Vulkan-Docs/doc/specs/vulkan/chapters/cmdbuffers.txt

// Copyright (c) 2015-2016 The Khronos Group Inc.
// Copyright notice at https://www.khronos.org/registry/speccopyright.html

[[commandbuffers]]
= Command Buffers

Command buffers are objects used to record commands which can: be
subsequently submitted to a device queue for execution. There are two levels
of command buffers - _primary command buffers_, which can: execute secondary
command buffers, and which are submitted to queues, and _secondary command
buffers_, which can: be executed by primary command buffers, and which are
not directly submitted to queues.

Recorded commands include commands to bind pipelines and descriptor sets to
the command buffer, commands to modify dynamic state, commands to draw (for
graphics rendering), commands to dispatch (for compute), commands to execute
secondary command buffers (for primary command buffers only), commands to
copy buffers and images, and other commands.

[[commandbuffers-statereset]]
Each command buffer manages state independently of other command buffers.
There is no inheritance of state across primary and secondary command
buffers, or between secondary command buffers. When a command buffer begins
recording, all state in that command buffer is undefined. When secondary
command buffer(s) are recorded to execute on a primary command buffer, the
secondary command buffer inherits no state from the primary command buffer,
and all state of the primary command buffer is undefined after an execute
secondary command buffer command is recorded. There is one exception to this
rule - if the primary command buffer is inside a render pass instance, then
the render pass and subpass state is not disturbed by executing secondary
command buffers. Whenever the state of a command buffer is undefined, the
application must: set all relevant state on the command buffer before any
state dependent commands such as draws and dispatches are recorded,
otherwise the behavior of executing that command buffer is undefined.

Unless otherwise specified, and without explicit synchronization, the
various commands submitted to a queue via command buffers may: execute in
arbitrary order relative to each other, and/or concurrently. Also, the
memory side-effects of those commands may: not be directly visible to other
commands without memory barriers. This is true within a command buffer, and
across command buffers submitted to a given queue. See
<<synchronization-events>>, <<synchronization-pipeline-barriers>> and
<<synchronization-memory-barriers>> about synchronization primitives
suitable to guarantee execution order and side-effect visibility between
commands on a given queue.

Each command buffer is always in one of three states:

  * _Initial state_: Before flink:vkBeginCommandBuffer. Either
    flink:vkBeginCommandBuffer has never been called, or the command buffer
    has been reset since it last recorded commands.
  * _Recording state_: Between flink:vkBeginCommandBuffer and
    flink:vkEndCommandBuffer. The command buffer is in a state where it can:
    record commands.
  * _Executable state_: After flink:vkEndCommandBuffer. The command buffer
    is in a state where it has finished recording commands and can: be
    executed.

_Resetting_ a command buffer is an operation that discards any previously
recorded commands and puts a command buffer in the initial state. Resetting
occurs as a result of flink:vkResetCommandBuffer or
flink:vkResetCommandPool, or as part of flink:vkBeginCommandBuffer (which
additionally puts the command buffer in the recording state).


[[commandbuffers-pools]]
== Command Pools

Command pools are opaque objects that command buffer memory is allocated
from, and which allow the implementation to amortize the cost of resource
creation across multiple command buffers. Command pools are
application-synchronized, meaning that a command pool mustnot: be used
concurrently in multiple threads. That includes use via recording commands
on any command buffers allocated from the pool, as well as operations that
allocate, free, and reset command buffers or the pool itself.

To create a command pool, call:

include::../protos/vkCreateCommandPool.txt[]

  * pname:device is the logical device that creates the command pool.
  * pname:pCreateInfo contains information used to create the command pool.
  * pname:pAllocator controls host memory allocation as described in the
    <<memory-allocation, Memory Allocation>> chapter.
  * pname:pCommandPool points to an sname:VkCommandPool handle in which the
    created pool is returned.

include::../validity/protos/vkCreateCommandPool.txt[]

The sname:VkCommandPoolCreateInfo structure is defined as:

include::../structs/VkCommandPoolCreateInfo.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:flags is a bitmask indicating usage behavior for the pool and
    command buffers allocated from it. Bits which can: be set include:
+
--
include::../enums/VkCommandPoolCreateFlagBits.txt[]

