Re: [RFC PATCH] dt-bindings: nvmem: add transformation support

[Rob Herring]

On Tue, 23 Nov 2021 14:44:25 +0100, Michael Walle wrote:
> This is my second attempt to solve the use case where there is only the
> base MAC address stored in an EEPROM or similar storage provider. This
> is the case for the Kontron sl28 board and multiple openwrt supported
> boards.
> 
> The first proposal [1] didn't find much appreciation and there wasn't
> any reply to my question or new proposal [2]. So here we are with my new
> proposal, that is more flexible and doesn't fix the ethernet mac only.
> This is just an RFC for the device tree representation for now to see if
> this is the correct way to tackle this.
> 
> I'm also aware of the latest post process hook support [3]. This doesn't
> fix the base mac address issue, but I think it also doesn't solve the
> case with swapped ethernet addresses in the general case. That hook will
> involve the driver to do the swapping, but how would the driver know
> if that swapping is actually required. Usually the interpretation of the
> content is opaque to the driver, after all it is the user/board
> manufacturer who does program the storage device. We might be lucky in
> the imx-ocotp case because the IMX reference manual actually states
> where and in which format the mac address is programmed.
> 
> Introduce a transformation property. This is intended to be just an
> enumeration of operations. If there will be a new operation, support for
> it has to be added to the nvmem core.
> 
> A transformation might have multiple output values, like in the base mac
> address case. It reads the mac address from the nvmem storage and
> generates multiple individual addresses, i.e. on our board we reserve 8
> consecutive addresses. These addresses then can be assigned to different
> network interfaces. To make it possible to reference different values we
> need to introduce an argument to the phandle. This additional argument
> is then an index into a list of values.
> 
> Example:
>   mac_addresses: base-mac-address@10 {
>     #nvmem-cell-cells = <1>;
>     reg = <10 6>;
>     transformation = <NVMEM_T_ETH_OFFSET 0 1 7>;
>   }
> 
>   &eth0 {
>     nvmem-cells = <&mac_addresses 0>;
>     nvmem-cell-names = "mac-address";
>   };
> 
>   &eth1 {
>     nvmem-cells = <&mac_addresses 2>;
>     nvmem-cell-names = "mac-address";
>   };
> 
> The NVMEM_T_ETH_OFFSET transformation takes N additional (dt) cells and
> will generate N values. In this example BASE_MAC+0, BASE_MAC+1, BASE_MAC+7.
> An nvmem consumer can then reference the nvmem cell with an index. So eth0
> will get BASE_MAC+0 and eth1 will get BASE_MAC+7.
> 
> This should be sufficient flexible for many different transformations
> without having to touch the bindings except for adding documentation and
> checks for new transformations.
> 
> I do have one question regarding "#nvmem-cell-cells" (aside from the
> awkward naming): is it allowed to have that property optional if there
> is no additional argument to the phandle?
> 
> [1] https://lore.kernel.org/all/20210414152657.12097-2-michael@xxxxxxxx/
> [2] https://lore.kernel.org/linux-devicetree/362f1c6a8b0ec191b285ac6a604500da@xxxxxxxx/
> [3] https://lore.kernel.org/lkml/20211013131957.30271-1-srinivas.kandagatla@xxxxxxxxxx/
> 
> Signed-off-by: Michael Walle <michael@xxxxxxxx>
> ---
>  .../devicetree/bindings/nvmem/nvmem.yaml      | 29 +++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 

My bot found errors running 'make DT_CHECKER_FLAGS=-m dt_binding_check'
on your patch (DT_CHECKER_FLAGS is new in v5.13):

yamllint warnings/errors:

dtschema/dtc warnings/errors:
Error: Documentation/devicetree/bindings/nvmem/nvmem.example.dts:53.35-36 syntax error
FATAL ERROR: Unable to parse input tree
make[1]: *** [scripts/Makefile.lib:373: Documentation/devicetree/bindings/nvmem/nvmem.example.dt.yaml] Error 1
make[1]: *** Waiting for unfinished jobs....
make: *** [Makefile:1413: dt_binding_check] Error 2

doc reference errors (make refcheckdocs):

See https://patchwork.ozlabs.org/patch/1558545

This check can fail if there are any dependencies. The base for a patch
series is generally the most recent rc1.

If you already ran 'make dt_binding_check' and didn't see the above
error(s), then make sure 'yamllint' is installed and dt-schema is up to
date:

pip3 install dtschema --upgrade

Please check and re-submit.