I'm trying to debug T440p's display problem, tested many times and also compared it with T420. Here are the results:

1, Run VGA Option ROMs+Legacy VGA text mode+VGA BIOS+SeaBIOS:

T420: Everything works fine

Can see SeaBIOS menu?: y

Can see Linux/Windows bootloader? (the Manjaro's start screen and the Windows 10's spinning circle): y

Can boot in Linux?: y

Can boot in Windows?: y

T440p:

Can see SeaBIOS menu?: y

Can see Linux/Windows bootloader?: n

Can boot in Linux?: y

Can boot in Windows?: n

2, libgfxinit+Legacy VGA text mode+SeaBIOS:

T420:

Can see SeaBIOS menu?: y

Can see Linux/Windows bootloader?: y

Can boot in Linux?: y

Can boot in Windows?: n

T440p:

Can see SeaBIOS menu?: y

Can see Linux/Windows bootloader?: n

Can boot in Linux?: y

Can boot in Windows?: n

3, libgfxinit+Legacy VGA text mode+SeaBIOS:

T420:

Can see SeaBIOS menu?: y

Can see Linux/Windows bootloader?: y

Can boot in Linux?: y

Can boot in Windows?: n

T440p:

Can see SeaBIOS menu?: y

Can see Linux/Windows bootloader?: n

Can boot in Linux?: y

Can boot in Windows?: n

4, libgfxinit+high resolution framebuffer+SeaBIOS:

T420:

Can see SeaBIOS menu?: y

Can see Linux/Windows bootloader?: y

Can boot in Linux?: y

Can boot in Windows?: n

T440p:

Can see SeaBIOS menu?: y

Can see Linux/Windows bootloader?: n

Can boot in Linux?: y

Can boot in Windows?: n

5, libgfxinit+Legacy VGA text mode+VGA BIOS+SeaBIOS:

T420:

Can see SeaBIOS menu?: n

Can see Linux/Windows bootloader?: n

Can boot in Linux?: y

Can boot in Windows?: n

T440p:

Can see SeaBIOS menu?: n

Can see Linux/Windows bootloader?: n

Can boot in Linux?: y

Can boot in Windows?: n

6, libgfxinit+high resolution framebuffer+VGA BIOS+SeaBIOS:

T420:

Can see SeaBIOS menu?: n

Can see Linux/Windows bootloader?: n

Can boot in Linux?: y

Can boot in Windows?: y

T440p:

Can see SeaBIOS menu?: n

Can see Linux/Windows bootloader?: n

Can boot in Linux?: y

Can boot in Windows?: n

So the conclusion is:

Under the first setting everything works fine for the T420. But the current T440p's port always can't display Linux/Windows's bootloader, and in the end it can't boot in Windows by all means. Has anyone using the T440p successfully boot in Windows under SeaBIOS? Is this a problem with my hardware or a general problem?

Is it in order to boot Windows the VGA BIOS is a must? I was suprised under the 6th setting T420 can boot in Windows, but add the VGA BIOS while using the libgfxinit make the SeaBIOS menu can't be displayed. Could this be fixed?

Lastly, I want to debug future. I heard it might be hard to debug VGA BIOS or Windows... What logs should I get and how to get them? Using "sudo cbmem -c" the log ends after SeaBIOS booted the disk. Using "dmesg | grep VGA" there are not much infomation.