Re: [PATCH V2] dt-bindings: nvmem: add U-Boot environment variables binding

[Rob Herring <robh@kernel.org>]

On Thu, Feb 17, 2022 at 02:02:35PM +0100, Rafał Miłecki wrote:
> From: Rafał Miłecki <rafal@milecki.pl>
> 
> U-Boot uses environment variables for storing device setup data. It
> usually needs to be accessed by a bootloader, kernel and often
> user-space.

How much of this is already in use vs. proposed? I know I've seen 
something, but that may have been a u-boot env string in 'label' and 
that's it.

> This binding allows describing environment data located in a raw flash
> partition. It's treated as NVMEM device and can be reused later for
> other storage devices.
> 
> Using DT should be cleaner than hardcoding & duplicating such info in
> multiple places. Bootloader & kernel can share DTS and user-space can
> try reading it too or just have correct data exposed by a kernel.
> 
> A custom "compatible" string allows system to automatically load
> relevant NVMEM driver but phandle can be also used for reading raw
> location.
> 
> Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
> ---
> V2: Update descriptions to don't make this binding MTD (flash partition)
>     specific. Mention multiple possible storage ways.
> ---
>  .../devicetree/bindings/nvmem/u-boot,env.yaml | 66 +++++++++++++++++++
>  MAINTAINERS                                   |  5 ++
>  2 files changed, 71 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/nvmem/u-boot,env.yaml
> 
> diff --git a/Documentation/devicetree/bindings/nvmem/u-boot,env.yaml b/Documentation/devicetree/bindings/nvmem/u-boot,env.yaml
> new file mode 100644
> index 000000000000..a53e34152c97
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/nvmem/u-boot,env.yaml
> @@ -0,0 +1,66 @@
> +# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
> +%YAML 1.2
> +---
> +$id: http://devicetree.org/schemas/nvmem/u-boot,env.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> +
> +title: U-Boot environment variables
> +
> +description: |
> +  U-Boot uses environment variables to store device parameters and
> +  configuration. They may be used for booting process, setup or keeping end user
> +  info.
> +
> +  Data is stored using U-Boot specific formats (variant specific header and NUL
> +  separated key-value pairs).
> +
> +  Environment data can be stored on various storage entities, e.g.:
> +  1. Raw flash partition
> +  2. UBI volume
> +
> +  This binding allows marking storage device (as containing env data) and
> +  specifying used format.
> +
> +  Right now only flash partition case is covered but it may be extended to e.g.
> +  UBI volumes in the future.
> +
> +maintainers:
> +  - Rafał Miłecki <rafal@milecki.pl>
> +
> +allOf:
> +  - $ref: nvmem.yaml#

What exactly is used from nvmem.yaml? Based on the example, nothing.

> +
> +properties:
> +  compatible:
> +    oneOf:
> +      - description: A standalone env data block
> +        const: u-boot,env

> +      - description: Two redundant blocks with active one flagged
> +        const: u-boot,env-redundant-bool
> +      - description: Two redundant blocks with active having higher counter
> +        const: u-boot,env-redundant-count

Aren't these 2 discoverable based on a flag or count property?

> +
> +  reg:
> +    maxItems: 1
> +
> +unevaluatedProperties: false
> +
> +examples:
> +  - |
> +    partitions {
> +        compatible = "fixed-partitions";
> +        #address-cells = <1>;
> +        #size-cells = <1>;
> +
> +        partition@0 {
> +            reg = <0x0 0x40000>;
> +            label = "u-boot";
> +            read-only;
> +        };
> +
> +        env: partition@40000 {
> +            compatible = "u-boot,env";
> +            reg = <0x40000 0x10000>;
> +            label = "u-boot-env";
> +        };
> +    };
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 66aa3a589f6a..55c56ce82856 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -19905,6 +19905,11 @@ W:	http://linuxtv.org
>  T:	git git://linuxtv.org/media_tree.git
>  F:	drivers/media/pci/tw686x/
>  
> +U-BOOT ENVIRONMENT VARIABLES
> +M:	Rafał Miłecki <rafal@milecki.pl>
> +S:	Maintained
> +F:	Documentation/devicetree/bindings/nvmem/u-boot,env.yaml
> +
>  UACCE ACCELERATOR FRAMEWORK
>  M:	Zhangfei Gao <zhangfei.gao@linaro.org>
>  M:	Zhou Wang <wangzhou1@hisilicon.com>
> -- 
> 2.34.1
> 
>