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

[Tom Rini]

On Mon, Apr 27, 2020 at 04:25:10PM -0600, Simon Glass wrote:
> 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.

To keep the threads in sync, I'm not sure it's worth diverging from the
Linux kernel build in this regard.

-- 
Tom
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