How to add extra dependencies to kernel modules
 

I'm trying to add fbtft kernel module as an external package in openwrt by copying all necessary files in /package/fbtft directory like this guy did https://github.com/GBert/openwrt-misc/tree/master/fbtft

The problem is that fbtft is compiled using -DCONFIG_FB_DEFERRED_IO=1 . This define is used by some framebuffer structures like fb_info from fb.h. If CONFIG_FB_DEFERRED_IO is defined then the structure has a different size. Now linux frame buffer files like fbsysfs.c, which uses this fb_info strucure, are not compiled with this CONFIG_FB_DEFERRED_IO flag which causes fb_info to have a different size. Fbtft uses fbsysfs.c to allocate memory for fb_info structure but the size is different between the two because they are not compiled with the same flags.

I tried to add FB_BROADSHEET and FB_AUO_K190X (which depend on FB_DEFERRED_IO) from "make kernel_menuconfig" but fbsysfs.c and other framebuffer files are still not compiled with the required flag (i can see this by looking at .fbsysfs.o.cmd file). How can I add -DCONFIG_FB_DEFERRED_IO=1 flag at compile time for all linux framebuffer files so they all have the correct fb_info structure ?

Thanks

Edit the .config file in the root of the kernel source. Change
to
then recompile the kernel.

I've attached the .config (renamed to config.txt) and the .o.cmd file after compilation.
It seems CONFIG_FB_DEFERRED_IO is set to y but if you look at the .o.cmd file you'll see that fbsysfs.c was not compiled with -DCONFIG_FB_DEFERRED_IO=1.
After I modified the .config file I ran make V=s in the root of openwrt source tree so I compiled not only the kernel.

Shouldn't I modify framebuffer Makefile and manually add -DCONFIG_FB_DEFERRED_IO=1 ?

If your build tools are not including .config in the compilation, then most modules will not work.

How can I know for sure if the .config file is used by the build tools ?

One more question: if CONFIG_FB_DEFERRED_IO=y in .config file all files should be compiled with -DCONFIG_FB_DEFERRED_IO=1 ? How does the build tool choose what files are compiled with what define flags ?