" Hi Rob, On Tue, 12 Oct 2021 at 09:05, Rob Herring <robh@xxxxxxxxxx> wrote: > > On Tue, Oct 12, 2021 at 8:41 AM Simon Glass <sjg@xxxxxxxxxxxx> wrote: > > > > Hi Rob, > > > > On Mon, 4 Oct 2021 at 13:30, Rob Herring <robh@xxxxxxxxxx> wrote: > > > > > > On Sun, Oct 03, 2021 at 12:51:53PM -0600, Simon Glass wrote: > > > > U-Boot makes use of the devicetree for its driver model. Devices are bound > > > > based on the hardware description in the devicetree. > > > > > > > > Since U-Boot is not an operating system, it has no command line or user > > > > space to provide configuration and policy information. This must be made > > > > available in some other way. > > > > > > > > Therefore U-Boot uses devicetree for configuration and run-time control > > > > and has done for approximately 9 years. This works extremely well in the > > > > project and is very flexible. However the bindings have never been > > > > incorporated in the devicetree bindings in the Linux tree. This could be > > > > a good time to start this work as we try to create standard bindings for > > > > communicating between firmware components. > > > > > > > > Add an initial binding for this node, covering just the config node, which > > > > is the main requirement. It is similar in concept to the chosen node, but > > > > used for passing information between firmware components, instead of from > > > > firmware to operating system. > > > > > > > > Signed-off-by: Simon Glass <sjg@xxxxxxxxxxxx> > > > > --- > > > > Please be kind in your review. Some words about why this is needed are > > > > included in the description in config.yaml file. > > > > > > > > The last attempt to add just one property needed by U-Boot went into the > > > > weeds 6 years ago, with what I see as confusion about the role of the > > > > chosen node in devicetree[1]. > > > > > > > > I am trying again in the hope of reaching resolution rather than just > > > > going around in circles with the 'devicetree is a hardware description' > > > > argument :-) > > > > > > > > Quoting from the introduction to latest devicetree spec[2]: > > > > > > > > >>> > > > > To initialize and boot a computer system, various software components > > > > interact. Firmware might perform low-level initialization of the system > > > > hardware before passing control to software such as an operating system, > > > > bootloader, or hypervisor. Bootloaders and hypervisors can, in turn, > > > > load and transfer control to operating systems. Standard, consistent > > > > interfaces and conventions facilitate the interactions between these > > > > software components. In this document the term boot program is used to > > > > generically refer to a software component that initializes the system > > > > state and executes another software component referred to as a client > > > > program. > > > > <<< > > > > > > > > This clearly envisages multiple software components in the firmware > > > > domain and in fact that is the case today. They need some way to > > > > communicate configuration data such as memory setup, runtime-feature > > > > selection and developer conveniences. Devicetree seems ideal, at least for > > > > components where the performance / memory requirements of devicetree are > > > > affordable. > > > > > > > > I hope that the Linux community (which owns the devicetree bindings) finds > > > > this initiative valuable and acceptable. > > > > > > Owns? I'm having a sale and can make you a good offer. Buy 1 binding, > > > get 2000 free. :) > > > > Yes, it's the price of that first binding that surely puts everyone off. > > > > (sorry for sitting on this for a week, my spam filter doesn't like > > some mailing lists and I'm working on it) > > > > > > > > > > > > > [1] https://lists.denx.de/pipermail/u-boot/2015-July/218585.html > > > > [2] https://github.com/devicetree-org/devicetree-specification/releases/tag/v0.3 > > > > > > > > .../devicetree/bindings/u-boot/config.yaml | 137 ++++++++++++++++++ > > > > 1 file changed, 137 insertions(+) > > > > create mode 100644 Documentation/devicetree/bindings/u-boot/config.yaml > > > > > > Might as well put this into dt-schema rather than the kernel. But might > > > get more review here first. > > > > OK, so does that mean a PR to https://github.com/robherring/dt-schema > > Wrong one: https://github.com/devicetree-org/dt-schema > > I need to update the readme there for the old one. OK thanks. > > > or is there a mailing list for it? I think I am missing some > > understanding here. > > You can send a PR or to a DT mailing list, but the mail list will get > more reviews (hopefully). devicetree-spec is better than devicetree as > it is not a firehose. OK. > > > > > > > > diff --git a/Documentation/devicetree/bindings/u-boot/config.yaml b/Documentation/devicetree/bindings/u-boot/config.yaml > > > > new file mode 100644 > > > > index 00000000000000..336577a17fdf5a > > > > --- /dev/null > > > > +++ b/Documentation/devicetree/bindings/u-boot/config.yaml > > > > @@ -0,0 +1,137 @@ > > > > +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause) > > > > +%YAML 1.2 > > > > +--- > > > > +$id: http://devicetree.org/schemas/u-boot/config.yaml# > > > > +$schema: http://devicetree.org/meta-schemas/core.yaml# > > > > + > > > > +title: U-Boot configuration node > > > > + > > > > +maintainers: > > > > + - Simon Glass <sjg@xxxxxxxxxxxx> > > > > + > > > > +description: | > > > > + The config node does not represent a real device, but serves as a place > > > > + for passing data between firmware elements, like memory maps. Data in the > > > > + config node does not represent the hardware. It is ignored by operating > > > > + systems. > > > > + > > > > + Purpose of config node > > > > + ---------------------- > > > > + > > > > + A common problem with firmware is that many builds are needed to deal with the > > > > + slight variations between different, related models. For example, one model > > > > + may have a TPM and another may not. Devicetree provides an excellent solution > > > > + to this problem, in that the devicetree to actually use on a platform can be > > > > + injected in the factory based on which model is being manufactured at the time. > > > > + > > > > + A related problem causing build proliferation is dealing with the differences > > > > + between development firmware, developer-friendly firmware (e.g. with all > > > > + security features present but with the ability to access the command line), > > > > + test firmware (which runs tests used in the factory), final production > > > > + firmware (before signing), signed firmware (where the signatures have been > > > > + inserted) and the like. Ideally all or most of these should use the same > > > > + U-Boot build, with just some options to determine the features available. For > > > > + example, being able to control whether the UART console or JTAG are available, > > > > + on any image, is a great debugging aid. > > > > + > > > > + When the firmware consists of multiple parts (various U-Boot phases, TF-A, > > > > + OP-TEE), it is helpful that all operate the same way at runtime, regardless of > > > > + how they were built. This can be achieved by passing the runtime configuration > > > > + (e.g. 