In the end, I want to develop/adapt a usb driver for a usb wifi card I bought because I wanted to use linux to use a browser on the calculator, but it seems my hardware revision (AA) has a screen linux doesn't support.
I don't know enough about device trees and display drivers, or even what display exactly it is, so I looked into usb driver development for the nspire.

I'm trying to make a usb driver and started by looking at the code of HIDn.
I first tried to copy it to my own code and set breakpoints, but it gives compilation errors.
I then tried to recompile vanilla HIDn, but the errors persistet:
nspire-gcc -Os -Wall -W -marm -c ums.c
ums.c:33:24: error: field ‘sc_ibuf’ has incomplete type
  struct s_usb_pipe_buf sc_ibuf;
                        ^~~~~~~
ums.c: In function ‘attach’:
ums.c:134:44: warning: passing argument 7 of ‘usbd_open_pipe_intr’ makes pointer from integer without a cast [-Wint-conversion]
                            &sc->sc_ibuf, sc->sc_isize, ums_intr,
                                          ~~^~~~~~~~~~
In file included from /mnt/c/Users/usertemp/Desktop/TI/Ndless/ndless-sdk/bin/../include/os.h:30,
                 from ums.c:1:
/mnt/c/Users/usertemp/Desktop/TI/Ndless/ndless-sdk/bin/../include/syscall-decls.h:189:149: note: expected ‘uint32_t *’ {aka ‘long unsigned int *’} but argument is of type ‘int’
 erface_handle p1, uint8_t p2, uint8_t p3, usbd_pipe_handle *p4, usbd_private_handle p5, void *p6, uint32_t *p8, usbd_callback p9, int p10);
                                                                                                   ~~~~~~~~~~^~

ums.c: In function ‘ums_register’:
ums.c:158:2: warning: implicit declaration of function ‘nl_relocdata’; did you mean ‘e_nl_relocdatab’? [-Wimplicit-function-declaration]
  nl_relocdata((unsigned*)methods, sizeof(methods)/sizeof(methods[0]) - 1);
  ^~~~~~~~~~~~
  e_nl_relocdatab
make: *** [Makefile:24: ums.o] Error 1

I found the code on the nspiretools-history github: https://github.com/ndless-nspire/nsptools-history/tree/master/hidn/trunk
It's also on TI-Planet: https://tiplanet.org/forum/archives_voir.php?id=12928

Maybe the usb api in the ndless-sdk got changed, because the last changes to HIDn on github are 7 years old.
It seems as if nl_relocdata and struct s_usb_pipe_buf got removed.
The binary version works on my cx cas, so it's still possible to write usb drivers.
Also I'm using Windows Subsystem for Linux with debian to compile.
If anyone knows how to fix this, I'd be happy.
(this is my first post and I'm not a native speaker, please forgive me if I make spelling mistakes)