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drm/i915/bdw: Document Logical Rings, LR contexts and Execlists

Browse source 
Add theory of operation notes to intel_lrc.c and comments to externally
visible functions.

v2: Add notes on logical ring context creation.

v3: Use kerneldoc.

v4: Integrate it in the DocBook template.

Signed-off-by: Thomas Daniel <thomas.daniel@intel.com> (v1)
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com> (v2, v3)
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Drop hunk about render ring init function since that's not
yet merged.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
This commit is contained in:
Oscar Mateo 2014-07-24 17:04:48 +01:00 committed by Daniel Vetter
parent c0ab1ae902
commit 73e4d07f8a
3 changed files with 237 additions and 1 deletions
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5
Documentation/DocBook/drm.tmpl
View file

@ -3919,6 +3919,11 @@ int num_ioctls;</synopsis>
!Pdrivers/gpu/drm/i915/i915_cmd_parser.c batch buffer command parser
!Idrivers/gpu/drm/i915/i915_cmd_parser.c
</sect2>
<sect2>
<title>Logical Rings, Logical Ring Contexts and Execlists</title>
!Pdrivers/gpu/drm/i915/intel_lrc.c Logical Rings, Logical Ring Contexts and Execlists
!Idrivers/gpu/drm/i915/intel_lrc.c
</sect2>
</sect1>
</chapter>
</part>

