From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
A little bit of documentation covering the topics of engine discovery,
context engine maps and virtual engines. It is not very detailed but
supposed to be a starting point of giving a brief high level overview of
general principles and intended use cases.
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
Documentation/gpu/i915.rst | 184 +++++++++++++++++++++++++++++++++++++
1 file changed, 184 insertions(+)
diff --git a/Documentation/gpu/i915.rst b/Documentation/gpu/i915.rst
index 42ce0196930a..8e5ab299c31f 100644
--- a/Documentation/gpu/i915.rst
+++ b/Documentation/gpu/i915.rst
@@ -335,6 +335,190 @@ for execution also include a list of all locations within buffers that
refer to GPU-addresses so that the kernel can edit the buffer correctly.
This process is dubbed relocation.
+Engine Discovery uAPI
+---------------------
+
+Engine discovery uAPI is a way of enumerating physical engines present in a GPU
+associated with an open i915 DRM file descriptor. This supersedes the old way of
+using `DRM_IOCTL_I915_GETPARAM` and engine identifiers like
+`I915_PARAM_HAS_BLT`.
+
+The need for this interface came starting with Icelake and newer GPUs, which
+started to establish a pattern of having multiple engines of a same class, where
+not all instances were always completely functionally equivalent.
+
+Entry point for this uapi is `DRM_IOCTL_I915_QUERY` with the
+`DRM_I915_QUERY_ENGINE_INFO` as the queried item id.
+
+Example for getting the list of engines:
+
+.. code-block:: C
+
+ struct drm_i915_query_engine_info *info;
+ struct drm_i915_query_item item = {
+ .query_id = DRM_I915_QUERY_ENGINE_INFO;
+ };
+ struct drm_i915_query query = {
+ .num_items = 1,
+ .items_ptr = (uintptr_t)&item,
+ };
+ int err, i;
+
+ // First query the size of the blob we need, this needs to be large
+ // enough to hold our array of engines. The kernel will fill out the
+ // item.length for us, which is the number of bytes we need.
+ //
+ // Alternatively a large buffer can be allocated straight away enabling
+ // querying in one pass, in which case item.length should contain the
+ // length of the provided buffer.
+ err = ioctl(fd, DRM_IOCTL_I915_QUERY, &query);
+ if (err) ...
+
+ info = calloc(1, item.length);
+ // Now that we allocated the required number of bytes, we call the ioctl
+ // again, this time with the data_ptr pointing to our newly allocated
+ // blob, which the kernel can then populate with info on all engines.
+ item.data_ptr = (uintptr_t)&info,
+
+ err = ioctl(fd, DRM_IOCTL_I915_QUERY, &query);
+ if (err) ...
+
+ // We can now access each engine in the array
+ for (i = 0; i < info->num_engines; i++) {
+ struct drm_i915_engine_info einfo = info->engines[i];
+ u16 class = einfo.engine.class;
+ u16 instance = einfo.engine.instance;
+ ....
+ }
+
+ free(info);
+
+Each of the enumerated engines, apart from being defined by its class and
+instance (see `struct i915_engine_class_instance`), also can have flags and
+capabilities defined as documented in i915_drm.h.
+
+For instance video engines which support HEVC encoding will have the
+`I915_VIDEO_CLASS_CAPABILITY_HEVC` capability bit set.
+
+Engine discovery only fully comes to its own when combined with the new way of
+addressing engines when submitting batch buffers using contexts with engine
+maps configured.
+
+Context Engine Map uAPI
+-----------------------
+
+Context engine map is a new way of addressing engines when submitting batch-
+buffers, replacing the existing way of using identifiers like `I915_EXEC_BLT`
+inside the flags field of `struct drm_i915_gem_execbuffer2`.
+
+To use it created GEM contexts need to be configured with a list of engines
+the user is intending to submit to. This is accomplished using the
+`I915_CONTEXT_PARAM_ENGINES` parameter and `struct i915_context_param_engines`.
+
+For such contexts the `I915_EXEC_RING_MASK` field becomes an index into the
+configured map.
