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Date:   Mon, 23 Nov 2020 13:53:32 +0100
From:   Maxime Ripard <maxime@cerno.tech>
To:     Icenowy Zheng <icenowy@aosc.io>
Cc:     Samuel Holland <samuel@sholland.org>, devicetree@vger.kernel.org,
        Jernej Skrabec <jernej.skrabec@siol.net>,
        linux-sunxi@googlegroups.com, linux-kernel@vger.kernel.org,
        Chen-Yu Tsai <wens@csie.org>, Rob Herring <robh+dt@kernel.org>,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [linux-sunxi] Re: [PATCH 3/3] arm64: allwinner: dts: a64: add DT
 for PineTab developer sample
Message-ID: <20201123125332.2p5z3ew7svszvyfs@gilmour>
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On Mon, Nov 23, 2020 at 07:25:47PM +0800, Icenowy Zheng wrote:
>=20
>=20
> =E4=BA=8E 2020=E5=B9=B411=E6=9C=8823=E6=97=A5 GMT+08:00 =E4=B8=8B=E5=8D=
=887:15:12, Maxime Ripard <maxime@cerno.tech> =E5=86=99=E5=88=B0:
> >Hi!
> >
> >On Fri, Nov 20, 2020 at 08:51:48PM -0600, Samuel Holland wrote:
> >> On 11/20/20 5:30 PM, Icenowy Zheng wrote:
> >> >>>>>>> +/ {
> >> >>>>>>> +	model =3D "PineTab Developer Sample";
> >> >>>>>>> +	compatible =3D "pine64,pinetab-dev", "allwinner,sun50i-a64";
> >> >>>>>>> +};
> >> >>>>>>
> >> >>>>>> Changing the DT and the compatible half-way through it isn't
> >ok. Please
> >> >>>>>> add a new DT with the newer revision like we did for the
> >pinephone
> >> >>>>>
> >> >>>>> We did this on Pine H64.
> >> >>>>
> >> >>>> What are you referring to? I couldn't find a commit where we did
> >what
> >> >>>> you suggested in that patch to the pine H64.
> >> >>>
> >> >>> Oh the situation is complex. On Pine H64, we didn't specify
> >anything at
> >> >>> start (which is the same here), the DT is originally
> >version-neutral
> >> >>> but then transitioned to model B, then reverted to model A. Here
> >the DT is always
> >> >>> for the sample.
> >> >>>
> >> >>> However, for Pine H64 there's model A/B names, but for PineTab
> >there's no
> >> >>> any samples that are sold, thus except who got the samples, all
> >PineTab
> >> >>> owners simply owns a "PineTab", not a "PineTab xxx version".
> >> >>
> >> >> It's fairly simple really, we can't really predict the future, so
> >any DT
> >> >> submitted is for the current version of whatever board there is.
> >This is
> >>=20
> >> I don't think that was the intention at all. The DT was submitted for
> >a
> >> future product, whatever that future product ends up being at the
> >time
> >> of its release. Since there are necessarily no users until the
> >product
> >> ships, there is no chance of breaking users by modifying the DT.
> >
> >There was no indication of that in the commit though
> >
> >> >> what we (somewhat messily) did for the PineH64, for the pinephone,
> >or
> >> >> really any other board that has several revisions
> >>=20
> >> Surely a non-public prototype doesn't count as a separate revision!
> >This
> >> sort of policy strongly discourages ever shipping a board with
> >> out-of-the-box mainline Linux support. Because if there any hardware
> >> bugs fixed between initial upstreaming and production, the
> >manufacture
> >> must come up with a new DT name.
> >>=20
> >> This is hostile to the users as well, because the "canonical" DT name
> >no
> >> longer matches the "canonical" (read: the only one ever available)
> >> version of the hardware.
> >>=20
> >> Do you want manufacturers to submit their initial board DT as
> >> "$BOARD-prototype.dts", just in case they have to make a change
> >before
> >> production? And only after the board is shipped (with out-of-tree
> >> patches, of course, to use $BOARD.dts, since the shipped board is
> >*not*
> >> the prototype) submit a "$BOARD.dts" to mainline?
> >>=20
> >> Maxime, can you clarify specifically what the line is where a device
> >> tree is "locked down" and further changes to the hardware require a
> >new
> >> name? First sample leaves the factory? $NUMBER units produced? First
> >> sold to the public for money?
> >
> >The first board that is shipped to a user. From the wiki, the version
> >supported here (I guess?) has been shipped to around 100 people, so it
