linux-kernel - Re: [PATCH v10 3/3] nvmem: core: Expose cells through sysfs

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Message-ID: <5f1221613fb71b87c01c82add9fe5097@milecki.pl>
Date:   Fri, 29 Sep 2023 07:18:32 +0200
From:   Rafał Miłecki <rafal@...ecki.pl>
To:     Miquel Raynal <miquel.raynal@...tlin.com>
Cc:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Michael Walle <michael@...le.cc>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Robert Marko <robert.marko@...tura.hr>,
        Luka Perkov <luka.perkov@...tura.hr>,
        linux-kernel@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>,
        Chen-Yu Tsai <wenst@...omium.org>,
        Daniel Golle <daniel@...rotopia.org>
Subject: Re: [PATCH v10 3/3] nvmem: core: Expose cells through sysfs

On 2023-09-28 17:31, Rafał Miłecki wrote:
> On 2023-09-22 19:48, 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 its position/size in the underlying
>> device. Unfortunately, these information are not accessible by users,
>> unless by fully re-implementing the parser logic in userland.
>> 
>> Let's expose the cells and their content through sysfs to avoid these
>> situations. Of course the relevant NVMEM sysfs Kconfig option must be
>> enabled for this support to be available.
>> 
>> Not all nvmem devices expose cells. Indeed, the .bin_attrs attribute
>> group member will be filled at runtime only when relevant and will
>> remain empty otherwise. In this case, as the cells attribute group 
>> will
>> be empty, it will not lead to any additional folder/file creation.
>> 
>> Exposed cells are read-only. There is, in practice, everything in the
>> core to support a write path, but as I don't see any need for that, I
>> prefer to keep the interface simple (and probably safer). The 
>> interface
>> is documented as being in the "testing" state which means we can later
>> add a write attribute if though relevant.
>> 
>> Signed-off-by: Miquel Raynal <miquel.raynal@...tlin.com>
> 
> Tested-by: Rafał Miłecki <rafal@...ecki.pl>
> 
> # hexdump -C /sys/bus/nvmem/devices/u-boot-env0/cells/ipaddr@15c
> 00000000  31 39 32 2e 31 36 38 2e  31 2e 31                 
> |192.168.1.1|
> 0000000b

The same test after converting U-Boot env into layout driver:

# hexdump -C /sys/bus/nvmem/devices/mtd1/cells/ipaddr@15c
00000000  31 39 32 2e 31 36 38 2e  31 2e 31                 
|192.168.1.1|
0000000b

Looks good!

-- 
Rafał Miłecki