linux-kernel - Re: [PATCH v6 1/3] ABI: sysfs-nvmem-cells: Expose cells through sysfs

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Date:   Mon, 31 Jul 2023 17:51:52 +0200
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     "John Thomson" <lists@...nthomson.fastmail.com.au>
Cc:     "Srinivas Kandagatla" <srinivas.kandagatla@...aro.org>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        "Thomas Petazzoni" <thomas.petazzoni@...tlin.com>,
        "Robert Marko" <robert.marko@...tura.hr>,
        "Luka Perkov" <luka.perkov@...tura.hr>,
        "Michael Walle" <michael@...le.cc>,
        "Randy Dunlap" <rdunlap@...radead.org>
Subject: Re: [PATCH v6 1/3] ABI: sysfs-nvmem-cells: Expose cells through
 sysfs

Hi John,

Srinivas, a question for you below.

lists@...nthomson.fastmail.com.au wrote on Sun, 23 Jul 2023 19:39:50
+0000:

> Hi Miquel,
> 
> On Mon, 17 Jul 2023, at 07:51, Miquel Raynal wrote:
> > The binary content of nvmem devices is available to the user so in the
> > easiest cases, finding the content of a cell is rather easy as it is
> > just a matter of looking at a known and fixed offset. However, nvmem
> > layouts have been recently introduced to cope with more advanced
> > situations, where the offset and size of the cells is not known in
> > advance or is dynamic. When using layouts, more advanced parsers are
> > used by the kernel in order to give direct access to the content of each
> > cell regardless of their position/size in the underlying device, but
> > these information were not accessible to the user.
> >
> > By exposing the nvmem cells to the user through a dedicated cell/ folder
> > containing one file per cell, we provide a straightforward access to
> > useful user information without the need for re-writing a userland
> > parser. Content of nvmem cells is usually: product names, manufacturing
> > date, MAC addresses, etc,
> >
> > Signed-off-by: Miquel Raynal <miquel.raynal@...tlin.com>
> > Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > ---
> >  Documentation/ABI/testing/sysfs-nvmem-cells | 19 +++++++++++++++++++
> >  1 file changed, 19 insertions(+)
> >  create mode 100644 Documentation/ABI/testing/sysfs-nvmem-cells
> >
> > diff --git a/Documentation/ABI/testing/sysfs-nvmem-cells 
> > b/Documentation/ABI/testing/sysfs-nvmem-cells
> > new file mode 100644
> > index 000000000000..b2d15a8d36e5
> > --- /dev/null
> > +++ b/Documentation/ABI/testing/sysfs-nvmem-cells
> > @@ -0,0 +1,19 @@
> > +What:		/sys/bus/nvmem/devices/.../cells/<cell-name>
> > +Date:		May 2023
> > +KernelVersion:	6.5
> > +Contact:	Miquel Raynal <miquel.raynal@...tlin.com>
> > +Description:
> > +		The "cells" folder contains one file per cell exposed by
> > +		the nvmem device. The name of the file is the cell name.  
> 
> Could we consider using a file within a folder (name defined by cell propertys) to access the cell bytes?
> Example (pick the best path and filename):
> /sys/bus/nvmem/devices/.../cells/<cell-name>/bytes
> 
> That way, it is much easier to expand this at a later stage,
> like adding an of_node link at
> /sys/bus/nvmem/devices/.../cells/<cell-name>/of_node
> or exposing other nvmem cell properties.

I have no strong opinion. Srinivas what do you prefer? I'm fine either
ways. I like the simplicity of the current approach more, but it's true
that it is more easy to make it grow if we follow John idea.

> This is particularly relevant given the cell-name alone does not always
> uniquely represent a cell on an nvmem device.
> https://lore.kernel.org/lkml/ZLaZ7fzUSsa0Igx1@makrotopia.org/

It seems like this is gonna be fixed by suffixing @<offset> to the
name, as anyway whatever solution we choose, it is gonna be needed.

> https://lore.kernel.org/lkml/e7173ab2-d3b2-4f75-beb8-32593b868774@www.fastmail.com/
> 
> > +		The length of the file is the size of the cell (when
> > +		known). The content of the file is the binary content of
> > +		the cell (may sometimes be ASCII, likely without
> > +		trailing character).
> > +		Note: This file is only present if CONFIG_NVMEM_SYSFS
> > +		is enabled.
> > +
> > +		Example::
> > +
> > +		  hexdump -C /sys/bus/nvmem/devices/1-00563/cells/product-name
> > +		  00000000  54 4e 34 38 4d 2d 50 2d  44 4e         |TN48M-P-DN|
> > +		  0000000a
> > -- 
> > 2.34.1  
> 
> Cheers,
> 


Thanks,
Miquèl