To create an Ansible playbook that appends the specified kernel parameters (`rootwait cgroup_enable=cpuset cgroup_memory=1 cgroup_enable=memory`) to the `/boot/firmware/cmdline.txt` file only if they are not already present, you can use the `lineinfile` Ansible module.

Here's how the playbook can be written:

### Ansible Playbook (`append_cmdline.yml`):

```yaml
---
- name: Ensure required parameters are in /boot/firmware/cmdline.txt
  hosts: raspberry_pi
  become: yes
  tasks:
    - name: Append kernel parameters to cmdline.txt if not present
      lineinfile:
        path: /boot/firmware/cmdline.txt
        line: "rootwait cgroup_enable=cpuset cgroup_memory=1 cgroup_enable=memory"
        insertafter: EOF
        state: present
        create: yes
        backup: yes  # This creates a backup of the file before making changes
      notify:
        - Reboot system

  handlers:
    - name: Reboot system
      reboot:
        msg: "Rebooting system to apply new kernel parameters"
        reboot_timeout: 600
        test_command: uptime
```

### Explanation of the Playbook:

- **`hosts: raspberry_pi`**: Targets the Raspberry Pi group from your inventory.
- **`become: yes`**: Ensures that the task runs with elevated privileges (sudo), which is necessary for modifying system files like `/boot/firmware/cmdline.txt`.
  
### Task:

- **`lineinfile` module**: 
  - `path: /boot/firmware/cmdline.txt`: This specifies the file to modify.
  - `line`: This is the exact string to ensure is present in the file.
  - `insertafter: EOF`: This ensures the line is appended at the end of the file if it is not already there.
  - `state: present`: Ensures that the specified line exists in the file.
  - `create: yes`: If the file doesn't exist, it will be created (though it's unlikely in this case because `/boot/firmware/cmdline.txt` should exist on a Raspberry Pi).
  - `backup: yes`: This creates a backup of the file before modifying it (important for safety).

### Handler:

- **`Reboot system` handler**: 
  - This handler is triggered if the task changes the file (i.e., if it appends the line). 
  - It will reboot the Raspberry Pi to apply the changes, which often requires a reboot to take effect.
  - `reboot_timeout: 600` specifies that the playbook will wait up to 600 seconds for the system to reboot successfully.
  - `test_command: uptime` ensures that the system is back up and running after the reboot.

### To run the playbook:

1. Save the playbook to a file, e.g., `append_cmdline.yml`.
2. Run the playbook with the following command:

   ```bash
   ansible-playbook -i inventory.ini append_cmdline.yml
   ```

This will ensure the specified parameters are added to `/boot/firmware/cmdline.txt` only if they are not already present and reboot the system if necessary. If you have any more questions or need further adjustments, feel free to ask!