[PATCH v3 2/2] drm/gpuvm: Add locking helpers

[Rob Clark]

On Fri, Jun 20, 2025 at 8:45 AM Rob Clark <robin.clark at oss.qualcomm.com> wrote:
>
> For UNMAP/REMAP steps we could be needing to lock objects that are not
> explicitly listed in the VM_BIND ioctl in order to tear-down unmapped
> VAs.  These helpers handle locking/preparing the needed objects.
>
> Note that these functions do not strictly require the VM changes to be
> applied before the next drm_gpuvm_sm_map_lock()/_unmap_lock() call.  In
> the case that VM changes from an earlier drm_gpuvm_sm_map()/_unmap()
> call result in a differing sequence of steps when the VM changes are
> actually applied, it will be the same set of GEM objects involved, so
> the locking is still correct.
>
> v2: Rename to drm_gpuvm_sm_*_exec_locked() [Danilo]
> v3: Expand comments to show expected usage, and explain how the usage
>     is safe in the case of overlapping driver VM_BIND ops.

Danilo, did you have any remaining comments on this?

BR,
-R

> Signed-off-by: Rob Clark <robin.clark at oss.qualcomm.com>
> ---
>  drivers/gpu/drm/drm_gpuvm.c | 126 ++++++++++++++++++++++++++++++++++++
>  include/drm/drm_gpuvm.h     |   8 +++
>  2 files changed, 134 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_gpuvm.c b/drivers/gpu/drm/drm_gpuvm.c
> index 0ca717130541..a811471b888e 100644
> --- a/drivers/gpu/drm/drm_gpuvm.c
> +++ b/drivers/gpu/drm/drm_gpuvm.c
> @@ -2390,6 +2390,132 @@ drm_gpuvm_sm_unmap(struct drm_gpuvm *gpuvm, void *priv,
>  }
>  EXPORT_SYMBOL_GPL(drm_gpuvm_sm_unmap);
>
> +static int
> +drm_gpuva_sm_step_lock(struct drm_gpuva_op *op, void *priv)
> +{
> +       struct drm_exec *exec = priv;
> +
> +       switch (op->op) {
> +       case DRM_GPUVA_OP_REMAP:
> +               if (op->remap.unmap->va->gem.obj)
> +                       return drm_exec_lock_obj(exec, op->remap.unmap->va->gem.obj);
> +               return 0;
> +       case DRM_GPUVA_OP_UNMAP:
> +               if (op->unmap.va->gem.obj)
> +                       return drm_exec_lock_obj(exec, op->unmap.va->gem.obj);
> +               return 0;
> +       default:
> +               return 0;
> +       }
> +}
> +
> +static const struct drm_gpuvm_ops lock_ops = {
> +       .sm_step_map = drm_gpuva_sm_step_lock,
> +       .sm_step_remap = drm_gpuva_sm_step_lock,
> +       .sm_step_unmap = drm_gpuva_sm_step_lock,
> +};
> +
> +/**
> + * drm_gpuvm_sm_map_exec_lock() - locks the objects touched by a drm_gpuvm_sm_map()
> + * @gpuvm: the &drm_gpuvm representing the GPU VA space
> + * @exec: the &drm_exec locking context
> + * @num_fences: for newly mapped objects, the # of fences to reserve
> + * @req_addr: the start address of the range to unmap
> + * @req_range: the range of the mappings to unmap
> + * @req_obj: the &drm_gem_object to map
> + * @req_offset: the offset within the &drm_gem_object
> + *
> + * This function locks (drm_exec_lock_obj()) objects that will be unmapped/
> + * remapped, and locks+prepares (drm_exec_prepare_object()) objects that
> + * will be newly mapped.
> + *
> + * The expected usage is:
> + *
> + *    vm_bind {
> + *        struct drm_exec exec;
> + *
> + *        // IGNORE_DUPLICATES is required, INTERRUPTIBLE_WAIT is recommended:
> + *        drm_exec_init(&exec, IGNORE_DUPLICATES | INTERRUPTIBLE_WAIT, 0);
> + *
> + *        drm_exec_until_all_locked (&exec) {
> + *            for_each_vm_bind_operation {
> + *                switch (op->op) {
> + *                case DRIVER_OP_UNMAP:
> + *                    ret = drm_gpuvm_sm_unmap_exec_lock(gpuvm, &exec, op->addr, op->range);
> + *                    break;
> + *                case DRIVER_OP_MAP:
> + *                    ret = drm_gpuvm_sm_map_exec_lock(gpuvm, &exec, num_fences,
> + *                                                     op->addr, op->range,
> + *                                                     obj, op->obj_offset);
