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

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Date:   Tue, 21 Dec 2021 15:27:23 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Rafał Miłecki <zajec5@...il.com>
Cc:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Dan Williams <dan.j.williams@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Rafał Miłecki <rafal@...ecki.pl>
Subject: Re: [PATCH 2/2] nvmem: expose NVMEM cells in sysfs

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.

thanks,

greg k-h