[PATCH v2 2/2] Makefile: Only build dtc if needed

[Simon Glass]

Hi Tom.

On Tue, 28 Apr 2020 at 08:19, Tom Rini <trini at konsulko.com> wrote:
>
> On Mon, Apr 27, 2020 at 04:10:06PM -0700, Vagrant Cascadian wrote:
> > On 2020-04-27, Simon Glass wrote:
> > > On Sun, 26 Apr 2020 at 18:58, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
> > >>
> > >> Am April 27, 2020 12:29:29 AM UTC schrieb Simon Glass <sjg at chromium.org>:
> > >> >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 2.5 seconds of elapsed time on a clean build for me.
> > >> >
> > >> >- Add a patch to bring back the dtc-version.sh script
> > >> >- Update the check to make sure libfdt is available if needed
> > >>
> > >> U -Boot has been set up to create reproducible builds. With this
> > >> patch dtc will have to be made a build dependency to provide
> > >> reproducibility. Cf. https://www.debian.org/doc/debian-policy/ch-source.html#reproducibility
> > >>
> > >> This may require changes in the packaging of U-Boot in Linux
> > >> distributions. Nothing to stop this patch, just something to keep in
> > >> mind.
> > >>
> > >> You presume that future versions of dtc will always be backward
> > >> compatible with U-Boot. Ok, we do the same for other tools like gcc
> > >> too (with surprises at each new major release).
> >
> > In general when packaging for Debian, the preference is to not use
> > embedded code copies if at all possible. This does require paying
> > attention to backwards and forwards compatibility issues a bit.
> >
> > A simple example: The security team in Debian generally likes to fix a
> > problem in a single source package, rather than an arbitrary number of
> > source packages that happen to share some embedded copy of the code from
> > who knows when...
> >
> > So at least from my perspective, I'd be happy to use the Debian packaged
> > dtc (a.k.a. device-tree-compiler), rather than the one embedded in
> > u-boot sources.
> >
> > Silently switching to the embedded copy sounds a little scary; I would
> > prefer for that to be explicit... a build flag to specify one way or the
> > other and failing is better that being too clever about autodetecting.
> >
> >
> > > Should we disable this check (and always build dtc) when doing a
> > > repeatable build? Is that SOURCE_DATE_EPOCH?
> >
> > And with my Reproducible Builds hat on, builds would ideally *always* be
> > reproducible, given the same sources and toolchain... several
> > distributions and software projects provide information sufficient to
> > reproduce the build environment:
> >
> >   https://reproducible-builds.org/docs/recording/
> >
> >
> > While SOURCE_DATE_EPOCH is definitely one sign that the builder is
> > explicitly attempting to be reproducible; It's a bit of a kludge to try
> > and be more reproducible just because SOURCE_DATE_EPOCH is
> > set. SOURCE_DATE_EPOCH should really only affect the behavior of date or
> > time related things; even better would be to not embded time related
>
> This is probably one of those cases where we should just continue to act
> like the linux kernel and always use and build our own copy of dtc.
> Then, when someone convinces the kernel folks to change their ways, we
> can adopt that.

It seems that Vagrant wants to use the system dtc by default and
require an explicit option to use the in-built dtc. I don't think that
would work well for most users though.

It is in my view somewhat mad to build dtc for every one of 1400
boards as I do today when running buildman.

Having said that it doesn't seem like we can come up with a default
behaviour that makes everyone happy, so the status quo is best.

But what about adding a build flag / envvar to select between:

- Use system default
- Build internal version
- Use system default if new enough, else build

Then we can satisfy distros as well as speed up the build for those that care.

I haven't heard from Marek on this thread, actually.

Regards,
Simon