drm/i915/execlists: Virtual engine bonding

Some users require that when a master batch is executed on one particular
engine, a companion batch is run simultaneously on a specific slave
engine. For this purpose, we introduce virtual engine bonding, allowing
maps of master:slaves to be constructed to constrain which physical
engines a virtual engine may select given a fence on a master engine.

For the moment, we continue to ignore the issue of preemption deferring
the master request for later. Ideally, we would like to then also remove
the slave and run something else rather than have it stall the pipeline.
With load balancing, we should be able to move workload around it, but
there is a similar stall on the master pipeline while it may wait for
the slave to be executed. At the cost of more latency for the bonded
request, it may be interesting to launch both on their engines in
lockstep. (Bubbles abound.)

Opens: Also what about bonding an engine as its own master? It doesn't
break anything internally, so allow the silliness.

v2: Emancipate the bonds
v3: Couple in delayed scheduling for the selftests
v4: Handle invalid mutually exclusive bonding
v5: Mention what the uapi does
v6: s/nbond/num_bonds/

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190521211134.16117-9-chris@chris-wilson.co.uk

1 files changed, 84 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index 68db6b28e606..5d2f8ba92b59 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -1504,8 +1504,92 @@ out_siblings:
 	return err;
 }
 
+static int
+set_engines__bond(struct i915_user_extension __user *base, void *data)
+{
+	struct i915_context_engines_bond __user *ext =
+		container_of_user(base, typeof(*ext), base);
+	const struct set_engines *set = data;
+	struct i915_engine_class_instance ci;
+	struct intel_engine_cs *virtual;
+	struct intel_engine_cs *master;
+	u16 idx, num_bonds;
+	int err, n;
+
+	if (get_user(idx, &ext->virtual_index))
+		return -EFAULT;
+
+	if (idx >= set->engines->num_engines) {
+		DRM_DEBUG("Invalid index for virtual engine: %d >= %d\n",
+			  idx, set->engines->num_engines);
+		return -EINVAL;
+	}
+
+	idx = array_index_nospec(idx, set->engines->num_engines);
+	if (!set->engines->engines[idx]) {
+		DRM_DEBUG("Invalid engine at %d\n", idx);
+		return -EINVAL;
+	}
+	virtual = set->engines->engines[idx]->engine;
+
+	err = check_user_mbz(&ext->flags);
+	if (err)
+		return err;
+
+	for (n = 0; n < ARRAY_SIZE(ext->mbz64); n++) {
+		err = check_user_mbz(&ext->mbz64[n]);
+		if (err)
+			return err;
+	}
+
+	if (copy_from_user(&ci, &ext->master, sizeof(ci)))
+		return -EFAULT;
+
+	master = intel_engine_lookup_user(set->ctx->i915,
+					  ci.engine_class, ci.engine_instance);
+	if (!master) {
+		DRM_DEBUG("Unrecognised master engine: { class:%u, instance:%u }\n",
+			  ci.engine_class, ci.engine_instance);
+		return -EINVAL;
+	}
+
+	if (get_user(num_bonds, &ext->num_bonds))
+		return -EFAULT;
+
+	for (n = 0; n < num_bonds; n++) {
+		struct intel_engine_cs *bond;
+
+		if (copy_from_user(&ci, &ext->engines[n], sizeof(ci)))
+			return -EFAULT;
+
+		bond = intel_engine_lookup_user(set->ctx->i915,
+						ci.engine_class,
+						ci.engine_instance);
+		if (!bond) {
+			DRM_DEBUG("Unrecognised engine[%d] for bonding: { class:%d, instance: %d }\n",
+				  n, ci.engine_class, ci.engine_instance);
+			return -EINVAL;
+		}
+
+		/*
+		 * A non-virtual engine has no siblings to choose between; and
+		 * a submit fence will always be directed to the one engine.
+		 */
+		if (intel_engine_is_virtual(virtual)) {
+			err = intel_virtual_engine_attach_bond(virtual,
+							       master,
+							       bond);
+			if (err)
+				return err;
+		}
+	}
+
+	return 0;
+}
+
 static const i915_user_extension_fn set_engines__extensions[] = {
 	[I915_CONTEXT_ENGINES_EXT_LOAD_BALANCE] = set_engines__load_balance,
+	[I915_CONTEXT_ENGINES_EXT_BOND] = set_engines__bond,
 };
 
 static int