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    SubjectRe: [PATCH 2/2] nvmem: expose NVMEM cells in sysfs
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    On 21.12.2021 15:27, Greg Kroah-Hartman wrote:
    > On Tue, Dec 21, 2021 at 02:52:05PM +0100, Rafał Miłecki wrote:
    >>> How are nvmem devices named?
    >>
    >> $ ls /sys/bus/nvmem/devices/
    >> brcm-nvram0
    >> mtd0
    >> mtd1
    >> u-boot-envvar0
    >
    > So no naming scheme at all.
    >
    > {sigh}
    >
    >>>> Example:
    >>>> $ cat /sys/bus/nvmem/devices/foo/cells/bootcmd
    >>>> tftp
    >>>> $ cat /sys/bus/nvmem/devices/foo/cells/bootdelay
    >>>> 5
    >>>>
    >>>> As you can see above NVMEM cells are not known at compilation time.
    >>>
    >>> Why do you want to expose these in a way that forces the kernel to parse
    >>> these key/value pairs? Why not just do it all in userspace like you can
    >>> today? What forces the kernel to do it and not a perl script?
    >>>
    >>>> So I believe the question is: how can I expose cells in sysfs?
    >>>
    >>> You can do this by dynamically creating the attributes on the fly, but
    >>> your show function is going to be rough and it's not going to be simple
    >>> to do so. One example will be the code that creates the
    >>> /sys/devices/system/machinecheck/machinecheckXX/bank* files.
    >>>
    >>> But I will push back again, why not just do it all in userspace? What
    >>> userspace tool is requiring the kernel to do this work for it?
    >>
    >> Environment data contains info that may be required by kernel.
    >>
    >> For example some home routers store two firmwares on flash. Kernel needs
    >> to read index of currently booted firmware to make sure MTD subsystem
    >> creates partitions correctly.
    >
    > You are talking about a kernel<->kernel api here, that's not what sysfs
    > is for at all.
    >
    >> Another example: MAC address. Ethernet subsystem supports reading MAC
    >> from NVMEM cell.
    >
    > Again, internal kernel api, nothing sysfs is ever involved in.
    >
    >> One could argue those tasks could be handled from userspace but that
    >> would get tricky. Sure - we have API for setting MAC address. However
    >> other cases (like setting active firmware partition and asking MTD to
    >> parse it into subpartitions) would require new user <-> kernel
    >> interfaces.
    >
    > Ok, but again, sysfs is for userspace to get access to these values.
    > That's what I'm concerned about. If you want to make an in-kernel api
    > for other subsystems to get these key/value pairs, wonderful, that has
    > nothing to do with sysfs.
    >
    > So I ask again, why do you want to expose these to userspace through
    > sysfs in a new format from what you have today. Who is going to use
    > that information and what is it going to be used for.

    Sorry, you asked why I need parsing in kernel and I focused on that part
    only.

    For user space there are also some relevant U-Boot environment entries
    (NVMEM cells). Examples: serial number, country code (e.g. WiFi
    purposes), default passwords (as printed on labels).

    So both: kernel and user space need to access U-Boot environment
    variables (NVMEM cells). Each for its own purposes.

    Kernel goes first so it needs its own parser of NVMEM content (data).

    User space can either: get NVMEM cells exposed by kernel OR parse NVMEM
    content on its own. I thought it'd be nice to avoid parsing code
    duplication in user space and let kernel expose NVMEM cells.

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