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

[Michal Simek <michal.simek@xilinx.com>]

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@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@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@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.
>>
>> 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