ABI: sysfs-nvmem-cells: Expose cells through sysfs

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 <[email protected]>
Reviewed-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Srinivas Kandagatla <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Documentation/ABI/testing/sysfs-nvmem-cells

@@ -0,0 +1,21 @@
+What:		/sys/bus/nvmem/devices/.../cells/<cell-name>
+Date:		May 2023
+KernelVersion:	6.5
+Contact:	Miquel Raynal <[email protected]>
+Description:
+		The "cells" folder contains one file per cell exposed by the
+		NVMEM device. The name of the file is: <name>@<where>, with
+		<name> being the cell name and <where> its location in the NVMEM
+		device, in hexadecimal (without the '0x' prefix, to mimic device
+		tree node names). 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@d
+		  00000000  54 4e 34 38 4d 2d 50 2d  44 4e         |TN48M-P-DN|
+		  0000000a