Re: [PATCH v4 4/7] drm/gpuvm: Add a helper to check if two VA can be merged

[Boris Brezillon <boris.brezillon@collabora.com>]

On Mon, 07 Jul 2025 21:06:50 +0200
"Danilo Krummrich" <dakr@kernel.org> wrote:

> On Mon Jul 7, 2025 at 9:00 PM CEST, Danilo Krummrich wrote:
> > On Mon Jul 7, 2025 at 7:04 PM CEST, Caterina Shablia wrote:  
> >> diff --git a/drivers/gpu/drm/drm_gpuvm.c b/drivers/gpu/drm/drm_gpuvm.c
> >> index 05978c5c38b1..dc3c2f906400 100644
> >> --- a/drivers/gpu/drm/drm_gpuvm.c
> >> +++ b/drivers/gpu/drm/drm_gpuvm.c
> >> @@ -2098,12 +2098,48 @@ op_unmap_cb(const struct drm_gpuvm_ops *fn, void *priv,
> >>  	return fn->sm_step_unmap(&op, priv);
> >>  }
> >>  
> >> +static bool can_merge(struct drm_gpuvm *gpuvm, const struct drm_gpuva *a,
> >> +		      const struct drm_gpuva *b)
> >> +{
> >> +	/* Only GEM-based mappings can be merged, and they must point to
> >> +	 * the same GEM object.
> >> +	 */
> >> +	if (a->gem.obj != b->gem.obj || !a->gem.obj)
> >> +		return false;
> >> +
> >> +	/* Let's keep things simple for now and force all flags to match. */
> >> +	if (a->flags != b->flags)
> >> +		return false;  
> 
> Forgot to mention, this can include driver specific flags. How do we know from
> the generic code whether this condition makes sense? *At least* it would need to
> be documented.

You're right, it should have been:

	if ((a->flags & DRM_GPUVA_MERGEABLE_FLAGS_MASK) !=
	    (b->flags & DRM_GPUVA_MERGEABLE_FLAGS_MASK))
		return false;

with DRM_GPUVA_COMMON_FLAGS_MASK set to the set of flags that matter
when merging.

> 
> However, I think it would be better to provide an optional callback for drivers
> to check whether merge makes sense or not. This doesn't mean we need drivers to
> do those common checks, this can remain here in the common code.

Seems a bit premature to me. Again, if there's a need for drivers to add
extra checks we can always add a callback at this point, but until
this is the case, I'd rather stick to these common checks.