[Mesa-dev] [PATCH 4/6] gallium: add way for drivers to create fences without flushing

[Marek Olšák]

There is one problem with this: It doesn't allow calling
pipe_context::fence_finish from another thread in a thread-safe
manner, and it implies that fence_finish is generally NOT thread-safe.
This thread safety is something I'd like to preserve.

I would say that a flush flag telling the driver not to flush would be
better, so that drivers which have per-screen fences and cheap flushes
can ignore it.

Marek

On Fri, Apr 1, 2016 at 10:29 PM, Rob Clark <robdclark at gmail.com> wrote:
> From: Rob Clark <robclark at freedesktop.org>
>
> Since current thing is kinda horrible for tilers.  And that issue will
> be even worse with EGL_ANDROID_native_fence_sync.
>
> Not wired up yet for gl syncobj, which can come later.  For now we just
> need this with EGL.
>
> Signed-off-by: Rob Clark <robclark at freedesktop.org>
> ---
>  src/gallium/include/pipe/p_context.h  | 24 ++++++++++++++++++++++++
>  src/gallium/state_trackers/dri/dri2.c | 29 ++++++++++++++++++++---------
>  2 files changed, 44 insertions(+), 9 deletions(-)
>
> diff --git a/src/gallium/include/pipe/p_context.h b/src/gallium/include/pipe/p_context.h
> index 1c97e82..02a946b 100644
> --- a/src/gallium/include/pipe/p_context.h
> +++ b/src/gallium/include/pipe/p_context.h
> @@ -457,6 +457,30 @@ struct pipe_context {
>                   unsigned flags);
>
>     /**
> +    * Create a fence without necessarily flushing rendering.  Note
> +    * that if the driver implements this, it must also implement
> +    * ctx->fence_finish() which will be used instead of
> +    * screen->fence_finish() to give the driver an opportunity to
> +    * flush.
> +    *
> +    * This allows drivers, in particular tilers, to defer flush
> +    * until someone actually wants to wait on a fence.
> +    *
> +    * \param fence  if not NULL, an old fence to unref and transfer a
> +    *    new fence reference to
> +    */
> +   void (*create_fence)(struct pipe_context *pipe,
> +                        struct pipe_fence_handle **fence);
> +
> +   /**
> +    * Wait for the fence to finish.
> +    * \param timeout in nanoseconds (may be PIPE_TIMEOUT_INFINITE).
> +    */
> +   boolean (*fence_finish)(struct pipe_context *pipe,
> +                           struct pipe_fence_handle *fence,
> +                           uint64_t timeout);
> +
> +   /**
>      * Create a view on a texture to be used by a shader stage.
>      */
>     struct pipe_sampler_view * (*create_sampler_view)(struct pipe_context *ctx,
> diff --git a/src/gallium/state_trackers/dri/dri2.c b/src/gallium/state_trackers/dri/dri2.c
> index fb0a180..b66d885 100644
> --- a/src/gallium/state_trackers/dri/dri2.c
> +++ b/src/gallium/state_trackers/dri/dri2.c
> @@ -1320,7 +1320,12 @@ dri2_create_fence(__DRIcontext *_ctx)
>     if (!fence)
>        return NULL;
>
> -   ctx->flush(ctx, &fence->pipe_fence, 0);
> +   if (ctx->create_fence) {
> +      debug_assert(ctx->fence_finish);
> +      ctx->create_fence(ctx, &fence->pipe_fence);
> +   } else {
> +      ctx->flush(ctx, &fence->pipe_fence, 0);
> +   }
>
>     if (!fence->pipe_fence) {
>        FREE(fence);
> @@ -1376,27 +1381,33 @@ static GLboolean
>  dri2_client_wait_sync(__DRIcontext *_ctx, void *_fence, unsigned flags,
>                        uint64_t timeout)
>  {
> +   struct pipe_context *ctx = dri_context(_ctx)->st->pipe;
>     struct dri2_fence *fence = (struct dri2_fence*)_fence;
>     struct dri_screen *driscreen = fence->driscreen;
>     struct pipe_screen *screen = driscreen->base.screen;
> +   struct pipe_fence_handle *pipe_fence = NULL;
>
> -   /* No need to flush. The context was flushed when the fence was created. */
> +   /* No need to flush. The context was flushed when the fence was created,
> +    * or the ctx implements ctx->fence_finish() which will take care of
> +    * flushing if required
> +    */
>
>     if (fence->pipe_fence)
> -      return screen->fence_finish(screen, fence->pipe_fence, timeout);
> +      pipe_fence = fence->pipe_fence;
>     else if (fence->cl_event) {
> -      struct pipe_fence_handle *pipe_fence =
> -         driscreen->opencl_dri_event_get_fence(fence->cl_event);
> -
> -      if (pipe_fence)
> -         return screen->fence_finish(screen, pipe_fence, timeout);
> -      else
> +      pipe_fence = driscreen->opencl_dri_event_get_fence(fence->cl_event);
> +      if (!pipe_fence)
>           return driscreen->opencl_dri_event_wait(fence->cl_event, timeout);
>     }
>     else {
>        assert(0);
>        return false;
>     }
> +
> +   if (ctx->fence_finish)
> +      return ctx->fence_finish(ctx, pipe_fence, timeout);
> +
> +   return screen->fence_finish(screen, pipe_fence, timeout);
>  }
>
>  static void
> --
> 2.5.5
>
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