freedreno: Add a nohw flag to skip submitting to the kernel.

For some CPU-side-only optimizations, it can be nice to disable rendering
so that we can see what the impact is even on cases where the GPU can't
quite keep up.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4996>

src/gallium/drivers/freedreno/freedreno_gmem.c

@@ -612,6 +612,9 @@ flush_ring(struct fd_batch *batch)
 	uint32_t timestamp;
 	int out_fence_fd = -1;
 
+	if (unlikely(fd_mesa_debug & FD_DBG_NOHW))
+		return;
+
 	fd_submit_flush(batch->submit, batch->in_fence_fd,
 			batch->needs_out_fence_fd ? &out_fence_fd : NULL,
 			&timestamp);

src/gallium/drivers/freedreno/freedreno_screen.c

@@ -90,6 +90,7 @@ static const struct debug_named_value debug_options[] = {
 		{"notile",    FD_DBG_NOTILE, "Disable tiling for all internal buffers"},
 		{"layout",    FD_DBG_LAYOUT, "Dump resource layouts"},
 		{"nofp16",    FD_DBG_NOFP16, "Disable mediump precision lowering"},
+		{"nohw",      FD_DBG_NOHW,   "Disable submitting commands to the HW"},
 		DEBUG_NAMED_VALUE_END
 };
 

src/gallium/drivers/freedreno/freedreno_util.h

@@ -90,6 +90,7 @@ enum fd_debug_flag {
 	FD_DBG_NOTILE       = BITFIELD_BIT(25),
 	FD_DBG_LAYOUT       = BITFIELD_BIT(26),
 	FD_DBG_NOFP16       = BITFIELD_BIT(27),
+	FD_DBG_NOHW         = BITFIELD_BIT(28),
 };
 
 extern int fd_mesa_debug;