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

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Date:   Tue, 1 Aug 2023 10:06:14 +0100
From:   Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
To:     Miquel Raynal <miquel.raynal@...tlin.com>,
        John Thomson <lists@...nthomson.fastmail.com.au>
Cc:     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



On 31/07/2023 16:51, Miquel Raynal wrote:
> 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.

Sounds sensible to me.


> 
>> 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.

we have to be careful here not to break the nvmem_cell_get() users.


--srini


> 
>> 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