Re: Refcounting dma_resv and using that for drm_exec support in TTM

[Thomas Hellström]

Hi, Christian,

On Fri, 2026-07-10 at 20:52 +0200, Christian König wrote:
> Hi everybody,
> 
> The idea of ref-counting dma_resv or ww_mutex came up multiple times
> from
> different people, but so far at least I have abandoned that as to
> complicated to implement considering how widely used that object is.
> 
> Thanks to AI I gave the task to refcount dma_resv to Claude Sonet 4
> just
> to check how horrible it would look like.
> 
> Well turns out that this is actually a cleanup we should most likely
> aim
> for and I'm really wondering why we haven't done it like this in the
> first place.
> 
> Not only resolves it a bunch of issues with dma_resv instances shared
> by
> multiple GEM objects (we just recently had a bunch of patches for
> that on
> the mailing list), but also allows TTM to implement it's delayed
> delete
> handling without any zombie resurrection or similar hacks.
> 
> The patch set is totally work in progress and only survives a smoke
> test
> with amdgpu, but I still wanted to check if the idea is valuable and
> should be looked in further.
> 
> Please comment and/or tear apart :)

Before starting to look at this, in the spirit of trying to aligning on
a "final" solution for ww transactions used in TTM, could we agree on
how we are supposed to pass the importer's ww transaction context to
the exporter's dma_map_attachment() operation to handle exporter
evictions while mapping, because that decides what abstaction we should
use for WW transations within TTM.

drm_exec_init()
xe_bo_validate() // IMporter
dma_buf_map()    // importer;
map_attachment() // callback in exporter
xe_bo_validate() // exporter tries to move exportert bo to VRAM but
runs out of memory.
ttm_bo_evict()   // exporter's evict needs importer's drm_exec

As mentioned a couple of times before, we can't pass a drm_exec to the
exporter because that would make dma-buf depend on drm.

So with this series it would be sufficient to move drm_exec to the dma-
buf layer and rename it, but I still think we should have a plan.

Thanks,
Thomas




> 
> Cheers,
> Christian.