Issues building RT patchset v6.18.13-rt4-rebase on top of v6.18.39

[David Runge <dave@sleepmap.de>]

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Hi,

on Arch Linux we have switched to a rebase workflow to offer the linux-rt [1]
and linux-rt-lts [2] kernels on latest kernels of the series, so that we can
also apply security and regression fixes.

For linux-rt-lts and with rebasing v6.18.13-rt4-rebase [3] onto v6.18.39 (and
later on applying our own distribution patches) [4], we run into a build failure
though.
(The rebase applied cleanly).

```
  CC [M]  drivers/misc/cardreader/rts5249.o
drivers/tty/serial/8250/8250_port.c:3225:7: error: conflicting types for ‘serial8250_fifo_wait_for_lsr_thre’; have ‘void(struct uart_8250_port *, struct nbcon_write_context *, unsigned int)’
 3225 |  void serial8250_fifo_wait_for_lsr_thre(struct uart_8250_port *up,
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from drivers/tty/serial/8250/8250_port.c:40:
drivers/tty/serial/8250/8250.h:189:6: note: previous declaration of ‘serial8250_fifo_wait_for_lsr_thre’ with type ‘void(struct uart_8250_port *, unsigned int)’
  189 | void serial8250_fifo_wait_for_lsr_thre(struct uart_8250_port *up, unsigned int count);
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ./include/linux/linkage.h:7,
                 from ./include/linux/printk.h:8,
                 from ./include/asm-generic/bug.h:22,
                 from ./arch/x86/include/asm/bug.h:108,
                 from ./arch/x86/include/asm/alternative.h:9,
                 from ./arch/x86/include/asm/barrier.h:5,
                 from ./include/linux/list.h:11,
                 from ./include/linux/module.h:12,
                 from drivers/tty/serial/8250/8250_port.c:14:
drivers/tty/serial/8250/8250_port.c:3245:22: error: conflicting types for ‘serial8250_fifo_wait_for_lsr_thre’; have ‘void(struct uart_8250_port *, struct nbcon_write_context *, unsigned int)’
 3245 | EXPORT_SYMBOL_NS_GPL(serial8250_fifo_wait_for_lsr_thre, "SERIAL_8250");
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/export.h:76:28: note: in definition of macro ‘__EXPORT_SYMBOL’
   76 |         extern typeof(sym) sym;                                 \
      |                            ^~~
drivers/tty/serial/8250/8250_port.c:3245:1: note: in expansion of macro ‘EXPORT_SYMBOL_NS_GPL’
 3245 | EXPORT_SYMBOL_NS_GPL(serial8250_fifo_wait_for_lsr_thre, "SERIAL_8250");
      | ^~~~~~~~~~~~~~~~~~~~
drivers/tty/serial/8250/8250.h:189:6: note: previous declaration of ‘serial8250_fifo_wait_for_lsr_thre’ with type ‘void(struct uart_8250_port *, unsigned int)’
  189 | void serial8250_fifo_wait_for_lsr_thre(struct uart_8250_port *up, unsigned int count);
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
make[6]: *** [scripts/Makefile.build:287: drivers/tty/serial/8250/8250_port.o] Error 1
make[5]: *** [scripts/Makefile.build:544: drivers/tty/serial/8250] Error 2
make[4]: *** [scripts/Makefile.build:544: drivers/tty/serial] Error 2
make[3]: *** [scripts/Makefile.build:544: drivers/tty] Error 2
make[3]: *** Waiting for unfinished jobs....
```

The culprit appears to be this commit:
'Reapply "serial: 8250: Switch to nbcon console"' [5]

I'm not sure who is currently working on the patchset for the 6.18 LTS branch,
but I guess it is worth exploring to drop/adjust the commit and release a new
patchset?
Is the patch still needed?

Best,
David

[1] https://archlinux.org/packages/extra/x86_64/linux-rt/
[2] https://archlinux.org/packages/extra/x86_64/linux-rt-lts/
[3] https://gitlab.archlinux.org/archlinux/packaging/upstream/linux-rt-lts/-/commits/v6.18.13-rt4-rebase
[4] https://gitlab.archlinux.org/archlinux/packaging/upstream/linux-rt-lts/-/commits/v6.18.39-rt4-arch1
[5] https://gitlab.archlinux.org/archlinux/packaging/upstream/linux-rt-lts/-/commit/c5661d54576a9bf12c2fd0eaa2a54baf48e45d4a
[6] https://wiki.linuxfoundation.org/realtime/start

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