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 Tue, 22 Jul 2025 20:17:14 +0100
Adrian Larumbe <adrian.larumbe@collabora.com> wrote:

> On 07.07.2025 17:04, Caterina Shablia wrote:
> > From: Boris Brezillon <boris.brezillon@collabora.com>
> >
> > We are going to add flags/properties that will impact the VA merging
> > ability. Instead of sprinkling tests all over the place in
> > __drm_gpuvm_sm_map(), let's add a helper aggregating all these checks
> > can call it for every existing VA we walk through in the
> > __drm_gpuvm_sm_map() loop.
> >
> > Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> > Signed-off-by: Caterina Shablia <caterina.shablia@collabora.com>
> > ---
> >  drivers/gpu/drm/drm_gpuvm.c | 47 +++++++++++++++++++++++++++++--------
> >  1 file changed, 37 insertions(+), 10 deletions(-)
> >
> > 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;
> > +
> > +	/* Order VAs for the rest of the checks. */
> > +	if (a->va.addr > b->va.addr)
> > +		swap(a, b);
> > +
> > +	/* We assume the caller already checked that VAs overlap or are
> > +	 * contiguous.
> > +	 */
> > +	if (drm_WARN_ON(gpuvm->drm, b->va.addr > a->va.addr + a->va.range))
> > +		return false;
> > +
> > +	/* We intentionally ignore u64 underflows because all we care about
> > +	 * here is whether the VA diff matches the GEM offset diff.
> > +	 */
> > +	return b->va.addr - a->va.addr == b->gem.offset - a->gem.offset;  
> 
> If we're reordering the VAs for the rest of the checks, when could underflow happen?

I think this comments predates the re-ordering (I originally tried not
to order VAs).

> 
> > +}
> > +
> >  static int
> >  __drm_gpuvm_sm_map(struct drm_gpuvm *gpuvm,
> >  		   const struct drm_gpuvm_ops *ops, void *priv,
> >  		   const struct drm_gpuvm_map_req *req)
> >  {
> >  	struct drm_gpuva *va, *next;
> > +	struct drm_gpuva reqva = {
> > +		.va.addr = req->va.addr,
> > +		.va.range = req->va.range,
> > +		.gem.offset = req->gem.offset,
> > +		.gem.obj = req->gem.obj,
> > +		.flags = req->flags,
> > +	};
> >  	u64 req_end = req->va.addr + req->va.range;
> >  	int ret;
> >
> > @@ -2116,12 +2152,9 @@ __drm_gpuvm_sm_map(struct drm_gpuvm *gpuvm,
> >  		u64 addr = va->va.addr;
> >  		u64 range = va->va.range;
> >  		u64 end = addr + range;
> > -		bool merge = !!va->gem.obj;
> > +		bool merge = can_merge(gpuvm, va, &reqva);
> >
> >  		if (addr == req->va.addr) {
> > -			merge &= obj == req->gem.obj &&
> > -				 offset == req->gem.offset;
> > -
> >  			if (end == req_end) {
> >  				ret = op_unmap_cb(ops, priv, va, merge);
> >  				if (ret)
> > @@ -2163,8 +2196,6 @@ __drm_gpuvm_sm_map(struct drm_gpuvm *gpuvm,
> >  			};
> >  			struct drm_gpuva_op_unmap u = { .va = va };
> >
> > -			merge &= obj == req->gem.obj &&
> > -				 offset + ls_range == req->gem.offset;
> >  			u.keep = merge;
> >
> >  			if (end == req_end) {
> > @@ -2196,10 +2227,6 @@ __drm_gpuvm_sm_map(struct drm_gpuvm *gpuvm,
> >  				break;
> >  			}
> >  		} else if (addr > req->va.addr) {
> > -			merge &= obj == req->gem.obj &&
> > -				 offset == req->gem.offset +
> > -					   (addr - req->va.addr);
> > -
> >  			if (end == req_end) {
> >  				ret = op_unmap_cb(ops, priv, va, merge);
> >  				if (ret)
> > --
> > 2.47.2  
> 
> 
> Adrian Larumbe