What is the legal test for disqualifying a judge?

^2. The statutory basis for disqualifying a judge is set forth in Code of Civil Procedure section 170.1. However, "a petition for writ of mandate is the exclusive method of obtaining review of a denial of a judicial disqualification motion." (People v. Mayfield (1997) 14 Cal.4th 668, 811, overruled on other grounds by People v. Scott (2015) 61 Cal.4th 363, 390, fn. 2.) "Accordingly, we address the issue of judicial disqualification solely under the rubric of due process." (People v. Freeman, supra, 47 Cal.4th at p. 1000.) Due process provides a narrower path to overturn a judgment since it requires actual bias, not merely the appearance of bias. (Id. at p. 1006.)