** ename:VK_COMMAND_POOL_CREATE_TRANSIENT_BIT indicates that command buffers
   allocated from the pool will be short-lived, meaning that they will be
   reset or freed in a relatively short timeframe. This flag may: be used by
   the implementation to control memory allocation behavior within the pool.
** ename:VK_COMMAND_POOL_CREATE_RESET_COMMAND_BUFFER_BIT controls whether
   command buffers allocated from the pool can: be individually reset. If
   this flag is set, individual command buffers allocated from the pool can:
   be reset either explicitly, by calling fname:vkResetCommandBuffer, or
   implicitly, by calling fname:vkBeginCommandBuffer on an executable
   command buffer. If this flag is not set, then fname:vkResetCommandBuffer
   and fname:vkBeginCommandBuffer (on an executable command buffer) mustnot:
   be called on the command buffers allocated from the pool, and they can:
   only be reset in bulk by calling fname:vkResetCommandPool.
--
  * pname:queueFamilyIndex designates a queue family as described in section
    <<devsandqueues-queueprops,Queue Family Properties>>. All command
    buffers created from this command pool must: be submitted on queues
    from the same queue family.

include::../validity/structs/VkCommandPoolCreateInfo.txt[]

Reset a command pool by calling:

include::../protos/vkResetCommandPool.txt[]

  * pname:device is the logical device that owns the command pool.
  * pname:commandPool is the command pool to reset.
  * pname:flags contains additional flags controlling the behavior of the
    reset.

include::../validity/protos/vkResetCommandPool.txt[]

Resetting a command pool recycles all of the resources from all of the
command buffers allocated from the command pool back to the command pool.
All command buffers that have been allocated from the command pool are put
in the initial state.

pname:flags is a bitmask controlling the operation. Bits which can: be
set include:

include::../enums/VkCommandPoolResetFlagBits.txt[]

If pname:flags includes ename:VK_COMMAND_POOL_RESET_RELEASE_RESOURCES_BIT,
resetting a command pool recycles all of the resources from the command pool
back to the system.

To destroy a command pool, call:

include::../protos/vkDestroyCommandPool.txt[]

  * pname:device is the logical device that destroys the command pool.
  * pname:commandPool is the handle of the command pool to destroy.
  * pname:pAllocator controls host memory allocation as described in the
    <<memory-allocation, Memory Allocation>> chapter.

include::../validity/protos/vkDestroyCommandPool.txt[]

When a pool is destroyed, all command buffers allocated from the pool are
implicitly freed and become invalid. Command buffers allocated from a given
pool do not need to be freed before destroying that command pool.


[[commandbuffer-allocation]]
== Command Buffer Allocation and Management

Command buffers are allocated by calling:

include::../protos/vkAllocateCommandBuffers.txt[]

  * pname:device is the logical device that owns the command pool.
  * pname:pAllocateInfo is a pointer to an instance of the
    sname:VkCommandBufferAllocateInfo structure describing parameters of the
    allocation.
  * pname:pCommandBuffers is a pointer to an array of sname:VkCommandBuffer
    handles in which the resulting command buffer objects are returned. The
    array must: be at least the length specified by the
    pname:commandBufferCount member of pname:pAllocateInfo. Each allocated
    command buffer begins in the initial state.

include::../validity/protos/vkAllocateCommandBuffers.txt[]

The sname:VkCommandBufferAllocateInfo structure is defined as:

include::../structs/VkCommandBufferAllocateInfo.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:commandPool is the name of the command pool that the command
    buffers allocate their memory from.
  * pname:level determines whether the command buffers are primary or
    secondary command buffers. Possible values include:
+
include::../enums/VkCommandBufferLevel.txt[]
  * pname:commandBufferCount is the number of command buffers to allocate
    from the pool.

include::../validity/structs/VkCommandBufferAllocateInfo.txt[]

Command buffers are reset by calling:

include::../protos/vkResetCommandBuffer.txt[]

  * pname:commandBuffer is the command buffer to reset. The command buffer
    can: be in any state, and is put in the initial state.
  * pname:flags is a bitmask controlling the reset operation. Bits which
    can: be set include:
+
--
include::../enums/VkCommandBufferResetFlagBits.txt[]

