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

[Michal Simek <michal.simek@xilinx.com>]

On 3/9/22 16:40, Rob Herring wrote:
> 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.

Also agree but isn't it enough to use proper label for it?

> 
>> 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.

Agree on this one.

Thanks,
Michal