> + *                    break;
> + *                }
> + *
> + *                drm_exec_retry_on_contention(&exec);
> + *                if (ret)
> + *                    return ret;
> + *            }
> + *        }
> + *    }
> + *
> + * This enables all locking to be performed before the driver begins modifying
> + * the VM.  This is safe to do in the case of overlapping DRIVER_VM_BIND_OPs,
> + * where an earlier op can alter the sequence of steps generated for a later
> + * op, because the later altered step will involve the same GEM object(s)
> + * already seen in the earlier locking step.  For example:
> + *
> + * 1) An earlier driver DRIVER_OP_UNMAP op removes the need for a
> + *    DRM_GPUVA_OP_REMAP/UNMAP step.  This is safe because we've already
> + *    locked the GEM object in the earlier DRIVER_OP_UNMAP op.
> + *
> + * 2) An earlier DRIVER_OP_MAP op overlaps with a later DRIVER_OP_MAP/UNMAP
> + *    op, introducing a DRM_GPUVA_OP_REMAP/UNMAP that wouldn't have been
> + *    required without the earlier DRIVER_OP_MAP.  This is safe because we've
> + *    already locked the GEM object in the earlier DRIVER_OP_MAP step.
> + *
> + * Returns: 0 on success or a negative error codec
> + */
> +int
> +drm_gpuvm_sm_map_exec_lock(struct drm_gpuvm *gpuvm,
> +                          struct drm_exec *exec, unsigned int num_fences,
> +                          u64 req_addr, u64 req_range,
> +                          struct drm_gem_object *req_obj, u64 req_offset)
> +{
> +       if (req_obj) {
> +               int ret = drm_exec_prepare_obj(exec, req_obj, num_fences);
> +               if (ret)
> +                       return ret;
> +       }
> +
> +       return __drm_gpuvm_sm_map(gpuvm, &lock_ops, exec,
> +                                 req_addr, req_range,
> +                                 req_obj, req_offset);
> +
> +}
> +EXPORT_SYMBOL_GPL(drm_gpuvm_sm_map_exec_lock);
> +
> +/**
> + * drm_gpuvm_sm_unmap_exec_lock() - locks the objects touched by drm_gpuvm_sm_unmap()
> + * @gpuvm: the &drm_gpuvm representing the GPU VA space
> + * @exec: the &drm_exec locking context
> + * @req_addr: the start address of the range to unmap
> + * @req_range: the range of the mappings to unmap
> + *
> + * This function locks (drm_exec_lock_obj()) objects that will be unmapped/
> + * remapped by drm_gpuvm_sm_unmap().
> + *
> + * See drm_gpuvm_sm_map_exec_lock() for expected usage.
> + *
> + * Returns: 0 on success or a negative error code
> + */
> +int
> +drm_gpuvm_sm_unmap_exec_lock(struct drm_gpuvm *gpuvm, struct drm_exec *exec,
> +                            u64 req_addr, u64 req_range)
> +{
> +       return __drm_gpuvm_sm_unmap(gpuvm, &lock_ops, exec,
> +                                   req_addr, req_range);
> +}
> +EXPORT_SYMBOL_GPL(drm_gpuvm_sm_unmap_exec_lock);
> +
>  static struct drm_gpuva_op *
>  gpuva_op_alloc(struct drm_gpuvm *gpuvm)
>  {
> diff --git a/include/drm/drm_gpuvm.h b/include/drm/drm_gpuvm.h
> index 2a9629377633..274532facfd6 100644
> --- a/include/drm/drm_gpuvm.h
> +++ b/include/drm/drm_gpuvm.h
> @@ -1211,6 +1211,14 @@ int drm_gpuvm_sm_map(struct drm_gpuvm *gpuvm, void *priv,
>  int drm_gpuvm_sm_unmap(struct drm_gpuvm *gpuvm, void *priv,
>                        u64 addr, u64 range);
>
> +int drm_gpuvm_sm_map_exec_lock(struct drm_gpuvm *gpuvm,
> +                         struct drm_exec *exec, unsigned int num_fences,
> +                         u64 req_addr, u64 req_range,
> +                         struct drm_gem_object *obj, u64 offset);
> +
> +int drm_gpuvm_sm_unmap_exec_lock(struct drm_gpuvm *gpuvm, struct drm_exec *exec,
> +                                u64 req_addr, u64 req_range);
> +
>  void drm_gpuva_map(struct drm_gpuvm *gpuvm,
>                    struct drm_gpuva *va,
>                    struct drm_gpuva_op_map *op);
> --
> 2.49.0
>