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[2603:6081:7b01:cbda:55fd:c014:b2c6:676a]) by smtp.gmail.com with ESMTPSA id p187sm9015qkd.101.2021.09.10.15.44.49 (version=TLS1_2 cipher=ECDHE-ECDSA-CHACHA20-POLY1305 bits=256/256); Fri, 10 Sep 2021 15:44:50 -0700 (PDT) Date: Fri, 10 Sep 2021 18:44:48 -0400 From: Tom Rini To: Mark Kettenis , Bin Meng , Rick Chen Cc: xypron.glpk@gmx.de, sjg@chromium.org, u-boot@lists.denx.de, seanga2@gmail.com, ilias.apalodimas@linaro.org, francois.ozog@linaro.org, bill.mills@linaro.org, marcel.ziswiler@toradex.com Subject: Re: [PATCH v3 3/3] RFC: doc: Add documentation about devicetree usage Message-ID: <20210910224448.GD12964@bill-the-cat> References: <20210909201033.755713-1-sjg@chromium.org> <20210909201033.755713-4-sjg@chromium.org> <20210910123420.GL12964@bill-the-cat> <5614507a14906226@bloch.sibelius.xs4all.nl> <20210910211737.GB12964@bill-the-cat> <561450ac28c24b92@bloch.sibelius.xs4all.nl> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="1Hs3rtFFTPjSXOg+" Content-Disposition: inline In-Reply-To: <561450ac28c24b92@bloch.sibelius.xs4all.nl> 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 --1Hs3rtFFTPjSXOg+ Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Sat, Sep 11, 2021 at 12:09:40AM +0200, Mark Kettenis wrote: > > Date: Fri, 10 Sep 2021 17:17:37 -0400 > > From: Tom Rini > >=20 > > On Fri, Sep 10, 2021 at 11:12:20PM +0200, Mark Kettenis wrote: > > > > Date: Fri, 10 Sep 2021 08:34:20 -0400 > > > > From: Tom Rini > > > >=20 > > > > 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 i= n 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= using > > > > > > 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= devicetree > > > > > > - 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= cmdline > > > > > > - 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 = devicetree > > > > > > 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 desi= gn' > > > > > > - Reword 'in-tree U-Boot devicetree' to 'devicetree source in U= -Boot' > > > > > > - Rewrite 'Devicetree generated on-the-fly in another project' = to cover > > > > > > 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/p= ackage/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, mostl= y, with some > > > > > > +mention of things that need to be fixed. It is not intended to= point the way to > > > > > > +what might be done in the future. That should be the subject o= f discussions on > > > > > > +the mailing list. > > > > > > + > > > > > > +U-Boot uses devicetree for runtime configuration and storing r= equired blobs or > > > > > > +any other information it needs to operate. It is possible to u= pdate the > > > > > > +devicetree separately from actually building U-Boot. This prov= ides a good degree > > > > > > +of control and flexibility for firmware that uses U-Boot in co= njunction with > > > > > > +other project. > > > > > > + > > > > > > +There are many reasons why it is useful to modify the devicetr= ee after 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 vidc= onsole) > > > > > > + > > > > > > +This section describes how to work with devicetree to accompli= sh your goals. > > > > > > + > > > > > > +See also :doc:`../devicetree/control` for a basic summary of t= he available > > > > > > +features. > > > > > > + > > > > > > + > > > > > > +Devicetree source > > > > > > +----------------- > > > > > > + > > > > > > +Every board in U-Boot must include a devicetree sufficient to = build and boot > > > > > > +that board on suitable hardware (or emulation). This is specif= ied using the > > > > > > +`CONFIG DEFAULT_DEVICE_TREE` option. > > > > > > + > > > > > > + > > > > > > +Current situation (August 2021) > > > > > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > > > > > + > > > > > > +As an aside, at present U-Boot allows `CONFIG_DEFAULT_DEVICE_T= REE` to be empty, > > > > > > +e.g. if `CONFIG_OF_BOARD` or `CONFIG_OF_PRIOR_STAGE` are used.