203
drivers/gpu/drm/i915/intel_lrc.c
View file

@ -28,13 +28,108 @@
*
*/
/*
/**
* DOC: Logical Rings, Logical Ring Contexts and Execlists
*
* Motivation:
* GEN8 brings an expansion of the HW contexts: "Logical Ring Contexts".
* These expanded contexts enable a number of new abilities, especially
* "Execlists" (also implemented in this file).
*
* One of the main differences with the legacy HW contexts is that logical
* ring contexts incorporate many more things to the context's state, like
* PDPs or ringbuffer control registers:
*
* The reason why PDPs are included in the context is straightforward: as
* PPGTTs (per-process GTTs) are actually per-context, having the PDPs
* contained there mean you don't need to do a ppgtt->switch_mm yourself,
* instead, the GPU will do it for you on the context switch.
*
* But, what about the ringbuffer control registers (head, tail, etc..)?
* shouldn't we just need a set of those per engine command streamer? This is
* where the name "Logical Rings" starts to make sense: by virtualizing the
* rings, the engine cs shifts to a new "ring buffer" with every context
* switch. When you want to submit a workload to the GPU you: A) choose your
* context, B) find its appropriate virtualized ring, C) write commands to it
* and then, finally, D) tell the GPU to switch to that context.
*
* Instead of the legacy MI_SET_CONTEXT, the way you tell the GPU to switch
* to a contexts is via a context execution list, ergo "Execlists".
*
* LRC implementation:
* Regarding the creation of contexts, we have:
*
* - One global default context.
* - One local default context for each opened fd.
* - One local extra context for each context create ioctl call.
*
* Now that ringbuffers belong per-context (and not per-engine, like before)
* and that contexts are uniquely tied to a given engine (and not reusable,
* like before) we need:
*
* - One ringbuffer per-engine inside each context.
* - One backing object per-engine inside each context.
*
* The global default context starts its life with these new objects fully
* allocated and populated. The local default context for each opened fd is
* more complex, because we don't know at creation time which engine is going
* to use them. To handle this, we have implemented a deferred creation of LR
* contexts:
*
* The local context starts its life as a hollow or blank holder, that only
* gets populated for a given engine once we receive an execbuffer. If later
* on we receive another execbuffer ioctl for the same context but a different
* engine, we allocate/populate a new ringbuffer and context backing object and
* so on.
*
* Finally, regarding local contexts created using the ioctl call: as they are
* only allowed with the render ring, we can allocate & populate them right
* away (no need to defer anything, at least for now).
*
* Execlists implementation:
* Execlists are the new method by which, on gen8+ hardware, workloads are
* submitted for execution (as opposed to the legacy, ringbuffer-based, method).
* This method works as follows:
*
* When a request is committed, its commands (the BB start and any leading or
* trailing commands, like the seqno breadcrumbs) are placed in the ringbuffer
* for the appropriate context. The tail pointer in the hardware context is not
* updated at this time, but instead, kept by the driver in the ringbuffer
* structure. A structure representing this request is added to a request queue
* for the appropriate engine: this structure contains a copy of the context's
* tail after the request was written to the ring buffer and a pointer to the
* context itself.
*
* If the engine's request queue was empty before the request was added, the
* queue is processed immediately. Otherwise the queue will be processed during
* a context switch interrupt. In any case, elements on the queue will get sent
* (in pairs) to the GPU's ExecLists Submit Port (ELSP, for short) with a
* globally unique 20-bits submission ID.
*
* When execution of a request completes, the GPU updates the context status
* buffer with a context complete event and generates a context switch interrupt.
* During the interrupt handling, the driver examines the events in the buffer:
* for each context complete event, if the announced ID matches that on the head
* of the request queue, then that request is retired and removed from the queue.
*
* After processing, if any requests were retired and the queue is not empty
* then a new execution list can be submitted. The two requests at the front of
* the queue are next to be submitted but since a context may not occur twice in
* an execution list, if subsequent requests have the same ID as the first then
* the two requests must be combined. This is done simply by discarding requests
* at the head of the queue until either only one requests is left (in which case
* we use a NULL second context) or the first two requests have unique IDs.
*
* By always executing the first two requests in the queue the driver ensures
* that the GPU is kept as busy as possible. In the case where a single context
* completes but a second context is still executing, the request for this second
* context will be at the head of the queue when we remove the first one. This
* request will then be resubmitted along with a new request for a different context,
* which will cause the hardware to continue executing the second request and queue
* the new request (the GPU detects the condition of a context getting preempted
* with the same context and optimizes the context switch flow by not doing
* preemption, but just sampling the new tail pointer).
*
*/
#include <drm/drmP.h>
@ -109,6 +204,17 @@ enum {
};
#define GEN8_CTX_ID_SHIFT 32
/**
* intel_sanitize_enable_execlists() - sanitize i915.enable_execlists
* @dev: DRM device.
* @enable_execlists: value of i915.enable_execlists module parameter.
*
* Only certain platforms support Execlists (the prerequisites being
* support for Logical Ring Contexts and Aliasing PPGTT or better),
* and only when enabled via module parameter.
*
* Return: 1 if Execlists is supported and has to be enabled.
*/
int intel_sanitize_enable_execlists(struct drm_device *dev, int enable_execlists)
{
WARN_ON(i915.enable_ppgtt == -1);
@ -123,6 +229,18 @@ int intel_sanitize_enable_execlists(struct drm_device *dev, int enable_execlists
return 0;
}
/**
* intel_execlists_ctx_id() - get the Execlists Context ID
* @ctx_obj: Logical Ring Context backing object.
*
* Do not confuse with ctx->id! Unfortunately we have a name overload
* here: the old context ID we pass to userspace as a handler so that
* they can refer to a context, and the new context ID we pass to the
* ELSP so that the GPU can inform us of the context status via
* interrupts.
*
* Return: 20-bits globally unique context ID.
*/
u32 intel_execlists_ctx_id(struct drm_i915_gem_object *ctx_obj)
{
u32 lrca = i915_gem_obj_ggtt_offset(ctx_obj);
@ -313,6 +431,13 @@ static bool execlists_check_remove_request(struct intel_engine_cs *ring,
return false;
}
/**
* intel_execlists_handle_ctx_events() - handle Context Switch interrupts
* @ring: Engine Command Streamer to handle.
*
* Check the unread Context Status Buffers and manage the submission of new
* contexts to the ELSP accordingly.
*/
void intel_execlists_handle_ctx_events(struct intel_engine_cs *ring)
{
struct drm_i915_private *dev_priv = ring->dev->dev_private;
@ -481,6 +606,23 @@ static int execlists_move_to_gpu(struct intel_ringbuffer *ringbuf,
return logical_ring_invalidate_all_caches(ringbuf);
}
/**
* execlists_submission() - submit a batchbuffer for execution, Execlists style
* @dev: DRM device.
* @file: DRM file.
* @ring: Engine Command Streamer to submit to.
* @ctx: Context to employ for this submission.
* @args: execbuffer call arguments.
* @vmas: list of vmas.
* @batch_obj: the batchbuffer to submit.
* @exec_start: batchbuffer start virtual address pointer.
* @flags: translated execbuffer call flags.
*
* This is the evil twin version of i915_gem_ringbuffer_submission. It abstracts
* away the submission details of the execbuffer ioctl call.
*
* Return: non-zero if the submission fails.
*/
int intel_execlists_submission(struct drm_device *dev, struct drm_file *file,
struct intel_engine_cs *ring,
struct intel_context *ctx,
@ -608,6 +750,15 @@ int logical_ring_flush_all_caches(struct intel_ringbuffer *ringbuf)
return 0;
}
/**
* intel_logical_ring_advance_and_submit() - advance the tail and submit the workload
* @ringbuf: Logical Ringbuffer to advance.
*
* The tail is updated in our logical ringbuffer struct, not in the actual context. What
* really happens during submission is that the context and current tail will be placed
* on a queue waiting for the ELSP to be ready to accept a new context submission. At that
* point, the tail *inside* the context is updated and the ELSP written to.
*/
void intel_logical_ring_advance_and_submit(struct intel_ringbuffer *ringbuf)
{
struct intel_engine_cs *ring = ringbuf->ring;
@ -781,6 +932,19 @@ static int logical_ring_prepare(struct intel_ringbuffer *ringbuf, int bytes)
return 0;
}
/**
* intel_logical_ring_begin() - prepare the logical ringbuffer to accept some commands
*
* @ringbuf: Logical ringbuffer.
* @num_dwords: number of DWORDs that we plan to write to the ringbuffer.
*
* The ringbuffer might not be ready to accept the commands right away (maybe it needs to
* be wrapped, or wait a bit for the tail to be updated). This function takes care of that
* and also preallocates a request (every workload submission is still mediated through
* requests, same as it did with legacy ringbuffer submission).
*
* Return: non-zero if the ringbuffer is not ready to be written to.
*/
int intel_logical_ring_begin(struct intel_ringbuffer *ringbuf, int num_dwords)
{
struct intel_engine_cs *ring = ringbuf->ring;
@ -1021,6 +1185,12 @@ static int gen8_emit_request(struct intel_ringbuffer *ringbuf)
return 0;
}
/**
* intel_logical_ring_cleanup() - deallocate the Engine Command Streamer
*
* @ring: Engine Command Streamer.
*
*/
void intel_logical_ring_cleanup(struct intel_engine_cs *ring)
{
struct drm_i915_private *dev_priv = ring->dev->dev_private;
@ -1215,6 +1385,16 @@ static int logical_vebox_ring_init(struct drm_device *dev)
return logical_ring_init(dev, ring);
}
/**
* intel_logical_rings_init() - allocate, populate and init the Engine Command Streamers
* @dev: DRM device.
*
* This function inits the engines for an Execlists submission style (the equivalent in the
* legacy ringbuffer submission world would be i915_gem_init_rings). It does it only for
* those engines that are present in the hardware.
*
* Return: non-zero if the initialization failed.
*/
int intel_logical_rings_init(struct drm_device *dev)
{
struct drm_i915_private *dev_priv = dev->dev_private;
@ -1377,6 +1557,14 @@ populate_lr_context(struct intel_context *ctx, struct drm_i915_gem_object *ctx_o
return 0;
}
/**
* intel_lr_context_free() - free the LRC specific bits of a context
* @ctx: the LR context to free.
*
* The real context freeing is done in i915_gem_context_free: this only
* takes care of the bits that are LRC related: the per-engine backing
* objects and the logical ringbuffer.
*/
void intel_lr_context_free(struct intel_context *ctx)
{
int i;
@ -1415,6 +1603,19 @@ static uint32_t get_lr_context_size(struct intel_engine_cs *ring)
return ret;
}
/**
* intel_lr_context_deferred_create() - create the LRC specific bits of a context
* @ctx: LR context to create.
* @ring: engine to be used with the context.
*
* This function can be called more than once, with different engines, if we plan
* to use the context with them. The context backing objects and the ringbuffers
* (specially the ringbuffer backing objects) suck a lot of memory up, and that's why
* the creation is a deferred call: it's better to make sure first that we need to use
* a given ring with the context.
*
* Return: non-zero on eror.
*/
int intel_lr_context_deferred_create(struct intel_context *ctx,
struct intel_engine_cs *ring)
{

