[PATCH] nvmem: add explicit config option to read OF fixed cells

[Rafał Miłecki]

On 21.02.2023 22:38, Martin Blumenstingl wrote:
> On Tue, Feb 21, 2023 at 3:50 PM Rafał Miłecki <zajec5 at gmail.com> wrote:
>>
>> From: Rafał Miłecki <rafal at milecki.pl>
>>
>> NVMEM subsystem looks for fixed NVMEM cells (specified in DT) by
>> default. This behaviour made sense in early days before adding support
>> for dynamic cells.
>>
>> With every new supported NVMEM device with dynamic cells current
>> behaviour becomes non-optimal. It results in unneeded iterating over DT
>> nodes and may result in false discovery of cells (depending on used DT
>> properties).
> I am not familiar with the recent changes around dynamic cells.
> Is there any discussion/summary that I can read to get up to speed?

Some NVMEM devices don't store specific data at hardcoded offsets. For
such devices we have drivers (to become: layouts) that parse their magic
content. They discover cells and register them and provide matching with
proper DT nodes.

For bindings see:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=084973e944bec21804f8afb0515b25434438699a
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a607a850ba1fa966cbb035544c1588e24a6307df

For example driver see:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6e977eaa8280e957b87904b536661550f2a6b3e8
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=207775f7e17b8fd0426a2ac4a5b81e4e1d71849e

For usage see:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c8442f0fb09ca3d842b9b23d1d0650f649fd10f8


> My main thought is: if there are many "fixed OF cells" implementations
> and only a few "dynamic" ones - does it make sense to flip the logic
> and introduce a new "use_dynamic_of_cells" flag instead?

The problem is that there are more cases than just two. We can have:
1. No cells at all
2. Fixed cells in DT
3. Dynamic cells with references in DT
4. Driver specified cells (specified within config)
5. Cells defined in a global table

So we need to reference DT cells explicitly (we can't just confirm /
deny *dynamic* cells).

Another solution would be to have "no_fixed_of_cells" but:

1. Personally I think negation is less clear / easy to follow

2. There may be actually more drivers with no fixed cells.
I think I modified 18 drivers. It seems devm_nvmem_register() is
referenced in 44 places. Few of them may be not actual users but it
still seems to be about equal.