+
+Example of creating such context and submitting against it:
+
+.. code-block:: C
+
+ I915_DEFINE_CONTEXT_PARAM_ENGINES(engines, 2) = {
+ .engines = { { I915_ENGINE_CLASS_RENDER, 0 },
+ { I915_ENGINE_CLASS_COPY, 0 } }
+ };
+ struct drm_i915_gem_context_create_ext_setparam p_engines = {
+ .base = {
+ .name = I915_CONTEXT_CREATE_EXT_SETPARAM,
+ },
+ .param = {
+ .param = I915_CONTEXT_PARAM_ENGINES,
+ .value = to_user_pointer(&engines),
+ .size = sizeof(engines),
+ },
+ };
+ struct drm_i915_gem_context_create_ext create = {
+ .flags = I915_CONTEXT_CREATE_FLAGS_USE_EXTENSIONS,
+ .extensions = to_user_pointer(&p_engines);
+ };
+
+ ctx_id = gem_context_create_ext(drm_fd, &create);
+
+ // We have now created a GEM context with two engines in the map:
+ // Index 0 points to rcs0 while index 1 points to bcs0. Other engines
+ // will not be accessible from this context.
+
+ ...
+ execbuf.rsvd1 = ctx_id;
+ execbuf.flags = 0; // Submits to index0, which is rcs0 for this context
+ gem_execbuf(drm_fd, &execbuf);
+
+ ...
+ execbuf.rsvd1 = ctx_id;
+ execbuf.flags = 1; // Submits to index0, which is bcs0 for this context
+ gem_execbuf(drm_fd, &execbuf);
+
+Virtual Engine uAPI
+-------------------
+
+Virtual engine is a concept where userspace is able to configure a set of
+physical engines, submit a batch buffer, and let the driver execute it on any
+engine from the set as it sees fit.
+
+This is primarily useful on parts which have multiple instances of a same class
+engine, like for example GT3+ Skylake parts with their two VCS engines.
+
+For instance userspace can enumerate all engines of a certain class using the
+previously described `Engine Discovery uAPI`_. After
+that userspace can create a GEM context with a placeholder slot for the virtual
+engine (using `I915_ENGINE_CLASS_INVALID` and `I915_ENGINE_CLASS_INVALID_NONE`
+for class and instance respectively) and finally using the
+`I915_CONTEXT_ENGINES_EXT_LOAD_BALANCE` extension place a virtual engine in the
+same reserved slot.
+
+Example of creating a virtual engine and submitting a batch buffer to it:
+
+.. code-block:: C
+
+ I915_DEFINE_CONTEXT_ENGINES_LOAD_BALANCE(virtual, 2) = {
+ .base.name = I915_CONTEXT_ENGINES_EXT_LOAD_BALANCE,
+ .engine_index = 0, // Place this virtual engine into engine map slot 0
+ .num_siblings = 2,
+ .engines = { { I915_ENGINE_CLASS_VIDEO, 0 },
+ { I915_ENGINE_CLASS_VIDEO, 1 }, },
+ };
+ I915_DEFINE_CONTEXT_PARAM_ENGINES(engines, 1) = {
+ .engines = { { I915_ENGINE_CLASS_INVALID,
+ I915_ENGINE_CLASS_INVALID_NONE } },
+ .extensions = to_user_pointer(&virtual), // Chains with the load_balance extension
+ };
+ struct drm_i915_gem_context_create_ext_setparam p_engines = {
+ .base = {
+ .name = I915_CONTEXT_CREATE_EXT_SETPARAM,
+ },
+ .param = {
+ .param = I915_CONTEXT_PARAM_ENGINES,
+ .value = to_user_pointer(&engines),
+ .size = sizeof(engines),
+ },
+ };
+ struct drm_i915_gem_context_create_ext create = {
+ .flags = I915_CONTEXT_CREATE_FLAGS_USE_EXTENSIONS,
+ .extensions = to_user_pointer(&p_engines);
+ };
+
+ ctx_id = gem_context_create_ext(drm_fd, &create);
+
+ // Now we have created a GEM context with its engine map containing a
+ // single virtual engine. Submissions to this slot can go either to
+ // vcs0 or vcs1, depending on the load balancing algorithm used inside
+ // the driver. The load balancing is dynamic from one batch buffer to
+ // another and transparent to userspace.
+
+ ...
+ execbuf.rsvd1 = ctx_id;
+ execbuf.flags = 0; // Submits to index0 which is the virtual engine
+ gem_execbuf(drm_fd, &execbuf);
+
Locking Guidelines
------------------
--
2.30.2
