Hello all, A new attempt at solving the long standing "add-on board" problem was recently posted[0]. The current out-of-tree solutions usually involve Device Tree Overlays. Recently, Overlays have started being accepted into the kernel repo, this makes now the perfect time to solve this issue. Here is my attempt at a generic solution. Problem statement ----------------- The Device tree(DT) system provides hardware descriptions to the Linux kernel (or other DT compatible SW). When the hardware is modular this description becomes dynamic. For this we use DT Overlays which take a base hardware description and append the description of the add-on hardware. Due to the design of DT, these DT overlays are specific to a given base hardware board. The add-on itself is usually not specific to a single board. Some examples of add-on ecosystems to consider: Beaglebone Cape Raspberry Pi HAT MikroBUS Click Boards Seeed Grove SparkFun Qwiic etc.. Some of these ecosystems already have more than a thousand(!) add-on boards. If a DT description needed to be written specific to each of the multitude of base boards that support each add-on, the combinatorial explosion would be unmanageable. We need to define a scheme that allow for creating and applying generic add-on overlays. Goals ----- * Each add-on board should be described by only one DT overlay. That DT overlay should be generic enough to apply to the DT of any base board that supports that add-on. * Some base boards have multiple instances of a given add-ons connector port. An add-on's overlay must apply to any available connection port without modification to the overlay. * Some connectors are stackable, stacked application of overlays shall function as expected. Chained connectors from one ecosystem to another shall be supported also (i.g. This thing[1] which connects to a BeagleBone Cape connector and then exposes a number of Grove connectors). * We should reuse as much existing infrastructure as possible (ideally no changes should be needed). The basic application of DT overlays is well supported and documented. * An overlay for an add-on board that is not compatible with the base board shall fail to apply at application time, not silently later. Incompatibility includes add-ons which require a function from a pin for which the matching pin on the base board cannot provide. We see this with some HATs and Capes where they use non-standard muxing of pins that only work for some subset of base boards. For instance, the BeaglePlay's Grove connector supports Digital/UART/I2C functions but not "Analog". So any Grove module that uses Analog pins should fail to apply. * Nothing in this solution should preclude runtime application of these DT overlays. Hardware auto-detection and runtime DT modification are orthogonal problems. Solution -------- This is a classic many-to-many problem, we propose to solve this the same as the database folks, with an associative(join) table like adapter overlay. We add an adapter overlay in-between the base board and the add-on. This adapter overlay prepares the base DTB for the application of an add-on targeting a specific connector. Adapting the base board's specifics to accept the generic connector names contained in the add-on overlay. There will be one adapter overlay per base board connector. We already have the infrastructure to implement these adapter overlays today. The DT overlay system makes use of a symbol table in the base DT and a fixup table in the overlay. The magic is in using the __fixups__ table to modify the __symbols__ table itself. Let's use the Grove connector[2] as an example. Grove is a good example target as it has * Low pin count (2 signal pins keeps the example gasket DTBOs simple, everything here can be extended to any number of signal pins) * Multiple connectors per base board * Has an add-on board that exposes more add-on board connectors * Each pin can have multiple functions depending on the base board * Moderately sized collection of add-on boards which contain parts already supported in Linux/DT To make an overlay generic we need a standard name scheme which we use across base boards. For the connector pins the pinmux phandle shall be: <connector-name>_<pin-name>_mux_<pin-function> All capitalized to make it easy to identify that this name is not the final phandle name, but will instead be fixed during overlay application. Each pin will have a definition for each function it can take, so pin1 in the Grove ecosystem has 4 possible functions, and pin2 has the same, therefor 8 definitions are needed in the connector's adapter overlay: /* Grove connector 0 Pin1 options */ GROVE_PIN1_MUX_I2C_SCL = &grove_pins_i2c; GROVE_PIN1_MUX_DIGITAL = &grove_pins_digital; /* GROVE_PIN1_MUX_ANALOG not available on this pin on this connector on this board */ ... GROVE_PIN2_MUX_UART_TX = &grove_pins_uart; etc.. (see patch [2/3] for a complete example) By listing each pin/function combination separately we allow for add-on boards that only use a subset of pins, or mix pin functions (pin1->digital and pin2->uart_tx). This also means is if a given base board does not support some function on a connector pin, then it is not defined and application of an