Question how to Get accurate instanceID cmd.DrawMeshInstanced?

I'm working on a grass render feature using draw mesh instanced to optimized performance, and many of my computation is based on instance ID.

Although cmd.DrawMeshInstanced() API can support up to 1023 instances, when I profile it in frame debugger, I can see the actual draw call sometimes break into 2 in editor, and even more draw calls on my phone:


When I tried to access instance ID (in my case I need it to vary from 0~1023), this draw call split causes trouble. Instance ID in vertex input structure got reset every draw call. And it seems there is some variable called "unity_BaseInstancedID" which can offset instance ID every draw call. I searched forum and notice this variable is handled by unity automatically.
I inspected this value on frame debugger, and it does not change, you can see it in following pic:


I came up with two solutions:
1. using mpb, and inject an int[], passing the id for each instance.
2. figure out the instancing limitation for the platform app is running on, and split the draw call myself, pass my own "my_BaseInstancedID".
But these are not elegant as instancing should be. I wonder if you guys have better ideas.