Hi Luca!
On Tue, 2024-09-17 at 10:53 +0200, Luca Ceresoli wrote:
> When device tree nodes are added, the I2C core tries to probe client
> devices based on the classic DT structure:
>
> i2c@abcd0000 {
> some-client@42 { compatible = "xyz,blah"; ... };
> };
>
> However for hotplug connectors described via device tree overlays there is
> additional level of indirection, which is needed to decouple the overlay
> and the base tree:
>
> --- base device tree ---
>
> i2c1: i2c@abcd0000 { compatible = "xyz,i2c-ctrl"; ... };
> i2c5: i2c@cafe0000 { compatible = "xyz,i2c-ctrl"; ... };
>
> connector {
> i2c-ctrl {
> i2c-parent = <&i2c1>;
> #address-cells = <1>;
> #size-cells = <0>;
> };
>
> i2c-sensors {
> i2c-parent = <&i2c5>;
> #address-cells = <1>;
> #size-cells = <0>;
> };
> };
>
> --- device tree overlay ---
>
> ...
> // This node will overlay on the i2c-ctrl node of the base tree
Why don't you overlay it right over &i2c1?
It should have worked since commit ea7513bbc041
("i2c/of: Add OF_RECONFIG notifier handler").
Doesn't it work for your use-case?
> i2c-ctrl {
> eeprom@50 { compatible = "atmel,24c64"; ... };
> };
> ...
>
> --- resulting device tree ---
>
> i2c1: i2c@abcd0000 { compatible = "xyz,i2c-ctrl"; ... };
> i2c5: i2c@cafe0000 { compatible = "xyz,i2c-ctrl"; ... };
>
> connector {
> i2c-ctrl {
> i2c-parent = <&i2c1>;
> #address-cells = <1>;
> #size-cells = <0>;
>
> eeprom@50 { compatible = "atmel,24c64"; ... };
> };
>
> i2c-sensors {
> i2c-parent = <&i2c5>;
> #address-cells = <1>;
> #size-cells = <0>;
> };
> };
>
> Here i2c-ctrl (same goes for i2c-sensors) represent the part of I2C bus
> that is on the hot-pluggable add-on. On hot-plugging it will physically
> connect to the I2C adapter on the base board. Let's call the 'i2c-ctrl'
> node an "extension node".
>
> In order to decouple the overlay from the base tree, the I2C adapter
> (i2c@abcd0000) and the extension node (i2c-ctrl) are separate
> nodes. Rightfully, only the former will probe into an I2C adapter, and it
> will do that perhaps during boot, long before overlay insertion.
>
> The extension node won't probe into an I2C adapter or any other device or
> bus, so its subnodes ('eeprom@50') won't be interpreted as I2C clients by
> current I2C core code. However it has an 'i2c-parent' phandle to point to
> the corresponding I2C adapter node. This tells those nodes are I2C clients
> of the adapter in that other node.
>
> Extend the i2c-core-of code to look for the adapter via the 'i2c-parent'
> phandle when the regular adapter lookup does not find one. This allows all
> clients to be probed: both those on the base board (described in the base
> device tree) and those on the add-on and described by an overlay.
>
> Signed-off-by: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>
> ---
>
> Note: while this patch works for normal hotplug and unplug, it has some
> weaknesses too, due to the implementation being in a OF change
> notifier. Two cases come to mind:
>
> 1. In the base device tree there must be _no_ nodes under the "extension
> node" (i2c-ctrl), or they won't be picked up as they are not
> dynamically added.
>
> 2. In case the I2C adapter is unbound and rebound, or it probes after
> overlay insertion, it will miss the OF notifier events and so it won't
> find the devices in the extension node.
>
> The first case is not a limiting factor: fixed I2C devices should just stay
> under the good old I2C adapter node.
>
> The second case is a limiting factor, even though not happening in "normal"
> use cases. I cannot see any solution without making the adapter aware of
> the "bus extensions" it has, so on its probe it can always go look for any
> devices there. Taking into account the case of multiple connectors each
> having an extension of the same bus, this may look as follows in device
> tree:
>
> --- base device tree ---
>
> i2c1: i2c@abcd0000 {
> compatible = "xyz,i2c-ctrl"; ...
> i2c-bus-extensions = <&i2c_ctrl_conn0, &i2c_ctrl_conn1>;
> };
>
> connector@0 {
> i2c_ctrl_conn0: i2c-ctrl {
> i2c-parent = <&i2c1>;
> #address-cells = <1>;
> #size-cells = <0>;
> };
> };
>
> connector@1 {
> i2c_ctrl_conn1: i2c-ctrl {
> i2c-parent = <&i2c1>;
> #address-cells = <1>;
> #size-cells = <0>;
> };
> };
>
> I'd love to have some feedback and opinions about the basic idea before
> digging into the details of this additional step.
>
> ---
>
> Changes in v4:
> - fix a typo in commit message
>
> This patch first appeared in v3.
> ---
> drivers/i2c/i2c-core-of.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/drivers/i2c/i2c-core-of.c b/drivers/i2c/i2c-core-of.c
> index a6c407d36800..71c559539a13 100644
> --- a/drivers/i2c/i2c-core-of.c
> +++ b/drivers/i2c/i2c-core-of.c
> @@ -170,6 +170,15 @@ static int of_i2c_notify(struct notifier_block *nb, unsigned long action,
> switch (of_reconfig_get_state_change(action, rd)) {
> case OF_RECONFIG_CHANGE_ADD:
> adap = of_find_i2c_adapter_by_node(rd->dn->parent);
> + if (adap == NULL) {
> + struct device_node *i2c_bus;
> +
> + i2c_bus = of_parse_phandle(rd->dn->parent, "i2c-parent", 0);
> + if (i2c_bus) {
> + adap = of_find_i2c_adapter_by_node(i2c_bus);
> + of_node_put(i2c_bus);
> + }
> + }
> if (adap == NULL)
> return NOTIFY_OK; /* not for us */
>
--
Alexander Sverdlin
Siemens AG
www.siemens.com
