So. According to this:
ROM:FF8100E0 MOV R12, SP
ROM:FF8100E4 STMFD SP!, {R4,R11,R12,LR,PC}
ROM:FF8100E8 SUB R11, R12, #4
ROM:FF8100EC SUB SP, SP, #0x14
ROM:FF8100F0 MOV R4, R0
ROM:FF8100F4 MOV R3, #0
ROM:FF8100F8 STR R3, [R11,#-0x18]
ROM:FF8100FC MRC p15, 0, R0,c1,c0
ROM:FF810100 ORR R0, R0, #0x1000
ROM:FF810104 ORR R0, R0, #4
ROM:FF810108 ORR R0, R0, #1
ROM:FF81010C MCR p15, 0, R0,c1,c0
ROM:FF810110 LDR R3, =unk_FFC33020
ROM:FF810114 STR R3, [R11,#-0x20]
ROM:FF810118 MOV R3, #0x1900
ROM:FF81011C STR R3, [R11,#-0x24]
ROM:FF810120 MOV R3, #0
ROM:FF810124 STR R3, [R11,#-0x1C]
my canon_data_len == FFC33020?