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

[Michal Simek]

On 2/15/22 15:57, Sean Anderson wrote:
> On 2/15/22 9:02 AM, Michal Simek wrote:
> > On 2/15/22 14:49, Rafał Miłecki wrote:
> > > From: Rafał Miłecki <rafal@xxxxxxxxxx>
> > >
> > > U-Boot uses environment variables for storing device setup data on
> > > flash. That data usually needs to be accessed by a bootloader, kernel
> > > and often user-space.
> > >
> > > This binding allows describing environment data location and its format
> > > clearly. In some/many cases it should be cleaner than hardcoding &
> > > duplicating that 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.
> > >
> > > Signed-off-by: Rafał Miłecki <rafal@xxxxxxxxxx>
> > > ---
> > >   .../devicetree/bindings/nvmem/u-boot,env.yaml | 58 +++++++++++++++++++
> > >   MAINTAINERS                                   |  5 ++
> > >   2 files changed, 63 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..a2b3a9b88eb8
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/nvmem/u-boot,env.yaml
> > > @@ -0,0 +1,58 @@
> > > +# 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 on flash in a U-Boot specific format (header and NUL separated
> > > +  key-value pairs).
> > > +
> > > +  This binding allows specifying data location and used format.
> > > +
> > > +maintainers:
> > > +  - Rafał Miłecki <rafal@xxxxxxxxxx>
> > > +
> > > +allOf:
> > > +  - $ref: nvmem.yaml#
> > > +
> > > +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
> >
> > I am not convinced that this is the best way how to do it. Because in u-boot 
> > implementation you would have to enable MTD partitions to get there.
> > And the whole parsing will take a lot of time.
> >
> > I think the way how I think this can be handled is.
> >
> > # I don't think that discussion with Simon was finished.
> > But for example (chosen or firmware node)
> > chosen {
> >      u-boot {
> >          u-boot,env = <&qspi &part0>;
> >          u-boot,env-redundant = <&qspi &part1>;
> >          #or
> >          u-boot,env = <&qspi 0 40000>;
> >          u-boot,env-redundant = <&qspi 40000 40000>;
>
> What about when the environment is on top of UBI?

I expect we should list all possible combinations and cover them here.

> >          #or
> >          u-boot,env = <&mmc 0 0 10000>; #device/start/size - raw mode
> >          u-boot,env = <&mmc 0 1>; # device/partition - as file to FS
>
> For emmc at least you will need another cell for the hardware partition.
> And of course, you can name the environment file whatever you want, so
> that needs to be recorded as well.
>
> IMO to do this properly you'd need to have a property corresponding to
> each of the major configs in the env menu.

Agree 100%. I just wanted to share how I think this should be done.
At the end we should be able to describe any combination in generic way.

Thanks,
Michal