> >doesn't really qualify for the "non-public prototype". We still have to
> >support these users, whether we like it or not.
> >
> >> Without some guidance, or a change in policy, this problem is going
> >to
> >> keep coming up again and again.
> >
> >There's a few things that are interleaved here. First, there's two hard
> >rules: never change the DT name and never change the compatible of a
> >board.
> >
> >The former would break any build system, boot script or documentation,
> >and the latter would break the DT compatibility.
> >
> >Aside from this, things get a bit blurrier since it's mostly about the
> >intent. I'm fine with having a DT for a non-public prototype if it's
> >clear from day one that it is. In this particular case, the DT just
> >stated that support for the pinetab was added. I guess anyone would
> >make
> >the assumption without any context that this would be for the (or a)
> >final design.
> >
> >Finally, I'd also advise against submitting the parts that are still
> >not
> >finalized though, because this is also fairly bad for the users. Let's
> >take the current situation as the example: from what I understand, the
> >screen changed half-way through the process, and the first one was
> >upstreamed. This means that any user that would use a kernel with that
> >bugfix would have the display working, while rolling back to 5.9 would
> >break the display, even though everyone claimed it was supported
> >out-of-the-box in mainline. This is a worse situation than not
> >supporting the display in the first place here.
> >
> >> You'll note that so far it has mostly affected Pine devices, and I
> >don't
> >> think that's because they make more board revisions than other
> >> manufacturers.
> >
> >Yes definitely. Pine devices may be worse though because of their
> >policy
> >of making their prototypes public. I guess most of the issues we had
> >also were due to poor communication: context on what were the Pine
> >intentions were missing, and thus we couldn't catch things that turned
> >out to be bad later on during review.
> >
> >> It's because they're actively involved in getting their
> >> boards supported upstream. For other manufacturers, it's some user
> >> sending in a device tree months after the hardware ships to the
> >public
> >> -- of course the hardware is more stable at that point.
> >
> >I mean, it's not black and white either. One could very well send the
> >DT
> >once the final design is done and that would still be upstreamed way
> >before reaching the first user.
> >
> >> I think Pine's behavior is something we want to encourage, not
> >> penalize.
> >
> >For the DT in particular, yes
> >
> >> > Okay. But I'm not satisfied with a non-public sample occupies
> >> > the pinetab name. Is rename it to pinetab-dev and add a
> >> > pinetab-retail okay?
> >>
> >> To me, naming the production version anything but "pinetab" isn't
> >> satisfying either.
> >
> >I understand where you're coming from, but the point I was making my
> >previous mail is precisely that it's not really possible.
> >
> >You want to name the early adopter version _the_ production version.
> >Let's assume the hardware changes again between the early adopter and
> >mass-production version. Which one will be _the_ production version?
> >The
> >early-adopter or the mass-produced one?
> >
> >There's really no good answer here, and both would suck in their own
> >way. The only way to deal with this is to simply avoid defining one
> >version as the one true board, and just loading the proper DT in u-boot
> >based on whatever clue we have of the hardware revision.
>=20
> Then will it be okay to rename current pinetab DT to pinetab-sample and
> then introduce new DTs all with suffixes?

No. From my previous mail:

> > there's two hard rules: never change the DT name and never change
> > the compatible of a board.
> >
> > The former would break any build system, boot script or
> > documentation, and the latter would break the DT compatibility.

Maxime

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