@ashi your answer to
“I saw GXBB(S905) files in the 4.9 kernel source. So why can’t it be supported?”
Amlogic decided not to continue supporting this older SoC early in their development of the 4.9 kernel. There is some consideration for GXBB(S905) in some parts that they added but as you assumed this is left overs from previous work ported to the 4.9 kernel. (Some of the code mentioning GXBB(S905) is also actually code that was already present in the kernel from community efforts before Amlogic started development on their version of the 4.9 kernel, and doesn’t work with the code Amlogic added.)
So in short there just isn’t enough of the code that would be required to to make it run included in the kernel, and it would be a lot of work to add the missing stuff.
Now some will remember hearing that GXBB and GXL are somewhat related, and do share some of the code that is required to support them. Which is a bit of a start to getting the 4.9 kernel booting on the S905. However not nearly enough is shared, and there is a lot of places in the GXL code paths that you need to account for these differences as well. I made an attempt at creating the missing device trees and filling in some of the missing bits for the GXBB(S905). Unfortunately close is not close enough, without a lot more time, effort, and documentation then I had available.
There is also a good chance too, that even if you got it booting that you would then encounter two further problems.
One, the Amlogic media modules(The drivers for the the video decoders, etc…) have been being developed with no consideration for GXBB(S905) support for years now, and would probably take their own extended effort to make work properly with the GXBB(S905).
Two, the 4.9 vendor kernel has issues working with the older versions of the vendor uboot that likely was used on these older Amlogic devices. So A lot of effort could go in for something that may still not even boot on the majority of GXBB(S905) based TV boxes. (Some GXL devices have this issue as well, if they don’t have at least the version of the vendor uboot that would have been used with an Android 7 or newer.)