[PATCH] dtc: add ability to make nodes conditional on them being referenced

[Andre Przywara]

On Tue, 28 Jan 2020 10:07:18 -0700
Simon Glass <sjg at chromium.org> wrote:

Hi Simon,

> On Sat, 25 Jan 2020 at 16:18, André Przywara <andre.przywara at arm.com> wrote:
> >
> > On 25/01/2020 16:20, Tom Rini wrote:
> >
> > Hi Tom,
> >
> > thanks for having a look!
> >
> >  
> > > On Tue, Jan 21, 2020 at 10:23:17AM +0000, Andre Przywara wrote:
> > >  
> > >> From: Maxime Ripard <maxime.ripard at bootlin.com>
> > >>
> > >> This is needed when importing mainline DTs into U-Boot, as some started
> > >> using this /omit-if-no-ref/ tag, so won't compile with U-Boot's current
> > >> dtc copy. This is just a cherry-pick of the patch introducing this
> > >> feature.
> > >> Original commit message from Maxime:
> > >> ------------------
> > >> A number of platforms have a need to reduce the number of DT nodes,
> > >> mostly because of two similar constraints: the size of the DT blob, and
> > >> the time it takes to parse it.
> > >>
> > >> As the DT is used in more and more SoCs, and by more projects, some
> > >> constraints start to appear in bootloaders running from SRAM with an
> > >> order of magnitude of 10kB. A typical DT is in the same order of
> > >> magnitude, so any effort to reduce the blob size is welcome in such an
> > >> environment.
> > >>
> > >> Some platforms also want to reach very fast boot time, and the time it
> > >> takes to parse a typical DT starts to be noticeable.
> > >>
> > >> Both of these issues can be mitigated by reducing the number of nodes in
> > >> the DT. The biggest provider of nodes is usually the pin controller and
> > >> its subnodes, usually one for each valid pin configuration in a given
> > >> SoC.
> > >>
> > >> Obviously, a single, fixed, set of these nodes will be used by a given
> > >> board, so we can introduce a node property that will tell the DT
> > >> compiler to drop the nodes when they are not referenced in the tree, and
> > >> as such wouldn't be useful in the targetted system.
> > >>
> > >> Signed-off-by: Maxime Ripard <maxime.ripard at bootlin.com>
> > >> Reviewed-by: Rob Herring <robh at kernel.org>
> > >> Signed-off-by: Andre Przywara <andre.przywara at arm.com>
> > >> ---
> > >>  scripts/dtc/checks.c     | 13 +++++++++++++
> > >>  scripts/dtc/dtc-lexer.l  |  7 +++++++
> > >>  scripts/dtc/dtc-parser.y | 17 +++++++++++++++++
> > >>  scripts/dtc/dtc.h        |  4 ++++
> > >>  scripts/dtc/livetree.c   | 14 ++++++++++++++
> > >>  5 files changed, 55 insertions(+)  
> 
> Reviewed-by: Simon Glass <sjg at chromium.org>

Thanks!

> 
> Is this for SPL or U-Boot proper?

I don't think either of those was the original target here, as this showed up in the Linux tree first.

If you need an answer to your question: it would actually be U-Boot proper, as it originates from Maxime's work on some Allwinner A20 devices - and we don't use DT in the SPL on sunxi.

> I assume the former since you talk about SRAM.

I think the idea was to provide a generic solution for some specific problem. The observation was that the .dtb is growing with all those pinmux nodes, also reportedly boot time increased because of the parsing efforts.
So short of hacking/trimming the DT manually, this tag was introduced, to address the issue in a generic way.

> But changing dtc won't affect SPL at present, since the DT
> is not separately compiled for SPL.  Of course if nodes are not needed
> for U-Boot proper, removing them is good and may have SPL too. But we
> use dtoc to drop unwanted nodes (as you probably know), and U-Boot has
> its own tags for indicating what nodes should be present (since
> everything is omitted from SPL by default).

Understood. As mentioned, sunxi doesn't even use the DT at all in the SPL.

The reason I posted this patch is simply that some mainline Linux .dts files carry this tag now, and without U-Boot's dtc understanding it, those DTs fail to build.
So for the sake of copying .dts files verbatim from the kernel tree, we need dtc support for this tag.

Cheers,
Andre.