Re: [PATCH REBASED 2/2] dt-bindings: nvmem: cells: add MAC address cell

["Rafał Miłecki" <zajec5@gmail.com>]

On 1.02.2022 17:49, Rafał Miłecki wrote:
> On 2022-02-01 16:55, Rob Herring wrote:
>> On Wed, Jan 26, 2022 at 08:07:45AM +0100, Rafał Miłecki wrote:
>>> From: Rafał Miłecki <rafal@milecki.pl>
>>>
>>> This adds support for describing details of NVMEM cell containing MAC
>>> address. Those are often device specific and could be nicely stored in
>>> DT.
>>>
>>> Initial documentation includes support for describing:
>>> 1. Cell data format (e.g. Broadcom's NVRAM uses ASCII to store MAC)
>>> 2. Reversed bytes flash (required for i.MX6/i.MX7 OCOTP support)
>>> 3. Source for multiple addresses (very common in home routers)
>>>
>>> Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
>>> ---
>>>  .../bindings/nvmem/cells/mac-address.yaml     | 94 +++++++++++++++++++
>>>  1 file changed, 94 insertions(+)
>>>  create mode 100644 Documentation/devicetree/bindings/nvmem/cells/mac-address.yaml
>>>
>>> diff --git a/Documentation/devicetree/bindings/nvmem/cells/mac-address.yaml b/Documentation/devicetree/bindings/nvmem/cells/mac-address.yaml
>>> new file mode 100644
>>> index 000000000000..f8d19e87cdf0
>>> --- /dev/null
>>> +++ b/Documentation/devicetree/bindings/nvmem/cells/mac-address.yaml
>>> @@ -0,0 +1,94 @@
>>> +# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
>>> +%YAML 1.2
>>> +---
>>> +$id: http://devicetree.org/schemas/nvmem/cells/mac-address.yaml#
>>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
>>> +
>>> +title: NVMEM cell containing a MAC address
>>> +
>>> +maintainers:
>>> +  - Rafał Miłecki <rafal@milecki.pl>
>>> +
>>> +properties:
>>> +  compatible:
>>> +    const: mac-address
>>> +
>>> +  format:
>>> +    description: |
>>> +      Some NVMEM cells contain MAC in a non-binary format.
>>> +
>>> +      ASCII should be specified if MAC is string formatted like:
>>> +      - "01:23:45:67:89:AB" (30 31 3a 32 33 3a 34 35 3a 36 37 3a 38 39 3a 41 42)
>>> +      - "01-23-45-67-89-AB"
>>> +      - "0123456789AB"
>>> +    enum:
>>> +      - ascii
>>> +
>>> +  reversed-bytes:
>>> +    type: boolean
>>> +    description: |
>>> +      MAC is stored in reversed bytes order. Example:
>>> +      Stored value: AB 89 67 45 23 01
>>> +      Actual MAC: 01 23 45 67 89 AB
>>> +
>>> +  base-address:
>>> +    type: boolean
>>> +    description: |
>>> +      Marks NVMEM cell as provider of multiple addresses that are relative to
>>> +      the one actually stored physically. Respective addresses can be requested
>>> +      by specifying cell index of NVMEM cell.
>>
>> While a base address is common, aren't there different ways the base is
>> modified.
>>
>> The problem with these properties is every new variation results in a
>> new property and the end result is something not well designed. A unique
>> compatible string, "#nvmem-cell-cells" and code to interpret the data is
>> more flexible.
>>
>> For something like this to fly, I need some level of confidence this is
>> enough for everyone for some time (IOW, find all the previous attempts
>> and get those people's buy-in). You have found at least 3 cases, but I
>> seem to recall more.
> 
> For base address I thought of dealing with base + offset only. I'm not
> sure what are other cases.
> 
> I read few old threads:
> https://lore.kernel.org/lkml/20211228142549.1275412-1-michael@walle.cc/T/
> https://lore.kernel.org/linux-devicetree/20211123134425.3875656-1-michael@walle.cc/
