dt-bindings: nvmem: add U-Boot environment variables binding

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>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220228131250.16943-1-zajec5@gmail.com

Documentation/devicetree/bindings/nvmem/u-boot,env.yaml

+# 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>
+
+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
+
+additionalProperties: 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>;
+        };
+    };

MAINTAINERS

@@ -19665,6 +19665,11 @@ 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>