[PATCH v3 0/4] NVMEM cells in sysfs

[Miquel Raynal]

Hello,

As part of a previous effort, support for dynamic NVMEM layouts was
brought into mainline, helping a lot in getting information from NVMEM
devices at non-static locations. One common example of NVMEM cell is the
MAC address that must be used. Sometimes the cell content is mainly (or
only) useful to the kernel, and sometimes it is not. Users might also
want to know the content of cells such as: the manufacturing place and
date, the hardware version, the unique ID, etc. Two possibilities in
this case: either the users re-implement their own parser to go through
the whole device and search for the information they want, or the kernel
can expose the content of the cells if deemed relevant. This second
approach sounds way more relevant than the first one to avoid useless
code duplication, so here is a series bringing NVMEM cells content to
the user through sysfs.

Here is a real life example with a Marvell Armada 7040 TN48m switch:

$ nvmem=/sys/bus/nvmem/devices/1-00563/
$ for i in `ls -1 $nvmem/cells/*`; do basename $i; hexdump -C $i | head -n1; done
country-code
00000000  54 57                                             |TW|
crc32
00000000  bb cd 51 98                                       |..Q.|
device-version
00000000  02                                                |.|
diag-version
00000000  56 31 2e 30 2e 30                                 |V1.0.0|
label-revision
00000000  44 31                                             |D1|
mac-address
00000000  18 be 92 13 9a 00                                 |......|
manufacture-date
00000000  30 32 2f 32 34 2f 32 30  32 31 20 31 38 3a 35 39  |02/24/2021 18:59|
manufacturer
00000000  44 4e 49                                          |DNI|
num-macs
00000000  00 40                                             |.@|
onie-version
00000000  32 30 32 30 2e 31 31 2d  56 30 31                 |2020.11-V01|
platform-name
00000000  38 38 46 37 30 34 30 2f  38 38 46 36 38 32 30     |88F7040/88F6820|
product-name
00000000  54 4e 34 38 4d 2d 50 2d  44 4e                    |TN48M-P-DN|
serial-number
00000000  54 4e 34 38 31 50 32 54  57 32 30 34 32 30 33 32  |TN481P2TW2042032|
vendor
00000000  44 4e 49                                          |DNI|

Here is a list of known limitations though:
* It is currently not possible to know whether the cell contains ASCII
  or binary data, so by default all cells are exposed in binary form.
* For now the implementation focuses on the read aspect. Technically
  speaking, in some cases, it could be acceptable to write the cells, I
  guess, but for now read-only files sound more than enough. A writable
  path can be added later anyway.
* The sysfs entries are created when the device probes, not when the
  NVMEM driver does. This means, if an NVMEM layout is used *and*
  compiled as a module *and* not installed properly in the system (a
  usermode helper tries to load the module otherwise), then the sysfs
  cells won't appear when the layout is actually insmod'ed because the
  sysfs folders/files have already been populated.

Changes in v3:
* Patch 1 is new: fix a style issue which bothered me when reading the
  core.
* Patch 2 is new: Don't error out when an attribute group does not
  contain any attributes, it's easier for developers to handle "empty"
  directories this way. It avoids strange/bad solutions to be
  implemented and does not cost much.
* Drop the is_visible hook as it is no longer needed.
* Stop allocating an empty attribute array to comply with the sysfs core
  checks (this check has been altered in the first commits).
* Fix a missing tab in the ABI doc.

Changes in v2:
* Do not mention the cells might become writable in the future in the
  ABI documentation.
* Fix a wrong return value reported by Dan and kernel test robot.
* Implement .is_bin_visible().
* Avoid overwriting the list of attribute groups, but keep the cells
  attribute group writable as we need to populate it at run time.
* Improve the commit messages.
* Give a real life example in the cover letter.

Miquel Raynal (4):
  sysfs: Improve readability by following the kernel coding style
  sysfs: Skip empty folders creation
  ABI: sysfs-nvmem-cells: Expose cells through sysfs
  nvmem: core: Expose cells through sysfs

 Documentation/ABI/testing/sysfs-nvmem-cells |  19 +++
 drivers/nvmem/core.c                        | 128 +++++++++++++++++++-
 fs/sysfs/group.c                            |  12 +-
 3 files changed, 151 insertions(+), 8 deletions(-)
 create mode 100644 Documentation/ABI/testing/sysfs-nvmem-cells

-- 
2.34.1