[Mesa-dev] [RFC 2/2] drm/i915: Select engines via class and instance in execbuffer2

[Tvrtko Ursulin]

From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>

Building on top of the previous patch which exported the concept
of engine classes and instances, we can also use this instead of
the current awkward engine selection uAPI.

This is primarily interesting for the VCS engine selection which
is a) currently done via disjoint set of flags, and b) the
current I915_EXEC_BSD flags has different semantics depending on
the underlying hardware which is bad.

Proposed idea here is to reserve 16-bits of flags, to pass in
the engine class and instance (8 bits each), and a new flag
named I915_EXEC_CLASS_INSTACE to tell the kernel this new engine
selection API is in use.

The new uAPI also removes access to the weak VCS engine
balancing as currently existing in the driver.

Example usage to send a command to VCS0:

  eb.flags = i915_execbuffer2_engine(DRM_I915_ENGINE_CLASS_VIDEO_DECODE, 0);

Or to send a command to VCS1:

  eb.flags = i915_execbuffer2_engine(DRM_I915_ENGINE_CLASS_VIDEO_DECODE, 1);

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Ben Widawsky <ben at bwidawsk.net>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter at intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Jon Bloomfield <jon.bloomfield at intel.com>
Cc: Daniel Charles <daniel.charles at intel.com>
Cc: "Rogozhkin, Dmitry V" <dmitry.v.rogozhkin at intel.com>
Cc: Oscar Mateo <oscar.mateo at intel.com>
Cc: "Gong, Zhipeng" <zhipeng.gong at intel.com>
Cc: intel-vaapi-media at lists.01.org
Cc: mesa-dev at lists.freedesktop.org
---
 drivers/gpu/drm/i915/i915_gem_execbuffer.c | 36 ++++++++++++++++++++++++++++++
 include/uapi/drm/i915_drm.h                |  7 +++++-
 2 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index af1965774e7b..7fc92ec839a1 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -1153,6 +1153,10 @@ i915_gem_check_execbuffer(struct drm_i915_gem_execbuffer2 *exec)
 	if (exec->flags & __I915_EXEC_UNKNOWN_FLAGS)
 		return false;
 
+	if ((exec->flags & I915_EXEC_CLASS_INSTANCE) &&
+	    (exec->flags & I915_EXEC_RING_MASK))
+		return false;
+
 	/* Kernel clipping was a DRI1 misfeature */
 	if (exec->num_cliprects || exec->cliprects_ptr)
 		return false;
@@ -1492,6 +1496,35 @@ gen8_dispatch_bsd_engine(struct drm_i915_private *dev_priv,
 	return file_priv->bsd_engine;
 }
 
+extern u8 user_class_map[DRM_I915_ENGINE_CLASS_MAX];
+
+static struct intel_engine_cs *
+eb_select_engine_class_instance(struct drm_i915_private *i915,
+				struct drm_i915_gem_execbuffer2 *args)
+{
+	struct intel_engine_cs *engine;
+	enum intel_engine_id id;
+	u16 class_instance;
+	u8 user_class, class, instance;
+
+	class_instance = (args->flags & I915_EXEC_CLASS_INSTANCE_MASK) >>
+			 I915_EXEC_CLASS_INSTANCE_SHIFT;
+
+	user_class = class_instance >> 8;
+	instance = class_instance & 0xff;
+
+	if (user_class >= DRM_I915_ENGINE_CLASS_MAX)
+		return NULL;
+	class = user_class_map[user_class];
+
+	for_each_engine(engine, i915, id) {
+		if (engine->class == class && engine->instance == instance)
+			return engine;
+	}
+
+	return NULL;
+}
+
 #define I915_USER_RINGS (4)
 
 static const enum intel_engine_id user_ring_map[I915_USER_RINGS + 1] = {
@@ -1510,6 +1543,9 @@ eb_select_engine(struct drm_i915_private *dev_priv,
 	unsigned int user_ring_id = args->flags & I915_EXEC_RING_MASK;
 	struct intel_engine_cs *engine;
 
+	if (args->flags & I915_EXEC_CLASS_INSTANCE)
+		return eb_select_engine_class_instance(dev_priv, args);
+
 	if (user_ring_id > I915_USER_RINGS) {
 		DRM_DEBUG("execbuf with unknown ring: %u\n", user_ring_id);
 		return NULL;
diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index 6058596a9f33..727a6dc4b029 100644
--- a/include/uapi/drm/i915_drm.h
+++ b/include/uapi/drm/i915_drm.h
@@ -906,7 +906,12 @@ struct drm_i915_gem_execbuffer2 {
  */
 #define I915_EXEC_FENCE_OUT		(1<<17)
 
-#define __I915_EXEC_UNKNOWN_FLAGS (-(I915_EXEC_FENCE_OUT<<1))
+#define I915_EXEC_CLASS_INSTANCE	(1<<18)
+
+#define I915_EXEC_CLASS_INSTANCE_SHIFT	(19)
+#define I915_EXEC_CLASS_INSTANCE_MASK	(0xffff << I915_EXEC_CLASS_INSTANCE_SHIFT)
+
+#define __I915_EXEC_UNKNOWN_FLAGS (-(35 << 1))
 
 #define I915_EXEC_CONTEXT_ID_MASK	(0xffffffff)
 #define i915_execbuffer2_set_context_id(eb2, context) \
-- 
2.9.3