Re: [PATCH v2 2/3] sam460ex: Remove FDT_PPC dependency from KConfig

[Peter Maydell <peter.maydell@linaro.org>]

On Mon, 4 Jan 2021 at 01:51, BALATON Zoltan <balaton@eik.bme.hu> wrote:
> Something is still not right with this, I've noticed that a few other
> boards also have this option selected but they don't need fdt.o that's
> gated by this option in meson.build. That fdt.o is only needed by PSERIES
> and POWERNV but removing FDT_PPC from other boards I get:
>
> ../hw/ppc/sam460ex.c:43:10: fatal error: libfdt.h: No such file or directory
>   #include <libfdt.h>
>
> so apparently this switch also pulls in the necessary CPPFLAGS or libfdt
> dependency. Is there a separate switch for that or we can only get it with
> fdt.o. Not a big deal just not trivial why we need an option that at first
> sight select a source file which we don't need. I think I'll drop this
> patch for now when resending the series.

This happens because hw/ppc/meson.build does:

ppc_ss.add(when: 'CONFIG_FDT_PPC', if_true: [files(
  'fdt.c',
), fdt])

ie if CONFIG_FDT_PPC is set then both
 * add fdt.c to the build
 * add the 'fdt' dependency (which brings in the CFLAGS and
   LDFLAGS necessary for libfdt).

So yes, at the moment for PPC there are only two options:
 * board doesn't use FDT at all
 * board uses FDT and gets hw/ppc/fdt.c linked in even if it
   doesn't use the functions there

Since fdt.c currently has just one not very large function (which
is only even present if TARGET_PPC64) this doesn't seem like a big
deal, but in theory the "need libfdt" and "need fdt.c" parts could
be decoupled.

thanks
-- PMM