Re: [PATCH 2/2] nvmem: expose NVMEM cells in sysfs

["Rafał Miłecki" <zajec5@gmail.com>]

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.