Ipega PG-9021 on Linux (Archlinux) working on Retroarch (Put this on ~/.config/retroarch/autoconfig/udev)

ipega classic gamepad.cfg

# -------------------------------------------------------------------------------------------------------------------------------------------------
# If RetroArch do NOT detect the controller when connected via Bluetooth and won't even recognize manual binding, 
# fix it by adding this udev rule (the line below WITHOUT the # symbol at the beginning!) to "/etc/udev/rules.d/10-local.rules" and reboot.
# SUBSYSTEM=="input", ATTRS{name}=="ipega classic gamepad", MODE="0666", ENV{ID_INPUT_JOYSTICK}="1"
# -------------------------------------------------------------------------------------------------------------------------------------------------

input_driver = "udev"
input_device = "ipega classic gamepad"
input_vendor_id = "32903"
input_product_id = "2012"

input_select_btn = "19"
input_start_btn = "20"
input_up_btn = "h0up"
input_down_btn = "h0down"
input_left_btn = "h0left"
input_right_btn = "h0right"
input_a_btn = "10"
input_x_btn = "13"
input_b_btn = "9"
input_y_btn = "12"
input_l_btn = "15"
input_r_btn = "16"
input_l2_axis = "+5"
input_r2_axis = "+4"
input_l3_btn = "22"
input_r3_btn = "23"

input_l_x_plus_axis = "+0"
input_l_x_minus_axis = "-0"
input_l_y_plus_axis = "+1"
input_l_y_minus_axis = "-1"
input_r_x_plus_axis = "+2"
input_r_x_minus_axis = "-2"
input_r_y_plus_axis = "+3"
input_r_y_minus_axis = "-3"

input_b_btn_label = "A"
input_y_btn_label = "X"
input_select_btn_label = "Select"
input_start_btn_label = "Start"
input_a_btn_label = "B"
input_x_btn_label = "Y"
input_l_btn_label = "LB"
input_r_btn_label = "RB"
input_l2_axis_label = "LT"
input_r2_axis_label = "RT"
input_up_btn_label = "D-Pad Up"
input_down_btn_label = "D-Pad Down"
input_left_btn_label = "D-Pad Left"
input_right_btn_label = "D-Pad Right"


input_l_x_plus_axis_label = "L Analog Right"
input_l_x_minus_axis_label = "L Analog Left"
input_l_y_plus_axis_label = "L Analog Down"
input_l_y_minus_axis_label = "L Analog Up"
input_r_x_plus_axis_label = "R Analog Right"
input_r_x_minus_axis_label = "R Analog Left"
input_r_y_plus_axis_label = "R Analog Down"
input_r_y_minus_axis_label = "R Analog Up"