30
drivers/gpu/drm/i915/intel_lrc.h
View file

@ -38,10 +38,21 @@ int intel_logical_rings_init(struct drm_device *dev);
int logical_ring_flush_all_caches(struct intel_ringbuffer *ringbuf);
void intel_logical_ring_advance_and_submit(struct intel_ringbuffer *ringbuf);
/**
* intel_logical_ring_advance() - advance the ringbuffer tail
* @ringbuf: Ringbuffer to advance.
*
* The tail is only updated in our logical ringbuffer struct.
*/
static inline void intel_logical_ring_advance(struct intel_ringbuffer *ringbuf)
{
ringbuf->tail &= ringbuf->size - 1;
}
/**
* intel_logical_ring_emit() - write a DWORD to the ringbuffer.
* @ringbuf: Ringbuffer to write to.
* @data: DWORD to write.
*/
static inline void intel_logical_ring_emit(struct intel_ringbuffer *ringbuf,
u32 data)
{
@ -66,6 +77,25 @@ int intel_execlists_submission(struct drm_device *dev, struct drm_file *file,
u64 exec_start, u32 flags);
u32 intel_execlists_ctx_id(struct drm_i915_gem_object *ctx_obj);
/**
* struct intel_ctx_submit_request - queued context submission request
* @ctx: Context to submit to the ELSP.
* @ring: Engine to submit it to.
* @tail: how far in the context's ringbuffer this request goes to.
* @execlist_link: link in the submission queue.
* @work: workqueue for processing this request in a bottom half.
* @elsp_submitted: no. of times this request has been sent to the ELSP.
*
* The ELSP only accepts two elements at a time, so we queue context/tail
* pairs on a given queue (ring->execlist_queue) until the hardware is
* available. The queue serves a double purpose: we also use it to keep track
* of the up to 2 contexts currently in the hardware (usually one in execution
* and the other queued up by the GPU): We only remove elements from the head
* of the queue when the hardware informs us that an element has been
* completed.
*
* All accesses to the queue are mediated by a spinlock (ring->execlist_lock).
*/
struct intel_ctx_submit_request {
struct intel_context *ctx;
struct intel_engine_cs *ring;