linux-kernel - Re: [PATCH 06/12] arm64: dts: zynqmp: Add label for zynqmp

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Message-ID: <YAoAN3JF27zWnjn5@pendragon.ideasonboard.com>
Date:   Fri, 22 Jan 2021 00:29:11 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Michal Simek <michal.simek@...inx.com>
Cc:     linux-kernel@...r.kernel.org, monstr@...str.eu, git@...inx.com,
        Kalyani Akula <kalyani.akula@...inx.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Manish Narani <manish.narani@...inx.com>,
        Rajan Vaja <rajan.vaja@...inx.com>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 06/12] arm64: dts: zynqmp: Add label for zynqmp_ipi

Hi Michal,

I've just realized I forgot to reply to this e-mail, sorry.

On Tue, Dec 08, 2020 at 08:26:41AM +0100, Michal Simek wrote:
> On 07. 12. 20 23:16, Laurent Pinchart wrote:
> > On Mon, Dec 07, 2020 at 10:39:25AM +0100, Michal Simek wrote:
> >> On 06. 12. 20 23:46, Laurent Pinchart wrote:
> >>> On Wed, Dec 02, 2020 at 03:06:05PM +0100, Michal Simek wrote:
> >>>> Add label which is used by bootloader for adding bootloader specific flag.
> >>>>
> >>>> Signed-off-by: Michal Simek <michal.simek@...inx.com>
> >>>> ---
> >>>>
> >>>> U-Boot needs to add u-boot,dm-pre-reloc; property
> >>>
> >>> I'm not entirely sure what best practice rules are in this area, but
> >>> shouldn't U-Boot locate the node by name instead of label ?
> >>
> >> Labels are not listed in dt binding and there are two approaches how to
> >> reference nodes. Via full path with node name or via labels.
> >> I do normally use labels which are much simple.
> > 
> > Note that labels require the DTB to be compiled with the -@ option,
> > otherwise they're not present in the binary.
> 
> U-Boot is using different concept. You can see that there are a lot of
> -u-boot.dtsi files in dts folders. These are automatically included to
> DTS before DTC is called. It means you don't need to build overlay to
> get merged.
> 
> >> And also if you take a look how dtb looks like (convert back to dts) you
> >> can see that for example aliases are using full path (just &label) but
> >> clocks/gic which is the part of <> is handled via phandles as numbers.
> >>
> >> And labels names can vary and shouldn't be the part of binding doc as
> >> far as I know. But I can be wrong of course.
> > 
> > The DT bindings should document the interface with the operating system,
> > and if applicable, the boot loader. If the boot loader requires a
> > particular label, then it becomes part of the ABI, and I think it should
> > be documented in the bindings.
> 
> We have been discussing with Rob some month ago but didn't have a time
> to do step further. Just keep it short Rob was ok to keep bootloader
> binding inside Linux repo.

I think that makes sense, DT bindings are meant to be OS-agnostic, so
boot loader requirements should be documented there.

> There is no hardcoding for a particular name. There is just a need to
> have any label. U-Boot needs to have one property(e.g.
> u-boot,dm-pre-reloc;) just to do early allocation.
> The name is just reference and none is really looking for it. It is just
> a way how to include it in much easier way.

Just to make sure I understand this issue correctly, does this mean that
you need to reference the node in a *-u-boot.dtsi file, and want a label
to do so ? The label name needs to be the same in the base file (taken
from the Linux source tree) and the *-u-boot.dtsi file (in the U-Boot
source tree) in that case. Isn't it the role of DT bindings to document
such requirements ?

-- 
Regards,

Laurent Pinchart