When pulling the recent pinctrl merge, I was surprised by how a
pinctrl-only pull request ended up rebuilding basically the whole
kernel.

The reason for that ended up being that <linux/device.h> included
<linux/pinctrl/devinfo.h>, so any change to that file ended up causing
pretty much every driver out there to be rebuilt.

The reason for that was because 'struct device' has this in it:

    #ifdef CONFIG_PINCTRL
        struct dev_pin_info     *pins;
    #endif

but we already avoid header includes for these kinds of things in that
header file, preferring to just use a forward-declaration of the
structure instead.  Exactly to avoid this kind of header dependency.

Since some drivers seem to expect that <linux/pinctrl/devinfo.h> header
to come in automatically, move the include to <linux/pinctrl/pinctrl.h>
instead.  It might be better to just make the includes more targeted,
but I'm not going to review every driver.

It would definitely be good to have a tool for finding and minimizing
header dependencies automatically - or at least help with them.  Right
now we almost certainly end up having way too many of these things, and
it's hard to test every single configuration.

FWIW, you can get a sense of the "hotness" of a header file with something
like this after doing a full build:

    find . -name '.*.o.cmd' -print0 |
        xargs -0 tail --lines=+2 |
        grep -v 'wildcard ' |
        tr ' \\' '\n' |
        sort | uniq -c | sort -n | less -S

which isn't exact (there are other things in those '*.o.cmd' than just
the dependencies, and the "--lines=+2" only removes the header), but
might a useful approximation.

With this patch, <linux/pinctrl/devinfo.h> drops to "only" having 833
users in the current x86-64 allmodconfig.  In contrast, <linux/device.h>
has 14857 build files including it directly or indirectly.

Of course, the headers that absolutely _everybody_ includes (things like
<linux/types.h> etc) get a score of 23000+.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>