[Buildroot] [PATCH 3 of 4 RFC] manual: add section about depending on toolchain options

[Thomas De Schampheleire]

Hi Thomas,

On Wed, Sep 18, 2013 at 6:34 PM, Thomas Petazzoni
<thomas.petazzoni at free-electrons.com> wrote:
> Dear Thomas De Schampheleire,
>
> On Wed, 18 Sep 2013 13:15:41 +0200, Thomas De Schampheleire wrote:
>
>
> I agree with both. I believe the documentation should explicitly state
> that a BR2_USE_MMU dependency or an architecture dependency should not
> be mentioned in a comment (with the reason that you explain).

Ok, I will add that.

>
> However, for the reason that you explain, showing a comment when the
> problem is a lack of dynamic library is not completely good. On some
> noMMU architectures, there no support for dynamic libraries at all, so
> the user cannot "enable" that at all. But well, despite this, I believe
> it's better to show a comment when a package is not available due to
> the lack of shared libraries.

Currently there is one option PREFER_STATIC_LIB, with help text:
          Where possible, build and use static libraries for the
target.
          This potentially increases your code size and should only be
          used if you know what you do.
          The default is to build dynamic libraries and use those on
          the target filesystem.

However, my interpretation of the current usage of this symbol is that
it is not PREFER_STATIC_LIB, but rather something like
STATIC_LIB_ONLY. Because some packages are simply not available when
this is set, instead of building the package with dynamic linking.

The current name PREFER_STATIC_LIB is also unfortunate in the case
that dynamic linking is not available because the target does not
support it, e.g. because of no MMU (thanks for mentioning this, I
wasn't aware). A name STATIC_LIB_ONLY would also match better here.

Note that I'm not necessarily requesting we rename the symbol, but I
find the current situation confusing.

Maybe we need an extra symbol, ARCH_NEEDS_STATIC_LIB or similar (or
re-use a possible new !ARCH_HAS_MMU) in addition to a user choice
PREFER_STATIC_LIB. When a package cannot be linked statically, we only
check on ARCH_NEEDS_STATIC_LIB. If ARCH_NEEDS_STATIC_LIB is false, and
PREFER_STATIC_LIB is true, then we build the package dynamically.

Best regards,
Thomas