[PATCH V3 1/6] dt-bindings: nvmem: layouts: add U-Boot environment variables layout

[Rafał Miłecki]

On 4.01.2024 08:58, Miquel Raynal wrote:
> robh at kernel.org wrote on Wed, 3 Jan 2024 17:11:29 -0700:
>> On Thu, Dec 21, 2023 at 06:34:16PM +0100, Rafał Miłecki wrote:
>>> From: Rafał Miłecki <rafal at milecki.pl>
>>>
>>> U-Boot env data is a way of storing firmware variables. It's a format
>>> that can be used of top of various storage devices. Its binding should
>>> be an NVMEM layout instead of a standalone device.
>>>
>>> This patch adds layout binding which allows using it on top of MTD NVMEM
>>> device as well as any other. At the same time it deprecates the old
>>> combined binding.
>>
>> I don't understand the issue. From a DT perspective, there isn't. A
>> partition is not a device, but is describing the layout of storage
>> already.
> 
> Actually I think what Rafał wants to do goes in the right direction but
> I also understand from a binding perspective it may be a little
> confusing, even more if we consider "NVMEM" a Linux specific concept.
> 
> There is today a "u-boot env" NVMEM *device* description which
> almost sits at the same level as eg. an eeprom device. We cannot
> compare "an eeprom device" and "a u-boot environment" of course. But
> that's truly what is currently described.
> 
> * Current situation
> 
> 	Flash device -> U-Boot env layout -> NVMEM cells
> 
> * Improved situation
> 
> 	Any storage device -> NVMEM -> U-Boot env layout -> NVMEM cells
> 
> The latter is of course the most relevant description as we expect
> storage devices to expose a storage-agnostic interface (NVMEM in
> this case) which can then be parsed (by NVMEM layouts) in a storage
> agnostic way.
> 
> In the current case, the current U-Boot env binding tells people to
> declare the env layout on top of a flash device (only). The current
> description also expects a partition node which is typical to flash
> devices. Whereas what we should have described in the first place is a
> layout that applies on any kind of NVMEM device.
> 
> Bonus point: We've been working the last couple years on clarifying
> bindings, especially with mtd partitions (with the partitions{}
> container) and NVMEM layouts (with the nvmem-layout{} container).
> The switch proposed in this patch makes use of the latter, of course.

Thanks Miquèl for filling bits I missed in commit description. Despite
years in Linux/DT I still struggle with more complex designs
documentation.


As per Rob's comment I think I see his point and a possible design
confusion. If you look from a pure DT perspective then "partitions" and
"nvmem-layout" serve a very similar purpose. They describe device's data
content structure. For fixed structures we have very similar
"fixed-partitions" and "fixed-cells".

If we were to design those bindings today I'm wondering if we couldn't
have s/partitions/layout/ and s/nvmem-layout/layout/.

Rob: other than having different bindings for MTD vs. NVMEM layouts I
think they overall design makes sense. A single device may have content
structurized on more than 1 level:
1. You may have fixed layout at top level (multiple partitions)
2. Single partitions may have their own layouts (like U-Boot env data)

Maybe ideally above should look more like:

flash at 0 {
	compatible = "<flash-compatible>";

	layout {
		compatible = "fixed-layout";
		#address-cells = <1>;
		#size-cells = <1>;

		partition at 0 {
			reg = <0x0 0x40000>;
			label = "u-boot";
		};

		partition at 40000 {
			reg = <0x40000 0x10000>;
			label = "u-boot-env";

			layout {
				compatible = "u-boot,env-layout";
			};
		};

		partition at 50000 {
			reg = <0x50000 0x100000>;
			label = "u-boot";
		};
	};
};

but I can clearly see a use for nested "layout"s. As I said maybe we
just shouldn't be so open in calling those MTD or NVMEM devices as that
is kind of Linux specific.

I'm not sure if we should try renaming "nvmem-layout" to "layout" or
"partitions" in similar way at this point.