If pname:flags includes ename:VK_COMMAND_BUFFER_RESET_RELEASE_RESOURCES_BIT,
then most or all memory resources currently owned by the command buffer
should: be returned to the parent command pool. If this flag is not set,
then the command buffer may: hold onto memory resources and reuse them when
recording commands.
--

include::../validity/protos/vkResetCommandBuffer.txt[]

Command buffers are freed by calling:

include::../protos/vkFreeCommandBuffers.txt[]

  * pname:device is the logical device that owns the command pool.
  * pname:commandPool is the handle of the command pool that the command
    buffers were allocated from.
  * pname:commandBufferCount is the length of the pname:pCommandBuffers
    array.
  * pname:pCommandBuffers is an array of handles of command buffers to free.

include::../validity/protos/vkFreeCommandBuffers.txt[]


[[commandbuffers-recording]]
== Command Buffer Recording

To begin recording a command buffer, call:

include::../protos/vkBeginCommandBuffer.txt[]

  * pname:commandBuffer is the handle of the command buffer which is to be
    put in the recording state.
  * pname:pBeginInfo is an instance of the sname:VkCommandBufferBeginInfo
    structure, which defines additional information about how the command
    buffer begins recording.

include::../validity/protos/vkBeginCommandBuffer.txt[]

The sname:VkCommandBufferBeginInfo structure is defined as:

include::../structs/VkCommandBufferBeginInfo.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:flags is a bitmask indicating usage behavior for the command
    buffer. Bits which can: be set include:
+
--
include::../enums/VkCommandBufferUsageFlagBits.txt[]

** ename:VK_COMMAND_BUFFER_USAGE_ONE_TIME_SUBMIT_BIT indicates that each
   recording of the command buffer will only be submitted once, and the
   command buffer will be reset and recorded again between each submission.
** ename:VK_COMMAND_BUFFER_USAGE_RENDER_PASS_CONTINUE_BIT indicates that
   a secondary command buffer is considered to be entirely inside a render
   pass. If this is a primary command buffer, then this bit is ignored.
** Setting ename:VK_COMMAND_BUFFER_USAGE_SIMULTANEOUS_USE_BIT allows the
   command buffer to be resubmitted to a queue or recorded into a primary
   command buffer while it is pending execution.
--
  * pname:pInheritanceInfo is a pointer to a
    sname:VkCommandBufferInheritanceInfo structure, which is used if
    pname:commandBuffer is a secondary command buffer. If this is a primary
    command buffer, then this value is ignored.

include::../validity/structs/VkCommandBufferBeginInfo.txt[]

If the command buffer is a secondary command buffer, then the
sname:VkCommandBufferInheritanceInfo structure defines any state that will
be inherited from the primary command buffer:

include::../structs/VkCommandBufferInheritanceInfo.txt[]

  * pname:renderPass is a sname:VkRenderPass object that must: be
    <<renderpass-compatibility, compatible>> with the one that is bound when
    the sname:VkCommandBuffer is executed if the command buffer was
    allocated with the
    ename:VK_COMMAND_BUFFER_USAGE_RENDER_PASS_CONTINUE_BIT set.
  * pname:subpass is the index of the subpass within pname:renderPass that
    the sname:VkCommandBuffer will be rendering against if it was allocated
    with the ename:VK_COMMAND_BUFFER_USAGE_RENDER_PASS_CONTINUE_BIT set.
  * pname:framebuffer refers to the sname:VkFramebuffer object that the
    sname:VkCommandBuffer will be rendering to if it was allocated with
    the ename:VK_COMMAND_BUFFER_USAGE_RENDER_PASS_CONTINUE_BIT set. It can:
    be code:VK_NULL_HANDLE if the framebuffer is not known.
+
[NOTE]
.Note
====
Specifying the exact framebuffer that the secondary command buffer will be
executed with may: result in better performance at command buffer execution
time.
====
  * pname:occlusionQueryEnable indicates whether the command buffer can: be
    executed while an occlusion query is active in the primary command
    buffer. If this is ename:VK_TRUE, then this command buffer can: be
    executed whether the primary command buffer has an occlusion query
    active or not. If this is ename:VK_FALSE, then the primary command
    buffer mustnot: have an occlusion query active.
  * pname:queryFlags indicates the query flags that can: be used by an
    active occlusion query in the primary command buffer when this secondary
    command buffer is executed. If this value includes the
    ename:VK_QUERY_CONTROL_PRECISE_BIT bit, then the active query can:
    return boolean results or actual sample counts. If this bit is not set,
    then the active query mustnot: use the
    ename:VK_QUERY_CONTROL_PRECISE_BIT bit. If this is a primary command
    buffer, then this value is ignored.
  * pname:pipelineStatistics indicates the set of pipeline statistics that
    can: be counted by an active query in the primary command buffer when
    this secondary command buffer is executed. If this value includes a
    given bit, then this command buffer can: be executed whether the primary
    command buffer has a pipeline statistics query active that includes this
    bit or not. If this value excludes a given bit, then the active pipeline
    statistics query mustnot: be from a query pool that counts that
    statistic.

