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[2603:6081:7b01:cbda:55fd:c014:b2c6:676a]) by smtp.gmail.com with ESMTPSA id o23sm3921370qkk.40.2021.09.10.09.37.26 (version=TLS1_2 cipher=ECDHE-ECDSA-CHACHA20-POLY1305 bits=256/256); Fri, 10 Sep 2021 09:37:27 -0700 (PDT) Date: Fri, 10 Sep 2021 12:37:25 -0400 From: Tom Rini To: Heinrich Schuchardt Cc: Simon Glass , U-Boot Mailing List , Sean Anderson , Mark Kettenis , Ilias Apalodimas , =?iso-8859-1?Q?Fran=E7ois?= Ozog , Bill Mills , Marcel Ziswiler Subject: Re: [PATCH v3 3/3] RFC: doc: Add documentation about devicetree usage Message-ID: <20210910163725.GP12964@bill-the-cat> References: <20210909201033.755713-1-sjg@chromium.org> <20210909201033.755713-4-sjg@chromium.org> <20210910123420.GL12964@bill-the-cat> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="8HHiVeP/YFKAklVd" Content-Disposition: inline In-Reply-To: X-Clacks-Overhead: GNU Terry Pratchett User-Agent: Mutt/1.9.4 (2018-02-28) X-BeenThere: u-boot@lists.denx.de X-Mailman-Version: 2.1.34 Precedence: list List-Id: U-Boot discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: u-boot-bounces@lists.denx.de Sender: "U-Boot" X-Virus-Scanned: clamav-milter 0.103.2 at phobos.denx.de X-Virus-Status: Clean --8HHiVeP/YFKAklVd Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Sep 10, 2021 at 06:28:36PM +0200, Heinrich Schuchardt wrote: >=20 >=20 > On 9/10/21 2:34 PM, Tom Rini wrote: > > On Fri, Sep 10, 2021 at 10:38:17AM +0200, Heinrich Schuchardt wrote: > > >=20 > > >=20 > > > On 9/9/21 10:10 PM, Simon Glass wrote: > > > > At present some of the ideas and techniques behind devicetree in U-= Boot > > > > are assumed, implied or unsaid. Add some documentation to cover how > > > > devicetree is build, how it can be modified and the rules about usi= ng > > > > the various CONFIG_OF_... options. > > > >=20 > > > > Signed-off-by: Simon Glass > > > > Reviewed-by: Marcel Ziswiler > > > > --- > > > >=20 > > > > Changes in v3: > > > > - Fix typos linst suppled receive EFL > > > > - Drop 'and' before 'self-defeating' > > > > - Reword mention of control of QEMU's devicetree generation > > > > - Add mention of dropping CONFIG_OF_BOARD > > > > - Clarify the 'Once this bug is fixed' paragraph a bit > > > > - Expand ways that CONFIG_OF_PRIOR_STAGE can support the U-Boot dev= icetree > > > > - Add a note at the top explaining that his patch covers 'now', not= 'future' > > > > - Add note 'Note: Some boards use a devicetree in U-Boot which does= not match' > > > >=20 > > > > Changes in v2: > > > > - Fix typos per Sean (thank you!) and a few others > > > > - Add a 'Use of U-Boot /config node' section > > > > - Drop mention of dm-verity since that actually uses the kernel cmd= line > > > > - Explain that OF_BOARD will still work after these changes (in > > > > 'Once this bug is fixed...' paragraph) > > > > - Expand a bit on the reason why the 'Current situation' is bad > > > > - Clarify in a second place that Linux and U-Boot use the same devi= cetree > > > > in 'To be clear, while U-Boot...' > > > > - Expand on why we should have rules for other projects in > > > > 'Devicetree in another project' > > > > - Add a comment as to why devicetree in U-Boot is not 'bad design' > > > > - Reword 'in-tree U-Boot devicetree' to 'devicetree source in U-Boo= t' > > > > - Rewrite 'Devicetree generated on-the-fly in another project' to c= over > > > > points raised on v1 > > > > - Add 'Why does U-Boot have its nodes and properties?' > > > > - Add 'Why not have two devicetrees?' > > > >=20 > > > > doc/develop/index.rst | 1 + > > > > doc/develop/package/devicetree.rst | 583 +++++++++++++++++++++++= ++++++ > > > > doc/develop/package/index.rst | 1 + > > > > 3 files changed, 585 insertions(+) > > > > create mode 100644 