Hi,
Am 2022-08-28 17:05, schrieb Rafał Miłecki:
On 25.08.2022 23:44, Michael Walle wrote:
This is now the third attempt to fetch the MAC addresses from the VPD
for the Kontron sl28 boards. Previous discussions can be found here:
https://lore.kernel.org/lkml/20211228142549.1275412-1-michael@xxxxxxxx/
NVMEM cells are typically added by board code or by the devicetree.
But
as the cells get more complex, there is (valid) push back from the
devicetree maintainers to not put that handling in the devicetree.
I dropped the ball waiting for Rob's reponse in the
[PATCH 0/2] dt-bindings: nvmem: support describing cells
https://lore.kernel.org/linux-arm-kernel/0b7b8f7ea6569f79524aea1a3d783665@xxxxxxxx/T/
Before we go any further can we have a clear answer from Rob (or
Krzysztof now too?):
Is there any point in having bindings like:
compatible = "mac-address";
for NVMEM cells nodes? So systems (Linux, U-Boot) can handle them in a
more generic way?
Or do we prefer more conditional drivers code (or layouts code as in
this Michael's proposal) that will handle cells properly based on their
names?
What do you mean by "based on their names?".
I'm not arguing for any solution. I just want to make sure we choose
the
right way to proceed.
With the 'compatible = "mac-address"', how would you detect what kind
of transformation you need to apply? You could guess ascii, yes. But
swapping bytes? You cannot guess that. So you'd need additional
information
coming from the device tree. But Rob was quite clear that this shouldn't
be in the device tree:
| I still don't think trying to encode transformations of data into the
DT
| is right approach.
https://lore.kernel.org/linux-devicetree/YaZ5JNCFeKcdIfu8@xxxxxxxxxxxxxxxxxx/
He also mention that the compatible should be on the nvmem device level
and should use specific compatible strings:
https://lore.kernel.org/linux-devicetree/CAL_JsqL55mZJ6jUyQACer2pKMNDV08-FgwBREsJVgitnuF18Cg@xxxxxxxxxxxxxx/
And IMHO that makes sense, because it matches the hardware and not any
NVMEM cells which is the *content* of a memory device.
And as you see in the sl28vpd layout, it allows you to do much more,
like
checking for integrity, and make it future proof by supporting different
versions of this sl28vpd layout.
What if you use it with the u-boot,env? You wouldn't need it because
u-boot,env will already know how to interpret it as an ascii string
(and it also know the offset). In this sense, u-boot,env is already a
compatible string describing the content of a NVMEM device (and the
compatible string is at the device level).
Therefore, introduce NVMEM layouts. They operate on the NVMEM device
and
can add cells during runtime. That way it is possible to add complex
cells than it is possible right now with the offset/length/bits
description in the device tree. For example, you can have post
processing
for individual cells (think of endian swapping, or ethernet offset
handling). You can also have cells which have no static offset, like
the
ones in an u-boot environment. The last patches will convert the
current
u-boot environment driver to a NVMEM layout and lifting the
restriction
that it only works with mtd devices. But as it will change the
required
compatible strings, it is marked as RFC for now. It also needs to have
its device tree schema update which is left out here.
So do I get it right that we want to have:
1. NVMEM drivers for providing I/O access to NVMEM devices
2. NVMEM layouts for parsing & NVMEM cells and translating their
content
?
Correct.
I guess it sounds good and seems to be a clean solution.
Good to hear :)
One thing I believe you need to handle is replacing "cell_post_process"
callback with your layout thing.
I find it confusing to have
1. cell_post_process() CB at NVMEM device level
2. post_process() CB at NVMEM cell level
What is wrong with having a callback at both levels?
Granted, in this particular case (it is just used at one place), I still
think that it is the wrong approach to add this transformation in the
driver (in this particular case). The driver is supposed to give you
access to the SoC's fuse box, but it will magically change the content
of a cell if the nvmem consumer named this cell "mac-address" (which
you also found confusing the last time and I do too!).
The driver itself doesn't add any cells on its own, so I cannot register
a .post_process hook there. Therefore, you'd need that post_process hook
on every cell, which is equivalent to have a post_process hook at
device level.
Unless you have a better idea. I'll leave that up to NXP to fix that (or
leave it like that).
-michael
