linux-kernel - Re: nvmem: add driver for Microchip 24AA025E48 I2C eeprom / nodeID chip

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Message-ID: <ffa98f3a-daa3-d2b3-8401-767517db7490@ltec.ch>
Date:   Thu, 25 Jan 2018 15:37:31 +0100
From:   Felix Brack <fb@...c.ch>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: nvmem: add driver for Microchip 24AA025E48 I2C eeprom / nodeID
 chip

On 24.01.2018 17:02, Bartosz Golaszewski wrote:
> 2018-01-24 16:18 GMT+01:00 Felix Brack <fb@...c.ch>:
>>
>> On 24.01.2018 16:08, Andy Shevchenko wrote:
>>> On Wed, Jan 24, 2018 at 4:23 PM, Felix Brack <fb@...c.ch> wrote:
>>>> Hello,
>>>>
>>>> About three years ago I wrote a driver for Microchip's 24AA025E48 I2C
>>>> eeprom/nodeID chip. At that time I placed the source files in
>>>> drivers/misc/eeprom. I never posted the code.
>>>> I now plan to rewrite the driver from scratch. Is it correct to place
>>>> the source code in drivers/nvmem and is there a special mailing list to
>>>> which the patch should be posted when ready?
>>>>
>>>> many thanks and kind regards, Felix
>>>
>>> Does the existing driver [1] work for you (if you add ID there)?
>>>
>>> [1]: at24
>>>
>> Yes it does. Actually the driver I wrote 3 years ago is based on the
>> at24 driver. Lot's of code in my driver originates directly from the
>> at24 driver.
>>
>> --
>> regards Felix
> 
> Just from looking at the doc - it seems that it's a variant of
> 24mac402. You should be able to access the memory block by
> instantiating an 'atmel,24c02' device. As for the EUI-48 block -
> current at24 driver will not work as the MAC is located at a different
> offset in this chip. We need to figure out a portable way to specify
> the addresses of such special blocks (same with the serial number) in
> the at24 driver.
> 
> Anyways - don't write your own driver for that, just make sure at24 works.
> 
If no one offends against blowing up this driver by some more code, then
that's fine with me. What about a patch that does the following:

1. Extend the DT bindings by a new optional property block_offset
   denoting where to start reading/writing from/to this device.
2. Add a member block_offset to struct at24_platform_data{} to store
   the device read/write offset returned from the DT (similar to the
   already existing page_size member).
3. Complement at24_get_pdata() to read block_offset from the DT and
   store it.
4. Make at24_eeprom_write_i2c() and at24_eeprom_read_serial() (and
   eventually more?) respect the block_offset value.

Initializing the new block_offset member of struct at24_platform_data{}
to 0 should leave the code semantically unchanged.

Once this patch is in place I could use the 24mac402 device type with
block_offset set to 0xfa (and read-only set to true) to read the EUI-48
node address from Microchip's 24AA025E48. Probably the later addition of
a device named 24eui48 would also make sense (if not redundant ?).

Any comments are very appreciated, Felix