= This has > > > > > > +unfortunately created an enormous amount of confusion and some= wasted 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= project > > > > > > + > > > > > > +- There is no way to see even a sample devicetree for these pl= atform in U-Boot, > > > > > > + so it is hard to know what is going on, e.g. which devices a= re typically > > > > > > + present > > > > > > + > > > > > > +- The other project may not provide a way to support U-Boot's = requirements for > > > > > > + devicetree, such as the /config node. Note: On the U-Boot ma= iling list, this > > > > > > + was only discovered after weeks of discussion and confusion > > > > > > + > > > > > > +- For QEMU specifically, consulting two QEMU source files is r= equired, for which > > > > > > + there are no references in U-Boot documentation. The code is= generating 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, = which 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= larger 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 use= d by RISC-V qemu > > > > > > + boards. > > > > > > + > > > > > > +Note: It is not clear that we actually need both of these. Pos= sibly > > > > > > +`CONFIG_OF_BOARD` can be dropped. > > > > > > + > > > > > > +Once this bug is fixed, CONFIG_OF_BOARD and CONFIG_OF_PRIOR_ST= AGE will override > > > > >=20 > > > > > What does "bug" refer to? Above you describe the current design n= ot 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 ? > > >=20 > > > With CONFIG_OF_BOARD there is a function that returns the pointer to > > > the DTB, so you can do all sort of things with it. > > >=20 > > > With CONFIG_OF_PRIOR_STAGE there is a variable that you need to set in > > > low-level code to point at the DTB and there is a pre-defined function > > > that returns that pointer. > > >=20 > > > CONFIG_OF_BOARD is more flexible than CONFIG_OF_PRIOR_STAGE, but if > > > the only thing you want to do is to pass on a DTB that is passed in a > > > CPU register to U-Boot then CONFIG_OF_PRIOR_STAGE is probably easier > > > to use. > > >=20 > > > I'm not convinced there is a bug here. > >=20 > > Thanks for explaining. Couldn't CONFIG_OF_PRIOR_STAGE be rewritten as > > an implementation of CONFIG_OF_BOARD, possibly at the same or less > > overall code size? That I think is the potential bug. >=20 > Probably a little bit more code: >=20 > void * > board_fdt_blob_setup(void) > { > return (void *)(uintptr_t)prior_stage_fdt_address; > } Tiny bit more. Probably worth doing to make the choices clearer on which to select when? Bin, Rick, thoughts on this since riscv is the main user of CONFIG_OF_PRIOR_STAGE at this point? --=20 Tom --1Hs3rtFFTPjSXOg+ Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQGzBAABCgAdFiEEGjx/cOCPqxcHgJu/FHw5/5Y0tywFAmE739gACgkQFHw5/5Y0 tyyg6wv7BzZ+hXg2brFoGwQQma1MMglBMuLj7Z+a38jF4G0c0n+F/YhrJXX54cSH q77Qh+TeAD+7/C2S1ZrhuFVbA5Dq6cGBJ97EfZcLGDva69PF4/ApfmHu57bXW0+w hwVjtsgf0HU833cksNaMZXGR1kYay38wmyojO72lx0SB0RCLSHPM18ob+Gdjny/O dxZ3+KE+LEtqo7sLNYApeP8W3y4DmlccfUemvi0VpwN2G7bK6l7adMJRm+/1CGV/ R85+nBLlxXY4xXaA+Ftrim0v75jg1L3J8UuwxHUKO5hnJoY6sJ84Gya66zTtFH77 aiehEs4aVXpOsCoizUzKvqLfHe0TSoY0jK9VhrDrwvRlShu+5JQ5ZkgQyIlGmLHq e/FJWVP/kd4aBfdfOH4ZjOEyOqgnlJce04Xczn4fA6UFTBdLVt3fxP/+I8djhQFh DyKA0TzsviGaVEk/rGTUIQi/h6H6cC1WyDD11fgEw+u2BqKj/u3PW32leT53NHyj p0vXoioc =5iSE -----END PGP SIGNATURE----- --1Hs3rtFFTPjSXOg+--