Re: [RFC PATCH 4/5] hw/ppc/fdt: Drop dependency on libfdt

[Paolo Bonzini]

On 13/05/21 05:46, David Gibson wrote:
> > The patch makes sense in general.  The file is only needed for pseries and
> > powernv, not for e.g. e500 which does need fdt.
>
> Yes, agreed.
>
> > I would get rid of FDT_PPC completely.  First, before patch 3, you can move
> > fdt.c to PSERIES and POWERNV (it's too small to need its own Kconfig symbol)
> > and only leave
> >
> >     ppc_ss.add(when: 'CONFIG_FDT_PPC', if_true: fdt)
>
> Uh... why do we need even this?

To tell Meson that a board requires QEMU to be linked with libfdt.  This 
symbol is then renamed to CONFIG_LIBFDT once it can be used with all 
targets (rather than just hw/ppc).

> > Since you are at it, remove the silly #ifdef TARGET_PPC64 in the
> > hw/ppc/fdt.c file.
> >
> > Then in patch 3:
> >
> > - add to Kconfig.host
> >
> >      config FDT
> >         bool
> >
> >      config LIBFDT
> >         bool
> >         depends on FDT
>
> Um.. I'm not sure what semantic difference you're envisaging between
> FDT and LIBFDT.

"FDT" is set by meson.build if the library is available, LIBFDT is set 
by the board to link with the library.  In other words CONFIG_FDT is 
per-build, while CONFIG_LIBFDT is per-target.

If a board selects LIBFDT but the library is not available, minikconf 
will report a contradiction due to "CONFIG_PSERIES=y" -> "config PSERIES 
select LIBFDT" -> "config LIBFDT depends on FDT" ->  "CONFIG_FDT=n".

> > - for all the boards I listed in my review, add "select LIBFDT" in addition
> > to "depends on FDT".

This is actually unnecessary---"depends on FDT" is not needed in the 
boards because LIBFDT already has the dependency.

Paolo