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

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