[PATCH v4 02/40] drm/gpuvm: Allow VAs to hold soft reference to BOs

[Danilo Krummrich]

On Wed, May 14, 2025 at 10:53:16AM -0700, Rob Clark wrote:
> From: Rob Clark <robdclark at chromium.org>
> 
> Eases migration for drivers where VAs don't hold hard references to
> their associated BO, avoiding reference loops.
> 
> In particular, msm uses soft references to optimistically keep around
> mappings until the BO is distroyed.  Which obviously won't work if the
> VA (the mapping) is holding a reference to the BO.

Ick! This is all complicated enough. Allow drivers to bypass the proper
reference counting for GEM objects in the context of VM_BO structures seems like
an insane footgun.

I don't understand why MSM would need weak references here. Why does msm need
that, but nouveau, Xe, panthor, PowerVR do not?