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

Rafał Miłecki
2022-03-10T09:34:16Z

On 10.03.2022 09:45, Michal Simek wrote:
>
>
> 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?

Let me quote Rob here:

> 'label' is supposed to correspond to a sticker on a port or something
> human identifiable

^^ https://patchwork.ozlabs.org/comment/2812214/

"label" is already abused for naming MTD partitions, I don't think it's
a good idea to abuse it even more for different purposes. Also
"compatible" is a standard way for describing hardware blocks & various
entities (identifying them).

I'm also wondering if using "label" instead of "compatible" wouldn't
require breaking changes in some DT files. What if someone uses a random
"label" (e.g. "ub00tenv") and has user-space based on that MTD partition
name?
If we require changing "label" that will require people to also update
names in other places (user-space).
I'm not sure how valid is that argument, just wondering.