doc and Makefile: portability stuff

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>

doc/spec-sw.in

@@ -1261,6 +1261,32 @@ be made safer.
 
 @end itemize
 
+@c ##########################################################################
+@node Portability
+@chapter Portability
+
+This package should be portable. However I didn't test it on a wide
+variety of systems.  I used the core @code{spec.ko} on a 32-bit x86
+host, running version 2.6.32 and 3.4 of the kernel.
+
+The complete package compiles without any warning from version 2.6.37
+up to 3.5 (I didn't try later versions, yet).  Frame timestamping
+changed seriously after 2.6.36, so the @code{wr-nic.ko} driver is not
+easily backward portable.
+
+To allow use of the core @i{spec} driver, to drive custom mezzanines,
+the @i{Makefile} supports the configuration variable
+@code{CONFIG_WR_NIC}, which you may set to @code{n} before compiling:
+
+@smallexample
+   export CONFIG_WR_NIC=n
+@end smallexample
+
+With this in place, the package compiles without any warning on a
+32-bit PC from version 2.6.30 onwards, and I tested up to 3.5.
+If you remove MSI code, you can compile back to version 2.6.27; earlier
+than that, the backporting effort increases seriously.
+
 @c ##########################################################################
 @bye
 

kernel/Makefile

@@ -9,8 +9,13 @@ KBUILD_EXTRA_SYMBOLS := $(FMC_DRV)/Module.symvers
 ccflags-y += -I$M/include -I$(FMC_DRV)/include
 ccflags-y += $(WR_NIC_CFLAGS)
 
+# this is a bad hack. Sometimes we are a submodule, and wr-nic can
+# only compile with recent versions, so let the caller disable it
+# FIXME: this is incorrect if we get copied to the kernel proper.
+CONFIG_WR_NIC ?= m
+
 obj-m += spec.o
-obj-m += wr-nic.o
+obj-$(CONFIG_WR_NIC) += wr-nic.o
 
 spec-y = spec-pci.o
 spec-y += spec-fmc.o