we test Huawei P20(Mali G72-MP12), Huawei P30(Mali G76-MP10), Samsung S8(Mali G71) w/ Type-C DisplayPort alternate mode.
Found GPU can't output correct 640x400 as EDID request. Please refer the following test result. Is there any limitation of requested timing.
[GPU Compatibility Test Result]
In a mobile device the GPU isn't responsible for the display output; the GPU just writes back to memory and a separate display controller block handles emitting the frame to the display. Neither of these devices use an Arm display processor, so please raise these issues with the chipset provider or device manufacturer.
Kind regards, Pete