doc/develop/package/devicetree.rst > > > >=20 > > > > diff --git a/doc/develop/index.rst b/doc/develop/index.rst > > > > index 83c929babda..d5ad8f9fe53 100644 > > > > --- a/doc/develop/index.rst > > > > +++ b/doc/develop/index.rst > > > > @@ -36,6 +36,7 @@ Packaging > > > > :maxdepth: 1 > > > >=20 > > > > package/index > > > > + package/devicetree > > > >=20 > > > > Testing > > > > ------- > > > > diff --git a/doc/develop/package/devicetree.rst b/doc/develop/packa= ge/devicetree.rst > > > > new file mode 100644 > > > > index 00000000000..b1bd310d906 > > > > --- /dev/null > > > > +++ b/doc/develop/package/devicetree.rst > > > > @@ -0,0 +1,583 @@ > > > > +.. SPDX-License-Identifier: GPL-2.0+ > > > > + > > > > +Updating the devicetree > > > > +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D > > > > + > > > > +Note: This documentation describes how things are today, mostly, w= ith some > > > > +mention of things that need to be fixed. It is not intended to poi= nt the way to > > > > +what might be done in the future. That should be the subject of di= scussions on > > > > +the mailing list. > > > > + > > > > +U-Boot uses devicetree for runtime configuration and storing requi= red blobs or > > > > +any other information it needs to operate. It is possible to updat= e the > > > > +devicetree separately from actually building U-Boot. This provides= a good degree > > > > +of control and flexibility for firmware that uses U-Boot in conjun= ction with > > > > +other project. > > > > + > > > > +There are many reasons why it is useful to modify the devicetree a= fter building > > > > +it: > > > > + > > > > +- Configuration can be changed, e.g. which UART to use > > > > +- A serial number can be added > > > > +- Public keys can be added to allow image verification > > > > +- Console output can be changed (e.g. to select serial or vidconso= le) > > > > + > > > > +This section describes how to work with devicetree to accomplish y= our goals. > > > > + > > > > +See also :doc:`../devicetree/control` for a basic summary of the a= vailable > > > > +features. > > > > + > > > > + > > > > +Devicetree source > > > > +----------------- > > > > + > > > > +Every board in U-Boot must include a devicetree sufficient to buil= d and boot > > > > +that board on suitable hardware (or emulation). This is specified = using the > > > > +`CONFIG DEFAULT_DEVICE_TREE` option. > > > > + > > > > + > > > > +Current situation (August 2021) > > > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > > > + > > > > +As an aside, at present U-Boot allows `CONFIG_DEFAULT_DEVICE_TREE`= to be empty, > > > > +e.g. if `CONFIG_OF_BOARD` or `CONFIG_OF_PRIOR_STAGE` are used. Thi= s has > > > > +unfortunately created an enormous amount of confusion and some was= ted effort. > > > > +This was not intended and this bug will be fixed soon. > > > > + > > > > +Some of the problems created are: > > > > + > > > > +- It is not obvious that the devicetree is coming from another pro= ject > > > > + > > > > +- There is no way to see even a sample devicetree for these platfo= rm in U-Boot, > > > > + so it is hard to know what is going on, e.g. which devices are t= ypically > > > > + present > > > > + > > > > +- The other project may not provide a way to support U-Boot's requ= irements for > > > > + devicetree, such as the /config node. Note: On the U-Boot mailin= g list, this > > > > + was only discovered after weeks of discussion and confusion > > > > + > > > > +- For QEMU specifically, consulting two QEMU source files is requi= red, for which > > > > + there are no references in U-Boot documentation. The code is gen= erating a > > > > + devicetree, with some control from command-line args, but it is = not clear > > > > + how to add properties required by U-Boot. > > > > + > > > > +Specifically