overlay which uses that pin/function will correctly fail as expected. For the parent provider phandle, we use a similar naming scheme: <connector-name>_<pin-name>_<pin-function> Note we list this per-pin. Even though one IP/bus may service multiple pins, we cannot know this in a generic way. For instance some boards may have all GPIO functions served by one controller, others may have some pins on different controllers. Patch [3/3] is a complete example overlay for an add-on board[3]. So what does this all look like? Let's take an example of a BeaglePlay with two Grove connectors for which we have physically attached a Sunlight module to the first connector, and an Air Quality sensor to the second. Doing ahead of time command-line DT overlay application: ./fdtoverlay \ -o output.dtb \ -i k3-am625-beagleplay.dtb k3-am625-beagleplay-grove-connector0.dtbo grove-sunlight-sensor.dtbo \ k3-am625-beagleplay-grove-connector1.dtbo grove-air-quality.dtbo We start with the base board, then apply the adapter overlay for the specific connector we are going to attach the add-on. The next add-on overlay applied will attach to the connector most recently applied. This can be continued as needed, simply apply the next connector's adapter overlay, then the next add-on, rinse, repeat. Note that the connector adapter overlay is board specific, but the add-on overlay is completely generic. It can be applied to any base board. ./fdtoverlay \ -o output.dtb \ -i bcm2837-rpi-3-b.dtb \ grove-base-hat.dtbo \ grove-base-hat-connector0.dtbo grove-sunlight-sensor.dtbo \ grove-base-hat-connector1.dtbo grove-air-quality.dtbo Should work just the same for any board supporting that extender HAT, for instance the BeagleY-AI would be: ./fdtoverlay \ -o output.dtb \ -i k3-am67a-beagley-ai.dtb \ grove-base-hat.dtbo \ grove-base-hat-connector0.dtbo grove-sunlight-sensor.dtbo \ grove-base-hat-connector1.dtbo grove-air-quality.dtbo \ grove-base-hat-connector4.dtbo etc.. All of the above works just the same at boot time (U-Boot overlay support) or runtime using the in-kernel runtime overlay support (when that is enabled). For connectors with board detection I'd expect the detector to be described in the base board connector node. On board identification, the adapter overlay for that connector would be loaded by the detector driver followed by th overlay for the identified board. Although this is an RFC, the patches in this series are functional and meet all the above goals. They require no additional DT schema nor kernel/tooling modifications. Nested adapters (add-ons on top of add-on connectors) require a small fix in DTC which will be sent separately. Open items ---------- Variable cell count providers. The provider specifies the cell count and meaning. For GPIO this is handled very well, there is a standard 2 cell format (GPIO number and flags). Any device can request a controllers' 4th GPIO with active high output the exact same way for all controllers. Interrupts on the other hand have providers with one, two, and even three cells variations. There is no universal way to say "I want this controller's 4th IRQ line with rising edge triggering". These cells may need some level of indirection in the connector node itself to handle variable cell counts/meanings. Where to store the add-on overlay source files. These are not specific to any one board, nor even to one architecture. For now I put the grove-sunlight-sensor.dtb in arch/arm64/boot/dts/ti but it needs a better home acceptable by all boards. More testing, I currently have very few add-on boards to test with right now (but I did just put some on order). Hopefully I can get some more complex ones to really exercise this idea. Maybe a stack like the one in the 4th image here[4], a RPi HAT that exposes a couple MikroBUS connectors, that then have 4 Grove ports on that. This isn't perfect, but the Goals section should be applicable to any solution, and the adapter overlay concept hopefully can be reused as needed for whatever solution the community chooses. Thanks, Andrew [0] https://lore.kernel.org/linux-arm-kernel/20240627-mikrobus-scratch-spi-v5-0-9e6c148bf5f0@xxxxxxxxxxxxxxx/ [1] https://wiki.seeedstudio.com/Grove_Base_Cape_for_BeagleBone_v2/ [2] https://wiki.seeedstudio.com/Grove_System/ [3] https://wiki.seeedstudio.com/Grove-Sunlight_Sensor/ [4] https://www.tindie.com/products/pmunts/mikrobus-grove-adapter-3/ Andrew Davis (3): arm64: dts: ti: k3-am625-beagleplay: Add Grove connector pinmux options arm64: dts: ti: k3-am625-beagleplay: Add Grove connector adapter overlays arm64: dts: ti: grove: Add Grove Sunlight Sensor overlay arch/arm64/boot/dts/ti/Makefile | 5 +++ .../boot/dts/ti/grove-sunlight-sensor.dtso | 31 ++++++++++++++ .../k3-am625-beagleplay-grove-connector0.dtso | 41 +++++++++++++++++++ .../k3-am625-beagleplay-grove-connector1.dtso | 22 ++++++++++ .../arm64/boot/dts/ti/k3-am625-beagleplay.dts | 32 +++++++++++---- 5 files changed, 124 insertions(+), 7 deletions(-) create mode 100644 arch/arm64/boot/dts/ti/grove-sunlight-sensor.dtso create mode 100644 arch/arm64/boot/dts/ti/k3-am625-beagleplay-grove-connector0.dtso create mode 100644 arch/arm64/boot/dts/ti/k3-am625-beagleplay-grove-connector1.dtso -- 2.39.2
