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

[Rafał Miłecki]

On 15.02.2022 15:02, 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.

We'll need to find some consensus considering all points:
1. DT objectives
2. U-Boot needs
3. Linux needs

DT should mainly describe hardware / platform without focusing on a
single implementation details. If U-Boot env data is indeed stored in a
flash block (or blocks) / UBI volume, its binding should be just that.

If U-Boot requires MTD to parse proposed binding and it can't be
afforded at the same time - maybe it can come with different
implementation?


> I think the way how I think this can be handled is.

One minor note: I don't think you can have one "standard" format and one
"redundant" format. If env data is stored in two places - both use the
redundant format.


> # 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>;

1. Using &qspi seems reundant here, you can get parent flash device by
   walking DT.
2. Using "chosen" seems to be a /shortcut/ for getting env data
   location, I don't see any direct conflict with using "compatible"
   string as proposed in my binding.


>          #or
>          u-boot,env = <&qspi 0 40000>;
>          u-boot,env-redundant = <&qspi 40000 40000>;

Here you moved code describing partition from "partitions" into "chosen"
which seems incorrect to me. We already have bindings for partitions and
they should be children of flash node.


>          #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
>          #etc.
>      };
> };
>
>
> &qspi {
>      flash {
>          partitions {
>              compatible = "fixed-partitions";
>              #address-cells = <1>;
>              #size-cells = <1>;
>
>              part0: partition@0 {
>                  label = "u-boot-env";
>                  reg = <0x0 0x40000>;
>              };
>
>              part1: partition@40000 {
>                  label = "u-boot-env-redundant";
>                  reg = <0x40000 0x10000>;
>              };
>      };
> };

So my summary for this would be:
1. Let's use partitions for placing env data partition binding
2. Let's add minimal U-Boot setup into "chosen" if needed

Please consider this:

chosen {
	u-boot {
		u-boot,env = <&env0>, <&env1>;
	};
};

&qspi {
	flash {
	    partitions {
		compatible = "fixed-partitions";
		#address-cells = <1>;
		#size-cells = <1>;

		env0: partition@0 {
		    label = "u-boot-env";
		    reg = <0x0 0x40000>;
		};

		env1: partition@40000 {
		    label = "u-boot-env-redundant";
		    reg = <0x40000 0x10000>;
		};
	};
};

If you still need to access flash content directly, you can pretty
easily calculate offset from &env0 and &env1 nodes.