gallium: add fence_server_signal() v2

Calling this function will emit a fence signal operation into the
GPU's command stream.

v2: documentation typos

Signed-off-by: Andres Rodriguez <andresx7@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/docs/source/context.rst

@@ -567,7 +567,38 @@ by a single pipe_screen and is not shared with another process.
 (i.e. you shouldn't use it to flush caches explicitly if you want to e.g.
 use the resource for texturing)
 
+Fences
+^^^^^^
 
+``pipe_fence_handle``, and related methods, are used to synchronize
+execution between multiple parties. Examples include CPU <-> GPU synchronization,
+renderer <-> windowing system, multiple external APIs, etc.
+
+A ``pipe_fence_handle`` can either be 'one time use' or 're-usable'. A 'one time use'
+fence behaves like a traditional GPU fence. Once it reaches the signaled state it
+is forever considered to be signaled.
+
+Once a re-usable ``pipe_fence_handle`` becomes signaled, it can be reset
+back into an unsignaled state. The ``pipe_fence_handle`` will be reset to
+the unsignaled state by performing a wait operation on said object, i.e.
+``fence_server_sync``. As a corollary to this behaviour, a re-usable
+``pipe_fence_handle`` can only have one waiter.
+
+This behaviour is useful in producer <-> consumer chains. It helps avoid
+unecessarily sharing a new ``pipe_fence_handle`` each time a new frame is
+ready. Instead, the fences are exchanged once ahead of time, and access is synchronized
+through GPU signaling instead of direct producer <-> consumer communication.
+
+``fence_server_sync`` inserts a wait command into the GPU's command stream.
+
+``fence_server_signal`` inserts a signal command into the GPU's command stream.
+
+There are no guarantees that the wait/signal commands will be flushed when
+calling ``fence_server_sync`` or ``fence_server_signal``. An explicit
+call to ``flush`` is required to make sure the commands are emitted to the GPU.
+
+The Gallium implementation may implicitly ``flush`` the command stream during a
+``fence_server_sync`` or ``fence_server_signal`` call if necessary.
 
 Resource Busy Queries
 ^^^^^^^^^^^^^^^^^^^^^

src/gallium/include/pipe/p_context.h

@@ -525,6 +525,12 @@ struct pipe_context {
    void (*fence_server_sync)(struct pipe_context *pipe,
                              struct pipe_fence_handle *fence);
 
+   /**
+    * Insert commands to have the GPU signal a fence.
+    */
+   void (*fence_server_signal)(struct pipe_context *pipe,
+                               struct pipe_fence_handle *fence);
+
    /**
     * Create a view on a texture to be used by a shader stage.
     */