Re: [PATCH V2] dt-bindings: nvmem: add U-Boot environment variables binding

["Rafał Miłecki" <zajec5@gmail.com>]

On 25.02.2022 17:42, Rob Herring wrote:
> On Thu, Feb 17, 2022 at 02:02:35PM +0100, Rafał Miłecki wrote:
>> From: Rafał Miłecki <rafal@milecki.pl>
>>
>> U-Boot uses environment variables for storing device setup data. It
>> usually needs to be accessed by a bootloader, kernel and often
>> user-space.
> 
> How much of this is already in use vs. proposed? I know I've seen
> something, but that may have been a u-boot env string in 'label' and
> that's it.

[bootloader]
Right now U-Boot doesn't use any binding for describing env variables.
It's location is usually hardcoded, see (in U-Boot):
* CONFIG_ENV_ADDR
* CONFIG_ENV_SECT_SIZE
* CONFIG_ENV_ADDR_REDUND

[kernel]
There is no support for accessing U-Boot env data. This patch is the
first step for adding such a support.

[user-space]
OpenWrt uses bash script to store a list of devices and their U-Boot env
variables location. In a long term I'd like to replace it and use DT
info + possibly a kernel exposed NVMEM data.


>> 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@milecki.pl>
>> ---
>> V2: Update descriptions to don't make this binding MTD (flash partition)
>>      specific. Mention multiple possible storage ways.
>> ---
>>   .../devicetree/bindings/nvmem/u-boot,env.yaml | 66 +++++++++++++++++++
>>   MAINTAINERS                                   |  5 ++
>>   2 files changed, 71 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..a53e34152c97
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/nvmem/u-boot,env.yaml
>> @@ -0,0 +1,66 @@
>> +# 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@milecki.pl>
>> +
>> +allOf:
>> +  - $ref: nvmem.yaml#
> 
> What exactly is used from nvmem.yaml? Based on the example, nothing.

Nothing. I thought it's nice for a context. I'll drop it.


>> +
>> +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
> 
> Aren't these 2 discoverable based on a flag or count property?

U-Boot discovers that based on a type of flash device(s). In redundant
mode env data can be stored on one or two flash devices.

U-Boot conditions:

/* Check flag scheme compatibility */
if (DEVTYPE(dev_current) == MTD_NORFLASH &&
	DEVTYPE(!dev_current) == MTD_NORFLASH) {
	environment.flag_scheme = FLAG_BOOLEAN;
} else if (DEVTYPE(dev_current) == MTD_NANDFLASH &&
		DEVTYPE(!dev_current) == MTD_NANDFLASH) {
	environment.flag_scheme = FLAG_INCREMENTAL;
} else if (DEVTYPE(dev_current) == MTD_DATAFLASH &&
		DEVTYPE(!dev_current) == MTD_DATAFLASH) {
	environment.flag_scheme = FLAG_BOOLEAN;
} else if (DEVTYPE(dev_current) == MTD_UBIVOLUME &&
		DEVTYPE(!dev_current) == MTD_UBIVOLUME) {
	environment.flag_scheme = FLAG_INCREMENTAL;
} else if (DEVTYPE(dev_current) == MTD_ABSENT &&
		DEVTYPE(!dev_current) == MTD_ABSENT &&
		IS_UBI(dev_current) == IS_UBI(!dev_current)) {
	environment.flag_scheme = FLAG_INCREMENTAL;
} else {
	fprintf(stderr, "Incompatible flash types!\n");
	ret = -EINVAL;
	goto open_cleanup;
}

I thought it's better & more flexible to describe format explicitly in
the DT. That way vendors have more options - they can e.g. start using
incremental setup on NOR flash devices.


>> +
>> +  reg:
>> +    maxItems: 1
>> +
>> +unevaluatedProperties: 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>;
>> +            label = "u-boot-env";
>> +        };
>> +    };
>> diff --git a/MAINTAINERS b/MAINTAINERS
>> index 66aa3a589f6a..55c56ce82856 100644
>> --- a/MAINTAINERS
>> +++ b/MAINTAINERS
>> @@ -19905,6 +19905,11 @@ 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@milecki.pl>
>> +S:	Maintained
>> +F:	Documentation/devicetree/bindings/nvmem/u-boot,env.yaml
>> +
>>   UACCE ACCELERATOR FRAMEWORK
>>   M:	Zhangfei Gao <zhangfei.gao@linaro.org>
>>   M:	Zhou Wang <wangzhou1@hisilicon.com>
>> -- 
>> 2.34.1
>>
>>