[v4,5/8] kbuild: add fine grained build dependencies for exported symbols

Like with kconfig options, we now have the ability to compile in and
out individual EXPORT_SYMBOL() declarations based on the content of
include/generated/autoksyms.h.  However we don't want the entire
world to be rebuilt whenever that file is touched.

Let's apply the same build dependency trick used for CONFIG_* symbols
where the time stamp of empty files whose paths matching those symbols
is used to trigger fine grained rebuilds. In our case the key is the
symbol name passed to EXPORT_SYMBOL().

However, unlike config options, we cannot just use fixdep to parse
the source code for EXPORT_SYMBOL(ksym) because several variants exist
and parsing them all in a separate tool, and keeping it in synch, is
not trivially maintainable.  Furthermore, there are variants such as

	EXPORT_SYMBOL_GPL(pci_user_read_config_##size);

that are instanciated via a macro for which we can't easily determine
the actual exported symbol name(s) short of actually running the
preprocessor on them.

Storing the symbol name string in a special ELF section doesn't work
for targets that output assembly or preprocessed source.

So the best way is really to leverage the preprocessor by having it emit
a warning for each EXPORT_SYMBOL() instance and filtering those apart
from stderr by the build system. Then the list of symbols is simply fed
to fixdep to be merged with the other dependencies.

Because of the lowercasing performed by fixdep, there might be name
collisions triggering spurious rebuilds for similar symbols. But this
shouldn't be a big issue in practice. (This is the case for CONFIG_*
symbols and I didn't want to be different here, whatever the original
reason for doing so.)

To avoid needless build overhead, the exported symbol name gathering is
performed only when CONFIG_TRIM_UNUSED_KSYMS is selected.

Signed-off-by: Nicolas Pitre <nico@linaro.org>

Acked-by: Rusty Russell <rusty@rustcorp.com.au>

---
 include/linux/export.h | 16 ++++++++++++++--
 scripts/Kbuild.include | 28 ++++++++++++++++++++++++++++
 scripts/basic/fixdep.c |  1 +
 3 files changed, 43 insertions(+), 2 deletions(-)

-- 
2.5.0

On Thu, 3 Mar 2016, Michal Marek wrote:

> Dne 29.2.2016 v 04:48 Nicolas Pitre napsal(a):

> > +# Filter out exported kernel symbol names advertised as warning pragmas

> > +# by the preprocessor and write them to $(1). We must consider continuation

> > +# lines as well: they start with a blank, or the preceeding line ends with

> > +# a ':'. Anything else is passed through as is.

> > +# See also __KSYM_DEP() in include/linux/export.h.

> > +ksym_dep_filter = sed -n \

> > +	-e '1 {x; $$!d}' \

> > +	-e '/^ / {H; $$!d}' \

> > +	-e 'x; /:$$/ {x; H; $$!d; s/^/ /; x}' \

> > +	-e ':filter; /^.*KBUILD_AUTOKSYM_DEP: /! {p; b next}' \

> > +	-e 's//KSYM_/; s/\n.*//; w $(1)' \

> > +	-e ':next; $$!d' \

> > +	-e '1 q; s/^/ /; x; /^ /! b filter'

> 

> This is unreadable and it does not work with my gcc version. I get

> dependencies like

> 

>     $(wildcard include/config/ksym/simple/strtoull [enabled by default].h) \


That's too bad.  This had almost zero overhead as the preprocessor 
didn't have to be executed twice.

Probably adding s/ .*//; before w $(1) would fix that.  But I agree this 
is fragile.  What gcc version is this?

> Please use some other way, which does not require parsing the compiler

> diagnostic messages. A straightforward solution is to do something

> similar to genksyms: A separate preprocessor pass with -Dsomething that

> leaves the EXPORT_SYMBOL statements alone and just collect their occurences.


I went that route at some point. But here I need to extract those 
EXPORT_SYMBOL() instances for all if_changed_dep uses.  That includes 
cc_o_c, cc_s_c, cc_i_c, cc_lst_c, as_s_S, as_o_S, etc. etc.  I need the 
corresponding preprocessor flags without the usual target flags.  Let's 
see what I can do that is more readable than a sed script.

Oh and who said that a sed script had to, or could be readable?  ;-)


Nicolas