> https://lore.kernel.org/all/20210414152657.12097-2-michael@walle.cc/
> https://lore.kernel.org/linux-devicetree/362f1c6a8b0ec191b285ac6a604500da@walle.cc/
> 
> but didn't find other required /transformations/ except for offset and
> format. Even "reversed-bytes" wasn't widely discussed (or I missed that)
> and I just came with it on my own.
> 
> If anyone knows other cases: please share so we have a complete view.
> 
> 
> I tried to Cc all previously invovled people but it seems only me and
> Michael remained active in this subject. If anyone knows other
> interested please Cc them and let us know.
> 
> 
> Rob: instead of me and Michael sending patch after patch let me try to
> gather solutions I can think of / I recall. Please kindly review them
> and let us know what do you find the cleanest.
> 
> 
> 1. NVMEM specific "compatible" string
> 
> Example:
> 
> partition@f00000 {
>      compatible = "brcm,foo-cells", "nvmem-cells";
>      label = "calibration";
>      reg = <0xf00000 0x100000>;
>      ranges = <0 0xf00000 0x100000>;
>      #address-cells = <1>;
>      #size-cells = <1>;
> 
>      mac@100 {
>          reg = <0x100 0x6>;
>          [optional: #nvmem-cell-cells = <1>;]
>      };
> };
> 
> A minimalistic binding proposed by Michael. DT doesn't carry any
> information on NVMEM cell format. Specific drivers (e.g. one handling
> "brcm,foo-cells") have to know how to handle specific cell.
> 
> Cell handling conditional code can depend on cell node name ("mac" in
> above case) OR on value of "nvmem-cell-names" in cell consumer (e.g.
> nvmem-cell-names = "mac-address").
> 
> 
> 2. NVMEM specific "compatible" string + cells "compatible"s
> 
> Example:
> 
> partition@f00000 {
>      compatible = "brcm,foo-cells", "nvmem-cells";
>      label = "calibration";
>      reg = <0xf00000 0x100000>;
>      ranges = <0 0xf00000 0x100000>;
>      #address-cells = <1>;
>      #size-cells = <1>;
> 
>      mac@100 {
>          compatible = "mac-address";
>          reg = <0x100 0x6>;
>          [optional: #nvmem-cell-cells = <1>;]
>      };
> };
> 
> Similar to the first case but cells that require special handling are
> marked with NVMEM device specific "compatible" values. Details of handling
> cells are still hardcoded in NVMEM driver. Different cells with
> compatible = "mac-address";
> may be handled differencly - depending on parent NVMEM device.
> 
> 
> 3. Flexible properties in NVMEM cells
> 
> Example:
> 
> partition@f00000 {
>      compatible = "brcm,foo-cells", "nvmem-cells";
>      label = "calibration";
>      reg = <0xf00000 0x100000>;
>      ranges = <0 0xf00000 0x100000>;
>      #address-cells = <1>;
>      #size-cells = <1>;
> 
>      mac@100 {
>          compatible = "mac-address";
>          reg = <0x100 0x6>;
>          [optional: #nvmem-cell-cells = <1>;]
>      };
> 
>      mac@200 {
>          compatible = "mac-address";
>          reg = <0x200 0x6>;
>          reversed-bytes;
>          [optional: #nvmem-cell-cells = <1>;]
>      };
> 
>      mac@300 {
>          compatible = "mac-address";
>          reg = <0x300 0x11>;
>          format = "ascii";
>          [optional: #nvmem-cell-cells = <1>;]
>      };
> };
> 
> This moves details into DT and requires more shared properties. It helps
> avoiding duplicated code for common cases (like base MAC address).
> 
> It's what I proposed in the
> [PATCH 0/2] dt-bindings: nvmem: support describing cells

Rob: could you review for us 3 above examples, please?