[Mesa-dev] [PATCH 06/22] gallium: add semaphore_server_signal()

[Roland Scheidegger]

Am 22.12.2017 um 01:41 schrieb Andres Rodriguez:
> Calling this function will emit a semaphore signal operation into the
> GPU's command stream.
> 
> Signed-off-by: Andres Rodriguez <andresx7 at gmail.com>
> ---
>  src/gallium/docs/source/context.rst  | 31 +++++++++++++++++++++++++++++++
>  src/gallium/include/pipe/p_context.h |  6 ++++++
>  2 files changed, 37 insertions(+)
> 
> diff --git a/src/gallium/docs/source/context.rst b/src/gallium/docs/source/context.rst
> index a8ff3dc..bab9613 100644
> --- a/src/gallium/docs/source/context.rst
> +++ b/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)
>  
> +Semaphores
> +^^^^^^^^^^
>  
> +``pipe_semaphore_handle``, and related methods, are used to synchronize
> +execution between multiple parties. Examples include CPU <-> GPU syncrhonization,
> +rederer <-> windowing system, multiple external APIs, etc.
> +
> +A ``pipe_semaphore_handle`` can either be 'one time use' or 're-usable'. A 'one time use'
> +semaphore behaves like a traditional GPU fence. Once it reaches the signaled state it
> +is forever considered to be signaled.
> +
> +Once a re-usable ``pipe_semaphore_handle`` becomes signaled, it can be reset
> +back into an unsignaled state. The ``pipe_semaphore_handle`` will be reset to
> +the unsignaled state by performing a wait operation on said object, i.e.
> +``semaphore_server_sync``. As a corollary to this behaviour, a re-usable
> +``pipe_semaphore_handle`` can only have one waiter.

I've only skimmed through this, but how are one-time use semaphores
(which can have multiple waiters) and re-usable ones (only one waiter)
distinguished? Just by the use of semaphore_server_sync?

Roland



> +
> +This behaviour is useful in producer <-> consumer chains. It helps avoid
> +unecessarily sharing a new ``pipe_semaphore_handle`` each time a new frame is
> +ready. Instead, the semaphores are exchanged once ahead of time, and access is synchronized
> +through GPU signaling instead of direct producer <-> consumer communication.
> +
> +``semaphore_server_sync`` inserts a wait command into the GPU's command stream.
> +
> +``semaphore_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 ``semaphore_server_sync`` or ``semaphore_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
> +``semaphore_server_sync`` or ``semaphore_server_signal`` call if necessary.
>  
>  Resource Busy Queries
>  ^^^^^^^^^^^^^^^^^^^^^
> diff --git a/src/gallium/include/pipe/p_context.h b/src/gallium/include/pipe/p_context.h
> index 72e4b9d..5827491 100644
> --- a/src/gallium/include/pipe/p_context.h
> +++ b/src/gallium/include/pipe/p_context.h
> @@ -527,6 +527,12 @@ struct pipe_context {
>                                   struct pipe_semaphore_handle *semaphore);
>  
>     /**
> +    * Insert commands to have the GPU signal a semaphore.
> +    */
> +   void (*semaphore_server_signal)(struct pipe_context *pipe,
> +                                   struct pipe_semaphore_handle *semaphore);
> +
> +   /**
>      * Create a view on a texture to be used by a shader stage.
>      */
>     struct pipe_sampler_view * (*create_sampler_view)(struct pipe_context *ctx,
>