Messages in this thread | | | Date | Tue, 21 Dec 2021 16:09:13 +0100 | Subject | Re: [PATCH 2/2] nvmem: expose NVMEM cells in sysfs | From | Rafał Miłecki <> |
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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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