linux-kernel - Re: [PATCH v2 08/12] ARM: dts: imx6dl-pico: fix board compatibles

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Date:   Fri, 2 Oct 2020 10:41:19 +0200
From:   Marco Felsch <m.felsch@...gutronix.de>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     Ahmad Fatoum <a.fatoum@...gutronix.de>, devicetree@...r.kernel.org,
        Robert Jones <rjones@...eworks.com>,
        Stefan Riedmueller <s.riedmueller@...tec.de>,
        Anson Huang <Anson.Huang@....com>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        linux-kernel@...r.kernel.org, Li Yang <leoyang.li@....com>,
        Rob Herring <robh+dt@...nel.org>,
        NXP Linux Team <linux-imx@....com>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Andreas Kemnade <andreas@...nade.info>,
        Fabio Estevam <festevam@...il.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 08/12] ARM: dts: imx6dl-pico: fix board compatibles

Hi,

sorry for jumping in.

On 20-10-02 10:20, Krzysztof Kozlowski wrote:
> On Fri, Oct 02, 2020 at 09:41:28AM +0200, Ahmad Fatoum wrote:
> > Hello,
> > 
> > On 10/1/20 12:37 PM, Krzysztof Kozlowski wrote:
> > >> The existing binding doesn't cover these boards then and needs to be
> > >> extended, no? How about following patch?
> > > 
> > > What do you mean it doesn't cover? It was added exactly to handle them:
> > > +              - technexion,imx6q-pico-dwarf   # TechNexion i.MX6Q Pico-Dwarf
> > > +              - technexion,imx6q-pico-hobbit  # TechNexion i.MX6Q Pico-Hobbit
> > > +              - technexion,imx6q-pico-nymph   # TechNexion i.MX6Q Pico-Nymph
> > > +              - technexion,imx6q-pico-pi      # TechNexion i.MX6Q Pico-Pi
> > > 
> > 
> > Still they are unused. So I'd think these boards should be handled like boards
> > that predated bindings: a binding is written that doesn't break existing users.
> 
> OK, let's assume the binding is not correct and DTSes are good.
> 
> > 
> > >> [I guess we need to keep the two-compatible list they were originally
> > >>  in for compatibility even if it's unused among upstream device trees?]
> > > 
> > > You want to change both the binding (thus breaking the ABI) and update
> > > the DTS to reflect new ABI. Then why having a binding at all?
> > 
> > If we leave the old two-compatible enumeration intact, there is no ABI broken.
> 
> Just to clarify, because I don't get here the "no ABI broken" part:
> ABI is the binding, not the DTS. We can change intree DTS as we like,
> replace compatibles, add nodes, remove nodes. There is no stability
> requirement for DTS contents.
> 
> If we leave two-compatible binding intact, it is a broken binding since
> beginning. Removing non-working, fake ABI is not breaking it because it
> could never work.

The problem here is that it wasn't covered by the review and now we have
the mess. I see the DTB and the Bootloader as Firmware. Now imagine if
the bootloader for these boards had some dt-fixup logic which won't
apply anymore or if the bootloader board init won't get called anymore
since the bootloader folks used the compatible found in the DTS. This
can cause a regression if the old Bootloader tries to boot the new
Kernel+DTS.

> > > I would assume that either binding is correct or DTS. You propose that
> > > both are wrong and both need changes... in such case this is clearly
> > > broken.
> > 
> > IMO the DTS is the correct one. If you want to honor the author's intention
> > that each base board has a different compatible, it should be an extra
> > compatible and not replace the existing one that may be already in use.

Question is what was the author's intention? @Fabio do you have any
comments here?

> OK, we can go with DTS approach. I fixed few of such cases as well,
> assuming that DTS was intended and binding was incorrect. In such case
> all boards will be documented under one compatible technexion,imx6q-pico
> and DTS will not be changed.

Or keep the exisiting bindings and adding the new one. Therefore the
yaml needs to handle two cases for each imx6[qdl]:
  compatible = "technexion,imx6dl-pico-dwarf", "technexion,imx6dl-pico", "fsl,imx6dl";
and
  compatible = "technexion,imx6dl-pico", "fsl,imx6dl";

Regards,
  Marco