floop: fill zeroes to fix level instead of calculating realtime

Flexible loopback device originally fills zeroes according to the rate
it runs on each time input_frames_queued is called. This mechanism fills
zeroes in a irregular manner, causing flexible loopback device need to
frquently wake up instead of its original schedule.

Always fill the buffer to min_cb_level to ensure it was running in its
original schedule. This will also create a delay when next output stream
starts.

BUG=b:362679648
TEST=cras_test_client --pin_device <dev> -C /dev/null \
     and check wakeup time with cras_test_client --follow_a

Change-Id: I149b7b738474ef59ff567d75932cbb608c736a70
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/adhd/+/5892521
Tested-by: chromeos-cop-builder@chromeos-cop.iam.gserviceaccount.com <chromeos-cop-builder@chromeos-cop.iam.gserviceaccount.com>
Commit-Queue: Curtis Malainey <cujomalainey@chromium.org>
Reviewed-by: Curtis Malainey <cujomalainey@chromium.org>
Reviewed-by: Yu-Hsuan Hsu <yuhsuan@chromium.org>

cras/src/server/cras_floop_iodev.c

@@ -112,17 +112,15 @@ static int input_frames_queued(const struct cras_iodev* iodev,
   unsigned int frame_bytes = cras_get_format_bytes(iodev->format);
 
   /*
-   *	When there is no output stream, fill the buffer with frames until it
-   *	reaches the number of frames that is expected according to frame rate.
+   * When there is no output stream, fill the buffer with frames to
+   * min_cb_level. This creates a delay for 1 min_cb_level when another
+   * output stream attached.
    */
   if (floop->input_active && !floop->pair.output.streams) {
-    unsigned int frames_since_start = cras_frames_since_time(
-        &floop->dev_start_time, iodev->format->frame_rate);
     unsigned int frames_in_buffer = buf_queued(floop->buffer) / frame_bytes;
-    unsigned int frames_to_fill =
-        frames_since_start > (floop->read_frames + frames_in_buffer)
-            ? frames_since_start - (floop->read_frames + frames_in_buffer)
-            : 0;
+    unsigned int frames_to_fill = iodev->min_cb_level > frames_in_buffer
+                                      ? iodev->min_cb_level - frames_in_buffer
+                                      : 0;
     frames_to_fill =
         MIN(buf_writable(floop->buffer) / frame_bytes, frames_to_fill);
     if (frames_to_fill > 0) {