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

[Rafał Miłecki]

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@xxxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxxx>
> > > > ---
> > > > 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@xxxxxxxxxx>
> > > > +
> > > > +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@xxxxxxxxxx>
> > > > +S:    Maintained
> > > > +F:    Documentation/devicetree/bindings/nvmem/u-boot,env.yaml
> > > > +
> > > >    UACCE ACCELERATOR FRAMEWORK
> > > >    M:    Zhangfei Gao <zhangfei.gao@xxxxxxxxxx>
> > > >    M:    Zhou Wang <wangzhou1@xxxxxxxxxxxxx>
> > >
> > > 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.