// 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 <>, <> and <> 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 <> 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 <>. 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 <> 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 <> 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 <>. 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.