Re: [PATCH V3] dt-bindings: nvmem: add U-Boot environment variables binding

[Rob Herring <robh@kernel.org>]

On Wed, Mar 09, 2022 at 02:42:43PM +0100, Michal Simek wrote:
> 
> 
> On 2/28/22 14:12, 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.
> > 
> > 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.
> > V3: Drop
> >      allOf:
> >        - $ref: nvmem.yaml#
> >      as we don't use anything rom the nvmem.yaml. Thanks Rob.
> > ---
> >   .../devicetree/bindings/nvmem/u-boot,env.yaml | 62 +++++++++++++++++++
> >   MAINTAINERS                                   |  5 ++
> >   2 files changed, 67 insertions(+)
> >   create mode 100644 Documentation/devicetree/bindings/nvmem/u-boot,env.yaml
> > 
> > diff --git a/Documentation/devicetree/bindings/nvmem/u-boot,env.yaml b/Documentation/devicetree/bindings/nvmem/u-boot,env.yaml
> > new file mode 100644
> > index 000000000000..e70b2a60cb9a
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/nvmem/u-boot,env.yaml
> > @@ -0,0 +1,62 @@
> > +# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
> > +%YAML 1.2
> > +---
> > +$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>
> > +
> > +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
> > +
> > +  reg:
> > +    maxItems: 1
> > +
> > +additionalProperties: 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>;
> > +        };
> > +    };
> > diff --git a/MAINTAINERS b/MAINTAINERS
> > index db8052bc1d26..24fc181a7e6c 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -19958,6 +19958,11 @@ 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>
> 
> I think that parsing these partitions is quite sw intensive process and I
> can't still see the value to have compatible string here.

It's always good to know what a node represents.

> I would prefer to have just any link from u-boot node to partition instead.

That's a separate issue and having 'compatible' in no way disallows 
that.

Rob