linux-kernel - Re: [PATCH 1/6] dt-bindings: nvmem: add cell-type to nvmem cells

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Message-ID: <dbd1c20c-e3be-6c92-52a8-2ad76d0092d0@pengutronix.de>
Date:   Wed, 22 Sep 2021 14:31:49 +0200
From:   Ahmad Fatoum <a.fatoum@...gutronix.de>
To:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Joakim Zhang <qiangqing.zhang@....com>, robh+dt@...nel.org,
        shawnguo@...nel.org,
        Jan Lübbe <jlu@...gutronix.de>
Cc:     devicetree@...r.kernel.org, linux-imx@....com,
        kernel@...gutronix.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] dt-bindings: nvmem: add cell-type to nvmem cells

Hello Srini,

On 22.09.21 14:23, Srinivas Kandagatla wrote:
> 
> 
> On 22/09/2021 12:34, Ahmad Fatoum wrote:
>> Hi,
>>
>> On 08.09.21 12:02, Joakim Zhang wrote:
>>> From: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
>>>
>>> Some of the nvmem providers encode data for certain type of nvmem cell,
>>> example mac-address is stored in ascii or with delimiter or in reverse order.
>>>
>>> This is much specific to vendor, so having a cell-type would allow nvmem
>>> provider drivers to post-process this before using it.
>>
>> I don't agree with this assessment. Users of the OCOTP so far
>> used this specific encoding. Bootloaders decode the OCOTP this way, but this
>> encoding isn't really an inherent attribute of the OCOTP. A new NXP SoC
>> with a different OTP IP will likely use the same format. Users may even
>> use the same format on an EEPROM to populate a second off-SoC interface, .. etc.
>>
> 
> That is okay.

How would you go about using this same format on an EEPROM?

>> I'd thus prefer to not make this specific to the OCOTP as all:
>>
>>    * #define NVMEM_CELL_ENCODING_MAC_ADDRESS_IMX    /* ... */
>>
>>    * cell-type = <NVMEM_CELL_ENCODING_MAC_ADDRESS_IMX>;
>>
> 
> No, this is not okay, cell-type is just representing what is the cell type in a generic way, and its not intended to have any encoding information.
> 
> Encoding information should be derived from the provider specific bindings. If we start adding this info in generic binding we will endup with a mess.
> And this has been discussed in various instances.

A linux-nvmem list would be nice to collect such discussions.

>>    * and then the decoder is placed into some generic location, e.g.
>>     drivers/nvmem/encodings.c for Linux
> 
> This is fine, we could have a library to do these post-processing, but only if we have enough users. For now its better to keep it within provider drivers till we have more consumers of these functions.
> 
> 
> --srini
>>
>> That way, we can reuse this and future encodings across nvmem providers.
>> It's also more extendable: e.g. big endian fields on EEPROMs. Just stick
>> the cell-type in, document it in the binding and drivers supporting it
>> will interpret bytes appropriately.
>>
>> It's still a good idea to record the type as well as the encoding,
>> e.g. split the 32 bit encoding constant into two 16-bit values.
>> One is an enum of possible types (unknown, mac_address, IP address ... etc.)
>> and one is an enum of the available encodings.
>>
>> What do you think?
>>
>>>
>>> Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
>>> Signed-off-by: Joakim Zhang <qiangqing.zhang@....com>
>>> ---
>>>   Documentation/devicetree/bindings/nvmem/nvmem.yaml | 11 +++++++++++
>>>   include/dt-bindings/nvmem/nvmem.h                  |  8 ++++++++
>>>   2 files changed, 19 insertions(+)
>>>   create mode 100644 include/dt-bindings/nvmem/nvmem.h
>>>
>>> diff --git a/Documentation/devicetree/bindings/nvmem/nvmem.yaml b/Documentation/devicetree/bindings/nvmem/nvmem.yaml
>>> index b8dc3d2b6e92..8cf6c7e72b0a 100644
>>> --- a/Documentation/devicetree/bindings/nvmem/nvmem.yaml
>>> +++ b/Documentation/devicetree/bindings/nvmem/nvmem.yaml
>>> @@ -60,6 +60,11 @@ patternProperties:
>>>               - minimum: 1
>>>                 description:
>>>                   Size in bit within the address range specified by reg.
>>> +      cell-type:
>>> +        $ref: /schemas/types.yaml#/definitions/uint32
>>> +        maxItems: 1
>>> +        description:
>>> +          Type of nvmem, Use defines in dt-bindings/nvmem/nvmem.h.
>>>         required:
>>>         - reg
>>> @@ -69,6 +74,7 @@ additionalProperties: true
>>>   examples:
>>>     - |
>>>         #include <dt-bindings/gpio/gpio.h>
>>> +      #include <dt-bindings/nvmem/nvmem.h>
>>>           qfprom: eeprom@...000 {
>>>             #address-cells = <1>;
>>> @@ -98,6 +104,11 @@ examples:
>>>                 reg = <0xc 0x1>;
>>>                 bits = <2 3>;
>>>             };
>>> +
>>> +          mac_addr: mac-addr@90{
>>> +              reg = <0x90 0x6>;
>>> +              cell-type = <NVMEM_CELL_TYPE_MAC_ADDRESS>;
>>> +          };
>>>         };
>>>     ...
>>> diff --git a/include/dt-bindings/nvmem/nvmem.h b/include/dt-bindings/nvmem/nvmem.h
>>> new file mode 100644
>>> index 000000000000..eed0478f6bfd
>>> --- /dev/null
>>> +++ b/include/dt-bindings/nvmem/nvmem.h
>>> @@ -0,0 +1,8 @@
>>> +/* SPDX-License-Identifier: GPL-2.0 */
>>> +#ifndef __DT_NVMMEM_H
>>> +#define __DT_NVMMEM_H
>>> +
>>> +#define NVMEM_CELL_TYPE_UNKNOWN        0
>>> +#define NVMEM_CELL_TYPE_MAC_ADDRESS    1
>>> +
>>> +#endif /* __DT_NVMMEM_H */
>>>
>>
>>
> 
> 


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