[ET-VK] 3/n Split dispatches between multiple command buffers. Track previous semaphore in context. (#12523)

Stack from [ghstack](https://github.com/ezyang/ghstack) (oldest at
bottom):
* #12525
* __->__ #12523
* #12522
* #12519

This diff is the third part of a series of diffs aiming to split
dispatches between multiple command buffers. In this diff, we are
tracking the previous semaphore in the context.

A new member variable `prev_semaphore_` was added to the `Context`
class. This variable is used to store the semaphore of the previously
submitted command buffer.

Differential Revision:
[D78360037](https://our.internmc.facebook.com/intern/diff/D78360037/)

---------

Co-authored-by: Stephen Jia <[email protected]>

Author: 2 people

backends/vulkan/runtime/api/Context.cpp

@@ -39,6 +39,7 @@ Context::Context(vkapi::Adapter* adapter, const ContextConfig& config)
       // Command buffer submission
       cmd_mutex_{},
       cmd_(VK_NULL_HANDLE, VK_NULL_HANDLE, 0u),
+      prev_semaphore_(VK_NULL_HANDLE),
       submit_count_{0u},
       // Memory Management
       buffer_clearlist_mutex_{},
@@ -195,10 +196,21 @@ void Context::register_blit(
 }
 
 void Context::submit_cmd_to_gpu(VkFence fence_handle, const bool final_use) {
+  // Wait semaphore would be previous command buffer's signal semaphore
+  VkSemaphore wait_semaphore = prev_semaphore_;
+  // Signal semaphore for the the current command buffer
+  VkSemaphore signal_semaphore = cmd_.get_signal_semaphore();
+  // Next command buffer would wait on this command buffer's signal semaphore
+  prev_semaphore_ = signal_semaphore;
+
   if (cmd_) {
     cmd_.end();
     adapter_p_->submit_cmd(
-        queue_, cmd_.get_submit_handle(final_use), fence_handle);
+        queue_,
+        cmd_.get_submit_handle(final_use),
+        fence_handle,
+        wait_semaphore,
+        signal_semaphore);
 
     submit_count_ = 0u;
   }
@@ -214,6 +226,8 @@ void Context::flush() {
   if (cmd_) {
     cmd_.invalidate();
   }
+  // Reset previous command buffer semaphore
+  prev_semaphore_ = VK_NULL_HANDLE;
 
   std::lock_guard<std::mutex> bufferlist_lock(buffer_clearlist_mutex_);
   std::lock_guard<std::mutex> imagelist_lock(image_clearlist_mutex_);

backends/vulkan/runtime/api/Context.h

@@ -68,6 +68,8 @@ class Context final {
   // Command buffers submission
   std::mutex cmd_mutex_;
   vkapi::CommandBuffer cmd_;
+  // Semaphore for the previously submitted command buffer, if any
+  VkSemaphore prev_semaphore_;
   uint32_t submit_count_;
   // Memory Management
   std::mutex buffer_clearlist_mutex_;