[ptxdist] target package dependency to platform

[Alexander Dahl <ada@thorsis.com>]

Hei hei,

I'm stuck with a problem where I can not find an elegant solution for.  
Let me describe.

I have a layered BSP.  The base layer has different platformconfigs for 
hardware targets with different SoC architectures, let's say the 
PTXCONF_PLATFORM is "pla" on one and "plb" on the other.  (I can not build the 
whole stuff for one platform only, two platforms are strictly required.)

There is a more or less ordinary CMake based target application in the base 
layer, let's say in "base/rules/myapp.{in,make}", because that application is 
needed on both platforms.

The two upper layers select the different platforms with different toolchains 
and both also enable "myapp".  (Yes, I could also have it all in a single 
layer BSP with two working copies and building one platform per working copy.  
Doesn't change the problem.)

myapp depends on multiple libraries, some of them hardware specific.  So for 
platform "pla" it needs "libmyliba" and for platform "plb" it does not.

In "base/rules/myapp.make" I pass PTXCONF_PLATFORM as a build time option to 
CMake like this:

    MYAPP_CONF_OPT += -DHARDWARE_NAME=$(PTXCONF_PLATFORM)

Depending on "HARDWARE_NAME" I call `find_package(myliba)` in "CMakeLists.txt" 
of myapp.

I could do a "select LIBMYLIBA" in "base/rules/myapp.in" unconditionally, and 
that would make the build successful.  In the BSP for platform "pla" CMake 
would look for libmyliba and link and on the target it would be used at 
runtime just fine.  In the BSP for platform "plb" CMake would not look for it, 
would not complain, myapp would run happily on the target without using 
libmyliba just fine.

However, libmyliba would be built for platform "plb" and it would be copied 
over to "plb" target and take space, but never be used there.  Build time for 
it would be completely wasted.  Even worse, user might find it on target and 
wonder what it means, although it's useless on platform "plb".

I could _not_ add that "select LIBMYLIBA" and build would fail for target 
"pla" dependending on package build order.  Also dependencies in opkg packages 
would not be correct then.

What also does not work is this (in "base/rules/myapp.in"):

    select LIBMYLIBA if PLATFORM = "pla"

This is valid syntax from Kconfig point of view, but PLATFORM is just not 
defined here at all.  I assume configs/ptxconfig and configs/platform-pla/
platformconfig are somewhat distinct Kconfig spaces and one can not simply 
reference to Kconfig options from the other side.

Interestingly I can do `ptxdist print PTXCONF_PLATFORM` and `ptxdist print 
PTXCONF_MYAPP` and ptxdist prints variables from both Kconfig spaces.  Also I 
can successfully use both in base/rules/myapp.make and that's at least 
somewhat misleading. (Misleading as in: Why can I use both here but not 
there?)

That's where I'm stuck.  Of course I looked through ptxdist git sources to 
find hints on how to solve this dilemma, but without luck.

Would be happy if anyone could give a hint or suggest a solution.

Greets
Alex