The W2 will only include HSA contributions made through your employment. That means both "true" employer contributions & the amount you choose to contribute will be included in Box 12.
For the purposes of your income-tax return, the contributions that you made through payroll contributions are considered to be made "by your employer." This is because there wasn't any income-tax withholding on those amounts & you didn't pay FICA.
If you made any direct contributions to an HSA outside of your employment (e.g. writing a check), then you report those separately on your income-tax return and may receive an adjustment to income.
