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

[Michael Walle <michael@walle.cc>]

Hi Rob,

Am 2021-11-30 20:19, schrieb Rob Herring:
> On Tue, Nov 23, 2021 at 02:44:25PM +0100, Michael Walle wrote:
..

>> 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.
> 
> I still don't think trying to encode transformations of data into the 
> DT
> is right approach.
> 
>> 
>> 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.
> 
> The content and number of cells is supposed to be opaque to the client
> and interpreted by the provider. That's sort of true here, but not
> really because the interpretation is tied to 'transformation'. So I'm
> okay with adding cells, but not fixing the interpretation of them. A
> compatible should determine how the cells are interpreted.

What do you mean by "adding cells"? The additional argument to the
phandle?

So an example would be:

ethernet_base_mac: base-mac-address@100 {
     #nvmem-cell-cells = <1>;
     compatible = "nvmem-ethernet-address";
     reg = <0x100 0x6>;
};

&eth0 {
     nvmem-cells = <&ethernet_base_mac 0>;
     nvmem-cell-names = "mac-address";
};

&eth1 {
     nvmem-cells = <&ethernet_base_mac 7>;
     nvmem-cell-names = "mac-address";
};

Right? Any suggestions for a better compatible name?

>> 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?
> 
> We don't have any choice if we add "#nvmem-cell-cells". There's already
> cases without it.

Yes, that was the reason for the question. But I wasn't sure, whether
that is allowed.

-michael