[PATCH v4] drm/i915: Document the Virtual Engine uAPI
Tvrtko Ursulin
tvrtko.ursulin at linux.intel.com
Fri Jun 18 15:00:36 UTC 2021
From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
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.
v2:
* Have the text in uapi header and link from there.
v4:
* Link from driver-uapi.rst.
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Daniel Vetter <daniel at ffwll.ch>
---
Documentation/gpu/driver-uapi.rst | 21 ++++
include/uapi/drm/i915_drm.h | 188 ++++++++++++++++++++++++++++++
2 files changed, 209 insertions(+)
diff --git a/Documentation/gpu/driver-uapi.rst b/Documentation/gpu/driver-uapi.rst
index 4411e6919a3d..27d0fbe33e87 100644
--- a/Documentation/gpu/driver-uapi.rst
+++ b/Documentation/gpu/driver-uapi.rst
@@ -5,4 +5,25 @@ DRM Driver uAPI
drm/i915 uAPI
=============
+Engine Discovery uAPI
+---------------------
+
+.. kernel-doc:: include/uapi/drm/i915_drm.h
+ :doc: Engine Discovery uAPI
+
+Context Engine Map uAPI
+-----------------------
+
+.. kernel-doc:: include/uapi/drm/i915_drm.h
+ :doc: Context Engine Map uAPI
+
+Virtual Engine uAPI
+-------------------
+
+.. kernel-doc:: include/uapi/drm/i915_drm.h
+ :doc: Virtual Engine uAPI
+
+i915_drm.h
+----------
.. kernel-doc:: include/uapi/drm/i915_drm.h
+ :internal:
diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index a1cb4aa035a9..2f70c48567c0 100644
--- a/include/uapi/drm/i915_drm.h
+++ b/include/uapi/drm/i915_drm.h
@@ -1806,6 +1806,69 @@ struct drm_i915_gem_context_param_sseu {
__u32 rsvd;
};
+/**
+ * DOC: 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 after 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 index 0 which is the virtual engine
+ * gem_execbuf(drm_fd, &execbuf);
+ */
+
/*
* i915_context_engines_load_balance:
*
@@ -1882,6 +1945,61 @@ struct i915_context_engines_bond {
struct i915_engine_class_instance engines[N__]; \
} __attribute__((packed)) name__
+/**
+ * DOC: 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 index 0, which is rcs0 for this context
+ * gem_execbuf(drm_fd, &execbuf);
+ *
+ * ...
+ * execbuf.rsvd1 = ctx_id;
+ * execbuf.flags = 1; // Submits to index 0, which is bcs0 for this context
+ * gem_execbuf(drm_fd, &execbuf);
+ */
+
struct i915_context_param_engines {
__u64 extensions; /* linked chain of extension blocks, 0 terminates */
#define I915_CONTEXT_ENGINES_EXT_LOAD_BALANCE 0 /* see i915_context_engines_load_balance */
@@ -2375,6 +2493,76 @@ struct drm_i915_query_topology_info {
__u8 data[];
};
+/**
+ * DOC: 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.
+ */
+
/**
* struct drm_i915_engine_info
*
--
2.30.2
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