[U-Boot] Makefile: Only build dtc if needed

[Simon Glass]

Hi Tom,

On Mon, 27 Apr 2020 at 12:59, Tom Rini <trini at konsulko.com> wrote:
>
> On Sun, Apr 26, 2020 at 06:34:48PM -0600, Simon Glass wrote:
> > Hi Tom,
> >
> > On Sat, 26 Jan 2019 at 20:51, Tom Rini <trini at konsulko.com> wrote:
> > >
> > > On Tue, Nov 13, 2018 at 03:43:07PM -0700, Simon Glass wrote:
> > >
> > > > At present U-Boot always builds dtc if CONFIG_OF_CONTROL is defined. This
> > > > is wasteful when the system already has a suitable version available.
> > > >
> > > > Update the Makefile logic to build dtc only if the version available is
> > > > too old.
> > > >
> > > > This saves about 3 seconds of CPU time on a clean build for me.
> > > >
> > > > Signed-off-by: Simon Glass <sjg at chromium.org>
> > > > Reviewed-by: Marek Vasut <marek.vasut at gmail.com>
> > >
> > > In the case of new enough host dtc this then fails on sandbox_spl,
> > > chromebook_minnie and others where we need to build
> > > spl/dts/dt-platdata.c with a python import error over lacking _libfdt.
> >
> > I'm not sure if it has been long enough, but have the packages been
> > updated to include pylibfdt?
> >
> > I'm sending a new version of this patch that checks for that.
>
> I honestly don't know how far behind distributions lag here and then in
> turn how old something is before we say "sorry, you can't build there
> easily".

I think we should be able to deal with this with respect to dtc at
least, since we can always build it if needed. We just need to get the
conditions right.

Regards,
SImon