linux-kernel - Re: [PATCH v6 1/3] ABI: sysfs-nvmem-cells: Expose cells through sysfs

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Date:   Sun, 23 Jul 2023 19:39:50 +0000
From:   "John Thomson" <lists@...nthomson.fastmail.com.au>
To:     "Miquel Raynal" <miquel.raynal@...tlin.com>,
        "Srinivas Kandagatla" <srinivas.kandagatla@...aro.org>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Cc:     "Thomas Petazzoni" <thomas.petazzoni@...tlin.com>,
        "Robert Marko" <robert.marko@...tura.hr>,
        "Luka Perkov" <luka.perkov@...tura.hr>,
        "Michael Walle" <michael@...le.cc>,
        "Randy Dunlap" <rdunlap@...radead.org>
Subject: Re: [PATCH v6 1/3] ABI: sysfs-nvmem-cells: Expose cells through sysfs

Hi Miquel,

On Mon, 17 Jul 2023, at 07:51, Miquel Raynal wrote:
> The binary content of nvmem devices is available to the user so in the
> easiest cases, finding the content of a cell is rather easy as it is
> just a matter of looking at a known and fixed offset. However, nvmem
> layouts have been recently introduced to cope with more advanced
> situations, where the offset and size of the cells is not known in
> advance or is dynamic. When using layouts, more advanced parsers are
> used by the kernel in order to give direct access to the content of each
> cell regardless of their position/size in the underlying device, but
> these information were not accessible to the user.
>
> By exposing the nvmem cells to the user through a dedicated cell/ folder
> containing one file per cell, we provide a straightforward access to
> useful user information without the need for re-writing a userland
> parser. Content of nvmem cells is usually: product names, manufacturing
> date, MAC addresses, etc,
>
> Signed-off-by: Miquel Raynal <miquel.raynal@...tlin.com>
> Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
>  Documentation/ABI/testing/sysfs-nvmem-cells | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
>  create mode 100644 Documentation/ABI/testing/sysfs-nvmem-cells
>
> diff --git a/Documentation/ABI/testing/sysfs-nvmem-cells 
> b/Documentation/ABI/testing/sysfs-nvmem-cells
> new file mode 100644
> index 000000000000..b2d15a8d36e5
> --- /dev/null
> +++ b/Documentation/ABI/testing/sysfs-nvmem-cells
> @@ -0,0 +1,19 @@
> +What:		/sys/bus/nvmem/devices/.../cells/<cell-name>
> +Date:		May 2023
> +KernelVersion:	6.5
> +Contact:	Miquel Raynal <miquel.raynal@...tlin.com>
> +Description:
> +		The "cells" folder contains one file per cell exposed by
> +		the nvmem device. The name of the file is the cell name.

Could we consider using a file within a folder (name defined by cell propertys) to access the cell bytes?
Example (pick the best path and filename):
/sys/bus/nvmem/devices/.../cells/<cell-name>/bytes

That way, it is much easier to expand this at a later stage,
like adding an of_node link at
/sys/bus/nvmem/devices/.../cells/<cell-name>/of_node
or exposing other nvmem cell properties.

This is particularly relevant given the cell-name alone does not always
uniquely represent a cell on an nvmem device.
https://lore.kernel.org/lkml/ZLaZ7fzUSsa0Igx1@makrotopia.org/
https://lore.kernel.org/lkml/e7173ab2-d3b2-4f75-beb8-32593b868774@www.fastmail.com/

> +		The length of the file is the size of the cell (when
> +		known). The content of the file is the binary content of
> +		the cell (may sometimes be ASCII, likely without
> +		trailing character).
> +		Note: This file is only present if CONFIG_NVMEM_SYSFS
> +		is enabled.
> +
> +		Example::
> +
> +		  hexdump -C /sys/bus/nvmem/devices/1-00563/cells/product-name
> +		  00000000  54 4e 34 38 4d 2d 50 2d  44 4e         |TN48M-P-DN|
> +		  0000000a
> -- 
> 2.34.1

Cheers,

-- 
  John Thomson