'enable UART console', 'here are your public keys') along the chain > > > > + through each firmware stage. It is frustrating to have to replicate a bug on > > > > + production firmware which does happen on developer firmware, because they are > > > > + completely different builds. > > > > + > > > > + The config node provides useful functionality for this. It allows the different > > > > + controls to be 'factored out' of the U-Boot binary, so they can be controlled > > > > + separately from the initial source-code build. The node can be easily updated > > > > + by a build or factory tool and can control various features in U-Boot. It is > > > > + similar in concept to a Kconfig option, except that it can be changed after > > > > + U-Boot is built. > > > > + > > > > + The config node is similar in concept to /chosen (see chosen.txt) except that > > > > > > chosen.yaml now (in dt-schema). > > > > OK > > > > > > > > > + it is for passing information *into* and *between) firmware components, > > > > + instead of from firmware to the Operating System. Also, while operating > > > > + systems typically have a (sometimes extremely long) command line, U-Boot does > > > > + not support this, except with sandbox. The devicetree provides a more > > > > + structured approach in any case. > > > > > > What about having a /chosen/u-boot/ node instead? > > > > What is your rationale for doing that? > > Simply that /chosen is where the s/w configuration for the next stage > consuming the DT goes. Also, we already have bootcmd defined in chosen > and don't need it in a whole other place. OK I see. The spec says "The /chosen node does not represent a real device in the system but describes parameters chosen or specified by the system firmware at run time. It shall be a child of the root node." To my reading, this is not the same thing. I would prefer something like: "The /xxx node does not represent a real device in the system but describes parameters used by the system firmware at run time. It shall be a child of the root node." Anyway, we could use /chosen, and I can see it will make a lot of people happy. But I don't think it is a great plan. Here are my thoughts: 1. This node is built, packaged and set up by and used by U-Boot itself, at least in most cases, so U-Boot is not the next stage, but the current stage. Conceptually, using /chosen is confusing for U-Boot itself. 2.. bootcmd is the Operating System command line, whereas the one here is for U-Boot, specifically 3. U-Boot does not and should not change this node, but it does change /chosen. 4. If we move to livetree for writing in U-Boot, we'll want to flatten the tree (containing /chosen) before calling Linux. Having the config under the /chosen node is an added complication there. 5. It is slightly more efficient for U-Boot to put this at the top level - this matters in SPL. But I don't think this is huge concern. 6.. I very much think of /chosen as an operating system thing. Would this be the first use by firmware? 7. If we want to sign the U-Boot config then it is easier if it is in a separate node from /chosen, which is, after all, updated by U-Boot. I'd really like to discuss whether we can break out of the /chosen node straightjacket. Perhaps instead we should have something like /firmware or /chosen-fw with the U-Boot node under that? I can definitely see the concern about having lots of vendor-specific nodes at the top level for every component, though. Do you have any other ideas? > > Arguably, we don't even need another sub-node here. We could just say > a given component is responsible for consuming /chosen and then > updating it for the next component. The problem with that is if you > want all the configuration to coexist at the start. Overlapping > properties is also a problem. The only overlap in this case is > bootcmd, but you could handle that with a 'u-boot,bootcmd'. Not saying > we should do that though given we need to extend things beyond 2 > components. Are we trying to conserve nodes? They are not that expensive. I like the idea of separating our concerns between firmware and OS, or this is going to get mighty confusing. As you say it makes it hard to use a mostly static DT. > > > Should we perhaps have a vendor/ directory for vendor-specific tags? > > In the kernel tree, we already have bindings/soc/<vendor> and vendors > like to just dump all their stuff there when it belongs in a directory > for the function. OK so perhaps we should leave this file where it is. > > > Also, thinking ahead, I am interested in how we can add bindings for > > firmware-to-firmware communications. There are some settings that > > could be defined across projects (such as memory layout, security > > level/settings) and these should ideally be harmless to pass to the > > kernel (i.e. ignored by the kernel). It is possible that some of these > > could be used by the kernel but then we can always recreate them using > > kernel bindings as needed (and cross that bridge when we come to it). > > So this would be a set of bindings used by firmware components in > > general. We would not want to use "u-boot,xxx" in that case. > > Yes, that is also why I'm thinking about how do we extend /chosen. > More generally, it's just one stage to the next. firmware-to-firmware > is not really any different than bootloader to OS. /chosen serves that > purpose already, so the question is how to make chosen support > multiple components. The problem, as I see it, is that we don't have /chosen/operating-system, we just have /chosen So that namespace is already claimed for the OS. A clean break seems better to me. Just a thought...if we have /chosen-fw we could have subnodes for each firmware component, with the 'standard' bindings at the top level? Regards, Simon
