[RFC 1/5] drm/i915: Select engines via class and instance in execbuffer2

[Tvrtko Ursulin <tursulin@ursulin.net>]

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

Introduce a new way of selecting engines using the engine class
and instance concept.

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 8-bits of flags, to pass in the
engine instance, re-use the existing engine selection bits for
the class selection, and a new flag named
I915_EXEC_CLASS_INSTANCE 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(I915_ENGINE_CLASS_VIDEO_DECODE, 0);

Or to send a command to VCS1:

  eb.flags = i915_execbuffer2_engine(I915_ENGINE_CLASS_VIDEO_DECODE, 1);

v2:
 * Fix unknown flags mask.
 * Use I915_EXEC_RING_MASK for class. (Chris Wilson)

v3:
 * Add a map for fast class-instance engine lookup. (Chris Wilson)

v4:
 * Update commit to reflect v3.
 * Export intel_engine_lookup for other users. (Chris Wilson)
 * Split out some warns. (Chris Wilson)

v5:
 * Fixed shift and mask logic.
 * Rebased to be standalone.

v6:
 * Rebased back to follow engine info ioctl.
 * Rename helper to intel_engine_lookup_user. (Chris Wilson)

v7:
 * Rebased.

v8:
 * Rebased after engine class uAPI got in via separate route.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
---
 drivers/gpu/drm/i915/i915_gem_execbuffer.c | 13 +++++++++++++
 include/uapi/drm/i915_drm.h                | 12 +++++++++++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 435ed95df144..2e7ddd197cc4 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -2004,6 +2004,16 @@ gen8_dispatch_bsd_engine(struct drm_i915_private *dev_priv,
 	return file_priv->bsd_engine;
 }
 
+static struct intel_engine_cs *
+eb_select_engine_class_instance(struct drm_i915_private *i915, u64 eb_flags)
+{
+	u8 class = eb_flags & I915_EXEC_RING_MASK;
+	u8 instance = (eb_flags & I915_EXEC_INSTANCE_MASK) >>
+		      I915_EXEC_INSTANCE_SHIFT;
+
+	return intel_engine_lookup_user(i915, class, instance);
+}
+
 #define I915_USER_RINGS (4)
 
 static const enum intel_engine_id user_ring_map[I915_USER_RINGS + 1] = {
@@ -2022,6 +2032,9 @@ 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->flags);
+
 	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 8ca906148a96..9ed351f4a304 100644
--- a/include/uapi/drm/i915_drm.h
+++ b/include/uapi/drm/i915_drm.h
@@ -1051,7 +1051,12 @@ struct drm_i915_gem_execbuffer2 {
  */
 #define I915_EXEC_FENCE_ARRAY   (1<<19)
 
-#define __I915_EXEC_UNKNOWN_FLAGS (-(I915_EXEC_FENCE_ARRAY<<1))
+#define I915_EXEC_CLASS_INSTANCE	(1<<20)
+
+#define I915_EXEC_INSTANCE_SHIFT	(21)
+#define I915_EXEC_INSTANCE_MASK		(0xff << I915_EXEC_INSTANCE_SHIFT)
+
+#define __I915_EXEC_UNKNOWN_FLAGS (-((1 << 29) << 1))
 
 #define I915_EXEC_CONTEXT_ID_MASK	(0xffffffff)
 #define i915_execbuffer2_set_context_id(eb2, context) \
@@ -1059,6 +1064,11 @@ struct drm_i915_gem_execbuffer2 {
 #define i915_execbuffer2_get_context_id(eb2) \
 	((eb2).rsvd1 & I915_EXEC_CONTEXT_ID_MASK)
 
+#define i915_execbuffer2_engine(class, instance) \
+	(I915_EXEC_CLASS_INSTANCE | \
+	(class) | \
+	((instance) << I915_EXEC_INSTANCE_SHIFT))
+
 struct drm_i915_gem_pin {
 	/** Handle of the buffer to be pinned. */
 	__u32 handle;
-- 
2.14.1

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