Re: [PATCH v5 06/11] nvmem: Add bindings for simple nvmem framework

[maitysanchayan-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org]

Hello Srinivas,

On 15-05-21 17:44:12, Srinivas Kandagatla wrote:
> This patch adds bindings for simple nvmem framework which allows nvmem
> consumers to talk to nvmem providers to get access to nvmem cell data.
> 
> Signed-off-by: Maxime Ripard <maxime.ripard-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>
> [Maxime Ripard: intial version of eeprom framework]
> Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> ---
>  Documentation/devicetree/bindings/nvmem/nvmem.txt | 84 +++++++++++++++++++++++
>  1 file changed, 84 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/nvmem/nvmem.txt
> 
> diff --git a/Documentation/devicetree/bindings/nvmem/nvmem.txt b/Documentation/devicetree/bindings/nvmem/nvmem.txt
> new file mode 100644
> index 0000000..ecea654
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/nvmem/nvmem.txt
> @@ -0,0 +1,84 @@
> += NVMEM Data Device Tree Bindings =
> +
> +This binding is intended to represent the location of hardware
> +configuration data stored in NVMEMs.
> +
> +On a significant proportion of boards, the manufacturer has stored
> +some data on NVMEM, for the OS to be able to retrieve these information
> +and act upon it. Obviously, the OS has to know about where to retrieve
> +these data from, and where they are stored on the storage device.
> +
> +This document is here to document this.
> +
> += Data providers =
> +Contains bindings specific to provider drivers and data cells as children
> +to this node.
> +
> +Optional properties:
> + read-only: Mark the provider as read only.
> +
> += Data cells =
> +These are the child nodes of the provider which contain data cell
> +information like offset and size in nvmem provider.
> +
> +Required properties:
> +reg:	specifies the offset in byte within that storage device, start bit
> +	in the byte and the length in bits of the data we care about.
> +	There could be more then one offset-length pairs in this property.
> +
> +Optional properties:
> +
> +bit-offset: specifies the offset in bit within the address range specified
> +	by reg property. Can take values from 0-7.
> +nbits: specifies number of bits this cell occupies starting from bit-offset.
> +
> +For example:
> +
> +	/* Provider */
> +	qfprom: qfprom@00700000 {
> +		...
> +
> +		/* Data cells */
> +		tsens_calibration: calib@404 {
> +			reg = <0x404 0x10>;
> +		};
> +
> +		tsens_calibration_bckp: calib_bckp@504 {
> +			reg = <0x504 0x11>;
> +			bit-offset = 6;
> +			nbits = 128;
> +		};
> +
> +		pvs_version: pvs-version@6 {
> +			reg = <0x6 0x2>
> +			bit-offset = 7;
> +			nbits = 2;
> +		};
> +
> +		speed_bin: speed-bin@c{
> +			reg = <0xc 0x1>;
> +			bit-offset = 2;
> +			nbits	= 3;
> +
> +		};
> +		...
> +	};

We have a need of exposing information like SoC ID, revision and such
which is what this nvmem framework proposes to be suitable for. Till
now I was trying a different approach for the same [1].

The On Chip One Time Programmable block on the Vybrid can be handled
nicely with this nvmem framework however I had a few queries with
regards to the framework after trying it on a Vybrid VF61 SoC. 

1. From what I understand, getting the information in hex is the only
way possible as of now? Would it be possible to expose the information
as strings from different paths under /sys/class/nvmem/*/nvmem/?

For example, if I have a sub node as below

ocotp: ocotp@400a5000 {
	compatible = "fsl,vf610-ocotp";
	reg = <0x400a5000 0x1000>;

	ocotp_cfg0: cfg0@410 {
		reg = <0x410 0x1>;
	};

	ocotp_cfg1: cfg1@420 {
		reg = <0x420 0x1>;
	};
};

The values from the above register locations represented by the two
sub nodes above can perhaps be exposed as strings? Even if they are
not exposed as strings, making them available in separate paths, is
that something which can be done? So depending on the sub node as
above, /sys/class/nvmem/ocotp0/nvmem/cfg0/ would give values from
the registers specified.

Basically the output of /sys/class/nvmem/*/nvmem being restricted
to only the subnodes specified is what I was hoping to get along
with separate paths.

2. What if the required information is scattered across different memory
regions? In my case, the SoC ID is available from one OCOTP peripheral
block, the revision is in a separate ROM area at the start of the memory
map and some CPU information I am interested in is in another memory
region. I am not sure what would be the right way to approach it with
the nvmem framework.

[1] https://lkml.org/lkml/2015/6/5/189

Thanks & Regards,
Sanchayan Maity.

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