net: ethernet: ti-cpsw: fix linking built-in code to modules

net: ethernet: ti-cpsw: fix linking built-in code to modules

There are six variants of the cpsw driver, sharing various parts of
the code: davinci-emac, cpsw, cpsw-switchdev, netcp, netcp_ethss and
am65-cpsw-nuss.

I noticed that this means some files can be linked into more than
one loadable module, or even part of vmlinux but also linked into
a loadable module, both of which mess up assumptions of the build
system, and causes warnings:

scripts/Makefile.build:279: cpsw_ale.o is added to multiple modules: ti-am65-cpsw-nuss ti_cpsw ti_cpsw_new
scripts/Makefile.build:279: cpsw_priv.o is added to multiple modules: ti_cpsw ti_cpsw_new
scripts/Makefile.build:279: cpsw_sl.o is added to multiple modules: ti-am65-cpsw-nuss ti_cpsw ti_cpsw_new
scripts/Makefile.build:279: cpsw_ethtool.o is added to multiple modules: ti_cpsw ti_cpsw_new
scripts/Makefile.build:279: davinci_cpdma.o is added to multiple modules: ti_cpsw ti_cpsw_new ti_davinci_emac

Change this back to having separate modules for each portion that
can be linked standalone, exporting symbols as needed:

 - ti-cpsw-common.ko now contains both cpsw-common.o and
   davinci_cpdma.o as they are always used together

 - ti-cpsw-priv.ko contains cpsw_priv.o, cpsw_sl.o and cpsw_ethtool.o,
   which are the core of the cpsw and cpsw-new drivers.

 - ti-cpsw-sl.ko contains the cpsw-sl.o object and is used on
   ti-am65-cpsw-nuss.ko in addition to the two other cpsw variants.

 - ti-cpsw-ale.o is the one standalone module that is used by all
   except davinci_emac.

Each of these will be built-in if any of its users are built-in, otherwise
it's a loadable module if there is at least one module using it. I did
not bring back the separate Kconfig symbols for this, but just handle
it using Makefile logic.

Note: ideally this is something that Kbuild complains about, but usually
we just notice when something using THIS_MODULE misbehaves in a way that
a user notices.

Fixes: 99f6297182729 ("net: ethernet: ti: cpsw: drop TI_DAVINCI_CPDMA config option")
Link: https://lore.kernel.org/lkml/20240417084400.3034104-1-arnd@kernel.org/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patch.msgid.link/20260402184726.3746487-2-arnd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>