Chase H.Q.: Routine at CBA4

CBA4: Unknown routine or dead code

I can't find what calls this routine. I've checked various stages...
mystery_cba4	CBA4	LD C,A	C = A
	CBA5	LD A,(IY+$00)	A = IY[0]
	CBA8	INC IY	IY++
	CBAA	INC IXl	IX.low++
	CBAC	SUB (IY+$00)	A -= IY[0]
	CBAF	CP $E0	Jump if >= 240
	CBB1	JP NC,mystery_cba4_0	Jump if >= 240
	CBB4	CP $50	Exit via dr_start_backdrop_fill/dr_start_backdrop_fill if >= 80
	CBB6	JP NC,dr_start_backdrop_fill
mystery_cba4_0	CBB9	DEC L	L -= 2
	CBBA	DEC L	L -= 2
	CBBB	ADD A,C	A += C
	CBBC	JR Z,mystery_cba4	Jump if zero or negative
	CBBE	JP M,mystery_cba4	Jump if zero or negative
	CBC1	LD C,A	C = A
	CBC2	JP dfr_c915	Exit via dfr_c915