include::../validity/structs/VkCommandBufferInheritanceInfo.txt[]

A primary command buffer is considered to be pending execution from the time
it is submitted via fname:vkQueueSubmit until that submission completes.

A secondary command buffer is considered to be pending execution from the
time its execution is recorded into a primary buffer (via
fname:vkCmdExecuteCommands) until the final time that primary buffer's
submission to a queue completes. If, after the primary buffer completes, the
secondary command buffer is recorded to execute on a different primary
buffer, the first primary buffer mustnot: be resubmitted until after it is
reset with flink:vkResetCommandBuffer unless the secondary command buffer
was recorded with ename:VK_COMMAND_BUFFER_USAGE_SIMULTANEOUS_USE_BIT.

If ename:VK_COMMAND_BUFFER_USAGE_SIMULTANEOUS_USE_BIT is not set on a
secondary command buffer, that command buffer mustnot: be used more than
once in a given primary command buffer. Furthermore, if a secondary command
buffer without ename:VK_COMMAND_BUFFER_USAGE_SIMULTANEOUS_USE_BIT set is
recorded to execute in a primary command buffer with
ename:VK_COMMAND_BUFFER_USAGE_SIMULTANEOUS_USE_BIT set, the primary command
buffer mustnot: be pending execution more than once at a time.

[NOTE]
.Note
====
On some implementations, not using the
ename:VK_COMMAND_BUFFER_USAGE_SIMULTANEOUS_USE_BIT bit enables command
buffers to be patched in-place if needed, rather than creating a copy of the
command buffer.
====

If a command buffer is in the executable state and the command buffer was
allocated from a command pool with the
ename:VK_COMMAND_POOL_CREATE_RESET_COMMAND_BUFFER_BIT flag set, then
fname:vkBeginCommandBuffer implicitly resets the command buffer, behaving as
if fname:vkResetCommandBuffer had been called with
ename:VK_COMMAND_BUFFER_RESET_RELEASE_RESOURCES_BIT not set. It then puts
the command buffer in the recording state.

Once recording starts, an application records a sequence of commands
(ftext:vkCmd*) to set state in the command buffer, draw, dispatch, and other
commands.

To complete recording of a command buffer, call:

include::../protos/vkEndCommandBuffer.txt[]

  * pname:commandBuffer is the command buffer to complete recording. The
    command buffer must: have been in the recording state, and is moved to
    the executable state.

include::../validity/protos/vkEndCommandBuffer.txt[]

If there was an error during recording, the application will be notified by
an unsuccessful return code returned by fname:vkEndCommandBuffer. If the
application wishes to further use the command buffer, the command buffer
must: be reset.

When a command buffer is in the executable state, it can: be submitted to a
queue for execution.


[[commandbuffers-submission]]
== Command Buffer Submission

Command buffers are submitted to a queue by calling:

include::../protos/vkQueueSubmit.txt[]

  * pname:queue is the handle of the queue that the command buffers will be
    submitted to.
  * pname:submitCount is the number of elements in the pname:pSubmits array.
  * pname:pSubmits is a pointer to an array of slink:VkSubmitInfo structures
    which describe the work to submit. All work described by pname:pSubmits
    must: be submitted to the queue before the command returns.
  * pname:fence is an optional handle to a fence. If pname:fence is not
    code:VK_NULL_HANDLE, the fence is signaled when execution of all command
    buffers specified in the sname:VkSubmitInfo::pname:pCommandBuffers
    members of pname:pSubmits is complete, providing certain
    <<synchronization-implicit-ordering,implicit ordering guarantees>>. If
    pname:submitCount is zero but pname:fence is not code:VK_NULL_HANDLE,
    the fence will still be submitted to the queue and will become signaled
    when all work previously submitted to the queue has completed.

include::../validity/protos/vkQueueSubmit.txt[]

Each submission of work is represented by a sequence of command buffers,
each preceded by a list of semaphores upon which to wait before beginning
execution of specific stages of commands in the command buffers, and
followed by a second list of semaphores to signal upon completion of the
work contained in the command buffers.

