpu makes sense, that's
>    kinda how it was meant to be anyway. The gem prime helpers are a bit too
>    much midlayer for my taste (mostly because nvidia wanted to bypass the
>    EXPORT_SYMBOL_GPL of core dma-buf, hooray for legal bs). We can always
>    extract more helpers once there's more ttm based drivers doing this.

Yeah, I though to abstract that similar to the AGP backend.

Just moving some callbacks around in TTM should be sufficient to 
de-midlayer the whole thing.

Thanks,
Christian.

>
> Overall I like, there's some details to figure out first.
> -Daniel