Re: [PATCH v3 3/5] arm64: dts: ti: Add support for AM642 SoC

From: Nishanth Menon <nm@ti.com>
To: Tony Lindgren <tony@atomide.com>
Cc: Arnd Bergmann <arnd@kernel.org>, Suman Anna <s-anna@ti.com>,
	Arnd Bergmann <arnd@arndb.de>, Dave Gerlach <d-gerlach@ti.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	DTML <devicetree@vger.kernel.org>,
	Rob Herring <robh+dt@kernel.org>,
	Vignesh Raghavendra <vigneshr@ti.com>,
	Sekhar Nori <nsekhar@ti.com>,
	Kishon Vijay Abraham <kishon@ti.com>,
	Lokesh Vutla <lokeshvutla@ti.com>,
	Aswath Govindraju <a-govindraju@ti.com>
Subject: Re: [PATCH v3 3/5] arm64: dts: ti: Add support for AM642 SoC
Date: Mon, 25 Jan 2021 08:16:42 -0600	[thread overview]
Message-ID: <20210125141642.4yybjnklk3qsqjdy@steersman> (raw)
In-Reply-To: <YArMe2ahPxAjRHsY@atomide.com>

Arnd, Tony,
On 15:00-20210122, Tony Lindgren wrote:
> * Arnd Bergmann <arnd@kernel.org> [210122 11:24]:
> > On Thu, Jan 21, 2021 at 8:57 PM Suman Anna <s-anna@ti.com> wrote:
> > > On 1/21/21 12:39 PM, Nishanth Menon wrote:
> > > > On 12:13-20210121, Suman Anna wrote:
> > > >>
> > > >> Hmm, this is kinda counter-intuitive. When I see a dts node, I am expecting the
> > > >
> > > > What is counter intutive about a -next branch be tested against
> > > > linux-next tree?
> > >
> > > The -next process is well understood. FWIW, you are not sending your PR against
> > > -next branch, but against primarily a -rc1 or -rc2 baseline.
> > >
> > > As a developer, when I am submitting patches, I am making sure that things are
> > > functional against the baseline you use. For example, when I split functionality
> > > into a driver portions and dts portions, I need to make sure both those
> > > individual pieces boot fine and do not cause regressions, even though for the
> > > final functionality, you need both.
> > > >
> > > >
> > > > Now, if you want to launch a product with my -next branch - go ahead, I
> > > > don't intent it for current kernel version - you are on your own.
> > > >
> > > > If there is a real risk of upstream next-breaking - speakup with an
> > > > real example - All I care about is keeping upstream functional and
> > > > useable.
> > >
> > > This is all moot when your own tree doesn't boot properly. In this case, you are
> > > adding MMC nodes, but yet for a boot test, you are saying use linux-next for the
> > > nodes that were added or you need additional driver patches (which is not how
> > > maintainer-level trees are verified).
> > >
> > > Arnd,
> > > Can you please guide us here as to what is expected in general, given that the
> > > pull-request from Nishanth goes through you, and if there is some pre-existing
> > > norms around this?
> > 
> > There are two very different cases to consider, and I'm not sure which one
> > we have here:
> > 
> > - When submitting any changes to a working platform, each patch on
> >   a branch that gets merged needs to work incrementally, e.g. a device
> >   tree change merged through the soc tree must never stop a platform
> >   from booting without a patch that gets merged through another branch
> >   in the same merge window, or vice versa.
> >   As an extension of this, I would actually appreciate if we never do
> >   incompatible binding changes at all. If a driver patch enables a new
> >   binding for already supported hardware, a second patch changes
> >   the dts file to use the new binding, and a third patch removes the
> >   original binding, this could still be done without regressions over
> >   multiple merge windows, but it breaks the assumption that a new
> >   kernel can boot with an old dtb (or vice versa). This second one
> >   is a softer requirement, and we can make exceptions for particularly
> >   good reasons, but please explain those in the patch description and
> >   discuss with upstream maintainers before submitting patches that do
> >   this.
> > 
> > - For a newly added hardware support, having a runtime dependency
> >   on another branch is not a problem, we do that all the time: Adding
> >   a device node for an existing board (or a new board) and the driver
> >   code in another branch is not a regression because each branch
> >   only has incremental changes that improve hardware support, and
> >   it will work as soon as both are merged.
> >   You raised the point about device bindings, which is best addressed
> >   by having one commit that adds the (reviewed) binding document
> >   first, and then have the driver branch and the dts branch based on
> >   the same commit.
> > 
> > I hope that clarifies the case you are interested in, let me know if I
> > missed something for the specific case at hand.
> 
> Hmm and additionally few more mostly obvious things that have helped
> quite a bit:
> 
> - Each commit in each topic branch should compile and boot so git
>   bisect works
> 
> - Each topic branch should be ideally based on -rc1 to leave out
>   dependencies to other branches
> 
> - Aiming for a working and usable -rc1 is worth the effort in case
>   git bisect is needed for any top branches based on it :) Otherwise
>   you'll be wasting the -rc cycle chasing regressions..

Thank you both for your valuable insight and direction. much
appreciated.

*) for every patch that is integrated - I already insist on
bisectability, no warnings with W=2 , dtbs_check .... Including
putting additional tooling[1] in place for folks to use - which goes
and tests sparse, coccinelle etc.. The team has been pretty deligent
in making sure things are clean.
*) We also insist on testing with linux-next to maintain rc1
functionality
*) I also maintain the minimal boot requirements equivalent to kernelci
(example:[2]) for my -next branch as well.

Yes, this series introduces 0 regression, new nodes are being added
and the thing I missed for this window, which is insisting on getting
immutable tags from subsystem maintainers for dt-bindings patches they
have picked up, will be rectified in the future. For this window,
for the last time, I am going to depend a bit on the later merge of
arm-soc.

Thanks for the clarifications, once again.

[1] https://github.com/nmenon/kernel_patch_verify
[2] https://storage.kernelci.org/stable/linux-4.14.y/v4.14.217/arm/omap2plus_defconfig/gcc-8/lab-cip/baseline-beaglebone-black.txt
-- 
Regards,
Nishanth Menon
Key (0xDDB5849D1736249D) / Fingerprint: F8A2 8693 54EB 8232 17A3  1A34 DDB5 849D 1736 249D