[NOTE]
.Note
====
The exact definition of a submission is platform-specific, but is considered
a relatively expensive operation. In general, applications should: attempt
to batch work together into as few calls to fname:vkQueueSubmit as possible.
====

Each call to fname:vkQueueSubmit submits zero or more _batches_ of work to
the queue for execution. pname:submitCount is used to specify the number of
batches to submit. Each batch includes zero or more semaphores to wait upon,
and a corresponding set of stages that will wait for the semaphore to be
signaled before executing any work, followed by a number of command buffers
that will be executed, and finally, zero or more semaphores that will be
signaled after command buffer execution completes. Each batch is represented
as an instance of the slink:VkSubmitInfo structure stored in an array, the
address of which is passed in pname:pSubmits.

The sname:VkSubmitInfo structure is defined as:

include::../structs/VkSubmitInfo.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:waitSemaphoreCount is the number of semaphores upon which
    to wait before executing the command buffers for the batch.
  * pname:pWaitSemaphores is a pointer to an array of semaphores upon which
    to wait before executing the command buffers in the batch.
  * pname:pWaitDstStageMask is a pointer to an array of pipeline stages at
    which each corresponding semaphore wait will occur.
  * pname:commandBufferCount contains the number of command buffers to
    execute in the batch.
  * pname:pCommandBuffers is a pointer to an array of command buffers to
    execute in the batch. The command buffers submitted in a batch begin
    execution in the order they appear in pname:pCommandBuffers, but may:
    complete out of order.
  * pname:signalSemaphoreCount is the number of semaphores to be
    signaled once the commands specified in pname:pCommandBuffers have
    completed execution.
  * pname:pSignalSemaphores is a pointer to an array of semaphores which
    will be signaled when the command buffers for this batch have completed
    execution.

include::../validity/structs/VkSubmitInfo.txt[]


[[commandbuffers-submission-progress]]
== Queue Forward Progress

The application must: ensure that command buffer submissions will be able to
complete without any subsequent operations by the application on any queue.
After any call to fname:vkQueueSubmit, for every queued wait on a semaphore
there must: be a prior signal of that semaphore that won't be consumed by a
different wait on the semaphore.

Command buffers in the submission can: include flink:vkCmdWaitEvents
commands that wait on events that won't be signaled by earlier commands in
the queue. Such events must: be signaled by the application using
flink:vkSetEvent, and the fname:vkCmdWaitEvents commands that wait upon them
mustnot: be inside a render pass instance. Implementations may: have limits
on how long the command buffer will wait, in order to avoid interfering with
progress of other clients of the device. If the event isn't signaled within
these limits, results are undefined and may: include device loss.


[[commandbuffers-secondary]]
== Secondary Command Buffer Execution

A secondary command buffer mustnot: be directly submitted to a queue.
Instead, secondary command buffers are recorded to execute as part of a
primary command buffer with the command:

include::../protos/vkCmdExecuteCommands.txt[]

  * pname:commandBuffer is a handle to a primary command buffer that the
    secondary command buffers are submitted to, and must: be in the
    recording state.
  * pname:commandBufferCount is the length of the pname:pCommandBuffers
    array.
  * pname:pCommandBuffers is an array of secondary command buffer handles,
    which are recorded to execute in the primary command buffer in the order
    they are listed in the array.

include::../validity/protos/vkCmdExecuteCommands.txt[]

Once fname:vkCmdExecuteCommands has been called, any prior executions of the
secondary command buffers specified by pname:pCommandBuffers in any other
primary command buffer become invalidated, unless those secondary command
buffers were recorded with
ename:VK_COMMAND_BUFFER_USAGE_SIMULTANEOUS_USE_BIT.