on the changes in U-Boot: > > > > + > > > > +- `CONFIG_OF_BOARD` was added in rpi_patch_ for Raspberry Pi, whic= h does have > > > > + an in-tree devicetree, but this feature has since been used for = boards that > > > > + don't > > > > +- `CONFIG_OF_PRIOR_STAGE` was added in bcm_patch_ as part of a lar= ger Broadcom > > > > + change with a tag indicating it only affected one board, so the = change in > > > > + behaviour was not noticed at the time. It has since been used by= RISC-V qemu > > > > + boards. > > > > + > > > > +Note: It is not clear that we actually need both of these. Possibly > > > > +`CONFIG_OF_BOARD` can be dropped. > > > > + > > > > +Once this bug is fixed, CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE = will override > > >=20 > > > What does "bug" refer to? Above you describe the current design not a= bug. > >=20 > > The bug is that we have two options to provide seemingly the same > > functionality. Is there a functional difference between CONFIG_OF_BOARD > > and CONFIG_OF_PRIOR_STAGE ? Does this clarify your question? > > > > +(at runtime) the devicetree supplied with U-Boot, but will otherwi= se use > > > > +CONFIG_OF_SEPARATE for the in-tree build. So these two will become= options, > > > > +moving out of the 'choice' in `dts/Kconfig`. To be clear, the devi= cetree in the > > > > +U-Boot tree may be largely for documentation and build-testing pur= poses, if at > > > > +runtime the devicetree if provided by another project. But the in-= tree > > > > +devicetree is packaged with U-Boot as a fallback if it does not ge= t one from a > > > > +prior stage at runtime. This does not create two devicetrees that = need to be > > > > +merged, or anything like that. If the prior stage provides one, it= is used as > > > > +is, with the one provided by U-Boot being ignored. > > > > + > > > > +This means that there is a basic devicetree build in the U-Boot tr= ee, for > > > > +build-testing, consistency and documentation purposes, but at runt= ime U-Boot can > > > > +accept its devicetree from another source. > > >=20 > > > The incoming devicetree may not contain any U-Boot specific stuff. So > > > don't you need the buildtime devicetree for all of this information at > > > runtime? E.g. you were requesting to move certificate blobs into the > > > build-time devicetree. > >=20 > > This is wrong because (a) no, there's no functional reason the prior > > stage cannot populate / be pre-populated with what we need and (b) we're > > documenting what we have today. >=20 > The problem is not functional but organizational. The prior boot stage > may be burnt into PROM while U-Boot is on an SD-card. >=20 > Don't expect that on a board where you could install EDK II or U-Boot or > anything else the prior boot stage cares about U-Boot. >=20 > How could a prior boot stage possibly know years ahead what is not even > yet supported in U-Boot when the prior boot stage is created? I don't follow you, sorry. Or perhaps, if you %s/U-Boot/Linux/ the above, what's your answer then? --=20 Tom --8HHiVeP/YFKAklVd Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQGzBAABCgAdFiEEGjx/cOCPqxcHgJu/FHw5/5Y0tywFAmE7ib4ACgkQFHw5/5Y0 tyw68gwAh/q0mds8KHPX3pcWH7G8q9urWoSw5Yrj1e0ysOY6Hcl9YFb7lLhpH9tE 5Ws9MjHQvHHZV+W79CVT1DYlH3o7zkSH9c2M4xXlnxAOnIiGvxfFAaBZ+Ir/vWfg zOuy98yyaFJsHl3q1y0mvFzHPi17XLrGCfyadR282ZxJ4nABAzdrLASFQCTqWGU1 o1INBdvZaifpMgJOl6Bax8kKZvBx3oehKipG0W17VLT7odT23BR4cCIfTpMk1cGi 6UmkFPSuAC2k/7WV1iPqiIrRXs45gQu68mDaqalJ8OVtBEPuTnBi31LA8t3hjPf+ 8BGqTgdNLUv6zd5njQ7VArVHmugnH6hmCDAriP4OO8ekVzzlN4E5TZlTsDZq9mTS REVq/lwyG3MzFOEyUnPrg/t4ZNySuuL2AmCOgiMJ9j1UVuY6EmFkui1RYw9MUFH1 6b7g8gdqKuxk644INmkiJE6MJI0i+o+b7ylK2b4fDV31/W1SFcET3b6pMLaSNDp2 80f8OMal =fb/A -----END PGP SIGNATURE----- --8HHiVeP/YFKAklVd--