Re: [RFC PATCH] mfd: syscon: add child device support

[Rob Herring <robherring2@gmail.com>]

On Tue, May 27, 2014 at 8:55 AM, Philipp Zabel <p.zabel@pengutronix.de> wrote:
> For devices which have a complete register for themselves, it is possible to
> place them next to the syscon device with overlapping reg ranges. The same is

We want to avoid overlapping ranges.

> not possible for devices which only occupy bitfields in registers shared with
> other users.

You are addressing the latter case here?

> For devices that are completely controlled by bitfields in the syscon address
> range, such as multiplexers or voltage regulators, allow to put child devices
> into the syscon device node.
>
> Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
> ---
>  Documentation/devicetree/bindings/mfd/syscon.txt | 11 +++++++++++
>  drivers/mfd/syscon.c                             |  2 ++
>  2 files changed, 13 insertions(+)
>
> diff --git a/Documentation/devicetree/bindings/mfd/syscon.txt b/Documentation/devicetree/bindings/mfd/syscon.txt
> index fe8150b..a7e11d5 100644
> --- a/Documentation/devicetree/bindings/mfd/syscon.txt
> +++ b/Documentation/devicetree/bindings/mfd/syscon.txt
> @@ -9,10 +9,21 @@ using a specific compatible value), interrogate the node (or associated
>  OS driver) to determine the location of the registers, and access the
>  registers directly.
>
> +Optionally, devices that are only controlled through single syscon
> +registers or bitfields can also be added as child nodes to the syscon
> +device node. These devices can implicitly assume their parent node
> +as syscon provider without referencing it explicitly via phandle.
> +In this case, the syscon node should have #address-cells = <1> and
> +#size-cells = <0> and no ranges property.
> +

I'd like to see an example. What does reg in the child contain? The
register address?

What if the child device needs 3 bitfields from 3 different registers?
It seems to me that you could then want to describe every single
bitfield of a block in the DT. That seems like too much information in
DT much like trying to describe every single clock in DT.

Rob