Re: [RFC PATCH 1/3] eeprom: Add a simple EEPROM framework

[Srinivas Kandagatla <srinivas.kandagatla@linaro.org>]

Thanks Stephen for the comments.

On 23/02/15 23:11, Stephen Boyd wrote:
> On 02/22/15 16:57, Rob Herring wrote:
>> On Sun, Feb 22, 2015 at 8:32 AM, Maxime Ripard
>> <maxime.ripard@free-electrons.com> wrote:
>>> On Fri, Feb 20, 2015 at 04:01:55PM -0600, Rob Herring wrote:
>>>> [...]
>>>>
>>>>>>> += Data consumers =
>>>>>>> +
>>>>>>> +Required properties:
>>>>>>> +
>>>>>>> +eeproms: List of phandle and data cell specifier triplet, one triplet
>>>>>>> +        for each data cell the device might be interested in. The
>>>>>>> +        triplet consists of the phandle to the eeprom provider, then
>>>>>>> +        the offset in byte within that storage device, and the length
>>>>>>> +        in byte of the data we care about.
>>>>>>
>>>>>> The problem with this is it assumes you know who the consumer is and
>>>>>> that it is a DT node. For example, how would you describe a serial
>>>>>> number?
>>>>> Correct me if I miss understood.
>>>>> Is serial number any different?
>>>>> Am hoping that the eeprom consumer would be aware of offset and size of
>>>>> serial number in the eeprom
>>>>>
>>>>> Cant the consumer do:
>>>>>
>>>>> eeprom-consumer {
>>>>>          eeproms = <&at24 0 4>;
>>>>>          eeprom-names = "device-serial-number";
>>>> Yes, but who is "eeprom-consumer"?
>>> Any device that should lookup values in one of the EEPROM.
>>>
>>>> DT nodes generally describe a h/w block, but it this case, the
>>>> consumer depends on the OS, not the h/w.
>>> Not really, or at least, not more than any similar binding we
>>> currently have.
>>>
>>> The fact that a MAC-address for the first ethernet chip is stored at a
>>> given offset in a given eeprom has nothing to do with the OS.
>> So MAC address would be a valid use to link to a h/w device, but there
>> are certainly cases that don't correspond to a device.
>>
>>>> I'm not saying you can't describe where things are, but I don't
>>>> think you should imply who is the consumer and doing so is
>>>> unnecessarily complicated.
>>> If you only take the case of a serial number, indeed. If you take
>>> other usage into account, you can't really do without it.
>>>
>>>> Also, the layout of EEPROM is likely very much platform specific.
>>> Indeed, which is why it should be in the DT.
>> Agreed. I'm not saying the layout should not be.
>>
>>>> Some could have a more complex structure perhaps with key ids and
>>>> linked list structure.
>>> Then just request the size of the whole list, and parse it afterwards
>>> in your driver?
>>>
>>>> I would do something more simple that is just a list of keys and their
>>>> location like this:
>>>>
>>>> device-serial-number = <start size>;
>>>> key1 = <start size>;
>>>> key2 = <start size>;
>>> I'm sorry, but what's the difference?
>> It can describe the layout completely whether the fields are tied to a
>> h/w device or not.
>>
>> What I would like to see here is the entire layout described covering
>> both types of fields.
>>
>
> I was thinking the DT might be like this on the provider side:
>
>     qfprom@1000000 {
>        reg = <0x1000000 0x1000>;
>        ranges = <0 0x1000000 0x1000>;
>        compatible = "qcom,qfprom-msm8960"
>
>        pvs-data: pvs-data@40 {
>              compatible = "qcom,pvs-a";

These compatibles could be optional. As it might not be required for 
devices which are simple and do not require any special interpretation 
of eeprom data.

>              reg = <0x40 0x20>,
> 	    #eeprom-cells = <0>;

Do we still need eeprom-cells if we are moving to use reg property here?

>        };
>
>         tsens-data: tmdata@10 {
>              compatible = "qcom,tsens-data-msm8960";
>              reg = <0x10 4>, <0x16 4>;
> 	    #eeprom-cells = <0>;
>
>        };
>
>        serial-number: serial@50 {
>              compatible = "qcom,serial-msm8960";
>              reg = <0x50 4>, <0x60 4>;
> 	    #eeprom-cells = <0>;
>
>        };
>     };
>
> and then on the consumer side:
>
> 	device {
> 		eeproms = <&serial-number>;
> 		eeprom-names = "soc-rev-id";
> 	};
>

Yes, this looks good, and Sasha was also recommending something on these 
lines too. Also this addresses the use cases like serial number too.

>
> This would solve a problem where the consumer device is some standard
> off-the-shelf IP block that needs to get some SoC specific calibration
> data from the eeprom. I may want to interpret the bits differently
> depending on which eeprom is connected to my SoC. Sometimes that data
> format may be the same across many variations of the SoC (e.g. the
> qcom,pvs-a node) or it may be completely different for a given SoC (e.g.
> qcom,serial-msm8960 node). I imagine for other SoCs out there it could
> be different depending on which eeprom the board manufacturer decides to
> wire onto their board and how they choose to program the data.
>
> So this is where I think the eeprom-cells and offset + length starts to
> fall apart. It forces us to make up a bunch of different compatible
> strings for our consumer device just so that we can parse the eeprom
> that we decided to use for some SoC/board specific data. Instead I'd
> like to see some framework that expresses exactly which eeprom is on my
> board and how to interpret the bits in a way that doesn't require me to
> keep refining the compatible string for my generic IP block.
>
> I worry that if we put all those details in DT we'll end up having to
> describe individual bits like serial-number-bit-2, serial-number-bit-3,
> etc. because sometimes these pieces of data are scattered all around the
> eeprom and aren't contiguous or aligned on a byte boundary. It may be
> easier to just have a way to express that this is an eeprom with this
> specific layout and my device has data stored in there. Then the driver
> can be told what layout it is (via compatible or some other string based
> means if we're not using DT?) and match that up with some driver data if
> it needs to know how to understand the bits it can read with the
> eeprom_read() API.
Yes this sounds more sensible to let the consumer driver interpret the 
eeprom data than the eeprom framework itself.


--srini
>