[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 at milecki.pl>
>>
>> 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 at milecki.pl>
>> ---
>>   .../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 at milecki.pl>
>> +
>> +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 at 0 {
>                  label = "u-boot-env";
>                  reg = <0x0 0x40000>;
>              };
> 
>              part1: partition at 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 at 0 {
		    label = "u-boot-env";
		    reg = <0x0 0x40000>;
		};

		env1: partition at 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.