[Mesa-dev] [PATCH 2/4] atomic: Use CPU-side KMS fence synchronisation

[Chris Wilson]

On Tue, May 02, 2017 at 11:52:07AM +0100, Daniel Stone wrote:
> Atomic does not let us have multiple requests in flight at once; if we
> don't synchronise with the request completion on the CPU side, we can
> get -EBUSY from the atomic commit.
> 
> We already have everything required to do this, namely the out-fence
> from the last commit. Block on that with a CPU-side wait before we call
> atomic commit, to make sure we'll never attempt to queue an atomic
> commit before the previous one has completed.
> 
> Signed-off-by: Daniel Stone <daniels at collabora.com>
> ---
>  common.c     |  1 +
>  common.h     |  1 +
>  drm-atomic.c | 21 ++++++++++++++++++++-
>  3 files changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/common.c b/common.c
> index bf2f78e..a2cf630 100644
> --- a/common.c
> +++ b/common.c
> @@ -155,6 +155,7 @@ int init_egl(struct egl *egl, const struct gbm *gbm)
>  	get_proc_dpy(eglCreateSyncKHR, "EGL_KHR_fence_sync");
>  	get_proc_dpy(eglDestroySyncKHR, "EGL_KHR_fence_sync");
>  	get_proc_dpy(eglWaitSyncKHR, "EGL_KHR_fence_sync");
> +	get_proc_dpy(eglClientWaitSyncKHR, "EGL_KHR_fence_sync");
>  	get_proc_dpy(eglDupNativeFenceFDANDROID, "EGL_ANDROID_native_fence_sync");
>  
>  	printf("Using display %p with EGL version %d.%d\n",
> diff --git a/common.h b/common.h
> index 639bd87..0acf4c0 100644
> --- a/common.h
> +++ b/common.h
> @@ -79,6 +79,7 @@ struct egl {
>  	PFNEGLCREATESYNCKHRPROC eglCreateSyncKHR;
>  	PFNEGLDESTROYSYNCKHRPROC eglDestroySyncKHR;
>  	PFNEGLWAITSYNCKHRPROC eglWaitSyncKHR;
> +	PFNEGLCLIENTWAITSYNCKHRPROC eglClientWaitSyncKHR;
>  	PFNEGLDUPNATIVEFENCEFDANDROIDPROC eglDupNativeFenceFDANDROID;
>  
>  	void (*draw)(unsigned i);
> diff --git a/drm-atomic.c b/drm-atomic.c
> index 0f3c4f2..65caacd 100644
> --- a/drm-atomic.c
> +++ b/drm-atomic.c
> @@ -210,6 +210,7 @@ static int atomic_run(const struct gbm *gbm, const struct egl *egl)
>  
>  		if (drm.kms_out_fence_fd != -1) {
>  			kms_fence = create_fence(egl, drm.kms_out_fence_fd);
> +			assert(kms_fence);
>  
>  			/* driver now has ownership of the fence fd: */
>  			drm.kms_out_fence_fd = -1;
> @@ -220,7 +221,6 @@ static int atomic_run(const struct gbm *gbm, const struct egl *egl)
>  			 * the buffer that is still on screen.
>  			 */
>  			egl->eglWaitSyncKHR(egl->display, kms_fence, 0);
> -			egl->eglDestroySyncKHR(egl->display, kms_fence);
>  		}
>  
>  		egl->draw(i++);
> @@ -229,6 +229,7 @@ static int atomic_run(const struct gbm *gbm, const struct egl *egl)
>  		 * signaled when gpu rendering done
>  		 */
>  		gpu_fence = create_fence(egl, EGL_NO_NATIVE_FENCE_FD_ANDROID);
> +		assert(gpu_fence);
>  
>  		eglSwapBuffers(egl->display, egl->surface);
>  
> @@ -246,6 +247,24 @@ static int atomic_run(const struct gbm *gbm, const struct egl *egl)
>  			return -1;
>  		}
>  
> +		if (kms_fence) {
> +			EGLint status;
> +
> +			/* Wait on the CPU side for the _previous_ commit to
> +			 * complete before we post the flip through KMS, as
> +			 * atomic will reject the commit if we post a new one
> +			 * whilst the previous one is still pending.
> +			 */
> +			do {
> +				status = egl->eglClientWaitSyncKHR(egl->display,
> +								   kms_fence,
> +								   0,
> +								   EGL_FOREVER_KHR);
> +			} while (status != EGL_CONDITION_SATISFIED_KHR);
> +
> +			egl->eglDestroySyncKHR(egl->display, kms_fence);

Would it be an interesting exercise to use that as the in-fence for the GPU?
Not sure which is more in the spirit of kmscube.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre