freedreno: no-op render when we need a fence

If app tries to create a fence but there is no rendering to submit, we
need a dummy/no-op submit.  Use a string-marker for the purpose.. mostly
since it avoids needing to realize that the packet format changes in
later gen's (so one less place to fixup for a5xx).

Signed-off-by: Rob Clark <robdclark@gmail.com>

src/gallium/drivers/freedreno/freedreno_context.c

@@ -53,8 +53,17 @@ fd_context_flush(struct pipe_context *pctx, struct pipe_fence_handle **fence,
 		fd_bc_flush(&ctx->screen->batch_cache, ctx);
 	}
 
-	if (fence)
+	if (fence) {
+		/* if there hasn't been any rendering submitted yet, we might not
+		 * have actually created a fence
+		 */
+		if (!ctx->last_fence || ctx->batch->needs_out_fence_fd) {
+			ctx->batch->needs_flush = true;
+			fd_gmem_render_noop(ctx->batch);
+			fd_batch_reset(ctx->batch);
+		}
 		fd_fence_ref(pctx->screen, fence, ctx->last_fence);
+	}
 }
 
 /**

src/gallium/drivers/freedreno/freedreno_gmem.c

@@ -359,6 +359,20 @@ render_sysmem(struct fd_batch *batch)
 	fd_reset_wfi(batch);
 }
 
+static void
+flush_ring(struct fd_batch *batch)
+{
+	struct fd_context *ctx = batch->ctx;
+	int out_fence_fd = -1;
+
+	fd_ringbuffer_flush2(batch->gmem, batch->in_fence_fd,
+			batch->needs_out_fence_fd ? &out_fence_fd : NULL);
+
+	fd_fence_ref(&ctx->screen->base, &ctx->last_fence, NULL);
+	ctx->last_fence = fd_fence_create(ctx,
+			fd_ringbuffer_timestamp(batch->gmem), out_fence_fd);
+}
+
 void
 fd_gmem_render_tiles(struct fd_batch *batch)
 {
@@ -399,13 +413,22 @@ fd_gmem_render_tiles(struct fd_batch *batch)
 		ctx->stats.batch_gmem++;
 	}
 
-	int out_fence_fd = -1;
-	fd_ringbuffer_flush2(batch->gmem, batch->in_fence_fd,
-			batch->needs_out_fence_fd ? &out_fence_fd : NULL);
+	flush_ring(batch);
+}
 
-	fd_fence_ref(&ctx->screen->base, &ctx->last_fence, NULL);
-	ctx->last_fence = fd_fence_create(ctx,
-			fd_ringbuffer_timestamp(batch->gmem), out_fence_fd);
+/* special case for when we need to create a fence but have no rendering
+ * to flush.. just emit a no-op string-marker packet.
+ */
+void
+fd_gmem_render_noop(struct fd_batch *batch)
+{
+	struct fd_context *ctx = batch->ctx;
+	struct pipe_context *pctx = &ctx->base;
+
+	pctx->emit_string_marker(pctx, "noop", 4);
+	/* emit IB to drawcmds (which contain the string marker): */
+	ctx->emit_ib(batch->gmem, batch->draw);
+	flush_ring(batch);
 }
 
 /* tile needs restore if it isn't completely contained within the

src/gallium/drivers/freedreno/freedreno_gmem.h

@@ -62,6 +62,7 @@ struct fd_gmem_stateobj {
 struct fd_batch;
 
 void fd_gmem_render_tiles(struct fd_batch *batch);
+void fd_gmem_render_noop(struct fd_batch *batch);
 
 bool fd_gmem_needs_restore(struct fd_batch *batch, struct fd_tile *tile,
 		uint32_t buffers);