Re: [PATCH 0/7] of: overlay: Add support for export-symbols node feature

[Andrew Davis <afd@ti.com>]

On 12/9/24 11:03 AM, Herve Codina wrote:
> On Mon, 9 Dec 2024 10:47:50 -0600
> Andrew Davis <afd@ti.com> wrote:
> 
>> On 12/9/24 9:18 AM, Herve Codina wrote:
>>> Hi,
>>>
>>> At Linux Plumbers Conference 2024, we (me and Luca Ceresolli) talked
>>> about issues we have with runtime hotplug on non-discoverable busses
>>> with device tree overlays [1].
>>>
>>> On our system, a base board has a connector and addon boards can be
>>> connected to this connector. Both boards are described using device
>>> tree. The base board is described by a base device tree and addon boards
>>> are describe by overlays device tree. More details can be found at [2].
>>>
>>> This kind of use case can be found also on:
>>>     - Grove Sunlight Sensor [3]
>>>     - mikroBUS [4]
>>>
>>> One of the issue we were facing on was referencing resources available
>>> on the base board device tree from the addon overlay device tree.
>>>
>>> Using a nexus node [5] helps decoupling resources and avoid the
>>> knowledge of the full base board from the overlay. Indeed, with nexus
>>> node, the overlay need to know only about the nexus node itself.
>>>
>>> For instance, suppose a connector where a GPIO is connected at PinA. On
>>> the base board this GPIO is connected to the GPIO 12 of the SoC GPIO
>>> controller.
>>>
>>> The base board can describe this GPIO using a nexus node:
>>>       soc_gpio: gpio-controller {
>>>         #gpio-cells = <2>;
>>>       };
>>>
>>>       connector1: connector1 {
>>>           /*
>>>            * Nexus node for the GPIO available on the connector.
>>>            * GPIO 0 (Pin A GPIO) is connected to GPIO 12 of the SoC gpio
>>>            * controller
>>>            */
>>>           #gpio-cells = <2>;
>>>           gpio-map = <0 0 &soc_gpio 12 0>;
>>>           gpio-map-mask = <0xf 0x0>;
>>>           gpio-map-pass-thru = <0x0 0xf>;
>>>       };
>>>
>>> The connector pin A GPIO can be referenced using:
>>>     <&connector1 0 GPIO_ACTIVE_HIGH>
>>>
>>> This implies that the overlay needs to know about exact label that
>>> references the connector. This label can be different on a different
>>> board and so applying the overlay could failed even if it is used to
>>> describe the exact same addon board. Further more, a given base board
>>> can have several connectors where the exact same addon board can be
>>> connected. In that case, the same overlay cannot be used on both
>>> connector. Indeed, the connector labels have to be different.
>>>
>>> The export-symbols node introduced by this current series solves this
>>> issue.
>>>
>>> The idea of export-symbols is to have something similar to the global
>>> __symbols__ node but local to a specific node. Symbols listed in this
>>> export-symbols are local and visible only when an overlay is applied on
>>> a node having an export-symbols subnode.
>>>
>>> Using export-symbols, our example becomes:
>>>       soc_gpio: gpio-controller {
>>>         #gpio-cells = <2>;
>>>       };
>>>
>>>       connector1: connector1 {
>>>           /*
>>>            * Nexus node for the GPIO available on the connector.
>>>            * GPIO 0 (Pin A GPIO) is connected to GPIO 12 of the SoC gpio
>>>            * controller
>>>            */
>>>           #gpio-cells = <2>;
>>>           gpio-map = <0 0 &soc_gpio 12 0>;
>>>           gpio-map-mask = <0xf 0x0>;
>>>           gpio-map-pass-thru = <0x0 0xf>;
>>>
>>>           export-symbols {
>>>             connector = <&connector1>;
>>>           };
>>>       };
>>>
>>> With that export-symbols node, an overlay applied on connector1 node can
>>> have the symbol named 'connector' resolved to connector1. Indeed, the
>>> export-symbols node available at connector1 node is used when the
>>> overlay is applied. If the overlay has an unresolved 'connector' symbol,
>>> it will be resolved to connector1 thanks to export-symbols.
>>>
>>> Our overlay using the nexus node can contains:
>>>      node {
>>>         foo-gpio = <&connector 0 GPIO_ACTIVE_HIGH>;
>>>      };
>>> It used the GPIO 0 from the connector it is applied on.
>>>
>>> A board with two connectors can be described with:
>>>       connector1: connector1 {
>>>           ...
>>>           export-symbols {
>>>             connector = <&connector1>;
>>>           };
>>>       };
>>>
>>>       connector2: connector2 {
>>>           ...
>>>           export-symbols {
>>>             connector = <&connector2>;
>>>           };
>>>       };
>>>
>>> In that case, the same overlay with unresolved 'connector' symbol can be
>>> applied on both connectors and the correct symbol resolution (connector1
>>> or connector2) will be done.
>>>    
>>
>> I might be missing something, but how is the correct connector (connector1
>> or connector2) selected? Let's say I connect my addon board to connector2,
>> then I apply the addon board's overlay to the base DTB. What connector
>> just got referenced?
>>
> 
> A driver for the connector is needed.
> The driver applies the overlay using of_overlay_fdt_apply().
> The node the overlay has to be applied to is passed by the driver to
> of_overlay_fdt_apply().
> 

So every connector needs a driver? Most connectors are dumb connectors,
just a bunch of wires broken out to a header.

What if an addon board overlay uses multiple connectors?

If you need a connector-specific driver, and that driver needs to know
which node this overlay will be applied to, then why not just do a
fixup directly to the overlay in the driver?

Andrew

> Even if obsolete because I added one more parameter (export_symbols_name)
> in of_overlay_fdt_apply() in this current series, you can have a look at the
> following patch to see the connector driver:
>    https://lore.kernel.org/lkml/20240917-hotplug-drm-bridge-v4-8-bc4dfee61be6@bootlin.com/
> 
> Best regards,
> Hervé
>