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

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

[Rafał Miłecki]

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.

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#
+
+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
+
+  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

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

[Rob Herring]

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.

> 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.

> +
> +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?

> +
> +  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
> 
> 

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

[Rafał Miłecki]

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
>>
>>

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

[Alexander Dahl]

Hei hei,

just want to give a little more background here from embedded non
openwrt point of view.  See below.

Am Mon, Feb 28, 2022 at 12:32:11PM +0100 schrieb Rafał Miłecki:
> 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

And more … U-Boot has a variety of options to store the U-Boot env,
from the top of my head:

- at some offset in raw flash / mtd partition on NAND, NOR, serial dataflash, etc.
- at some offset on a block device, e.g. on eMMC
- as a file in a FAT partition
- in a UBI volume
- …

And yes, it's determined by some build option and hardcoded at compile
time.

> [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.

U-Boot source itself has userspace tools fw_setenv and fw_printenv
and those look into /etc/fw_env.config on how that env should be
accessed.

Greets
Alex

> 
> 
> > > 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
> > > 
> > > 
> 

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

[Rafał Miłecki]

On 7.03.2022 11:24, Alexander Dahl wrote:
> Hei hei,
> 
> just want to give a little more background here from embedded non
> openwrt point of view.  See below.
> 
> Am Mon, Feb 28, 2022 at 12:32:11PM +0100 schrieb Rafał Miłecki:
>> 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
> 
> And more … U-Boot has a variety of options to store the U-Boot env,
> from the top of my head:
> 
> - at some offset in raw flash / mtd partition on NAND, NOR, serial dataflash, etc.
> - at some offset on a block device, e.g. on eMMC
> - as a file in a FAT partition
> - in a UBI volume
> - …
> 
> And yes, it's determined by some build option and hardcoded at compile
> time.
> 
>> [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.
> 
> U-Boot source itself has userspace tools fw_setenv and fw_printenv
> and those look into /etc/fw_env.config on how that env should be
> accessed.

I wasn't 100% clear on that but OpenWrt actually uses those U-Boot tools

OpenWrt scripts I mentioned can be found there:
https://git.openwrt.org/?p=openwrt/openwrt.git;a=tree;f=package/boot/uboot-envtools/files;h=687c89806f72756053457a7ab7245bff1c4dcfe8;hb=HEAD

Random example:

linksys,wrt1900ac-v1)
	ubootenv_add_uci_config "/dev/mtd1" "0x0" "0x40000" "0x20000"
	;;
linksys,wrt3200acm|\
linksys,wrt32x)
	ubootenv_add_uci_config "/dev/mtd1" "0x0" "0x20000" "0x20000"
	;;

So those scripts simply generate /etc/fw_env.config required by
fw_printenv / fw_setenv. They have to match what's used by U-Boot at
compilation time.

My long term plan is to move that info to DTS and share it across
projects.